Public interest in the methods employed in food production increased considerably during the final decade of the twentieth century, fuelled primarily by concerns about food safety and quality, animal welfare and environmental impact. Discussions have been centred principally in the developed world, where agricultural systems tend to be most intensive. As a result of these concerns, the demand for organic food has increased dramatically, and support schemes to encourage farmers to convert to organic were introduced in the European Union in 1993.
Further evidence of this trend is provided by the adoption of EU Regulation 2092/91 in 1991, which effectively defined organic production in the EU. Similarly, the FAO Codex Alimentarius Guideline was published in 2000 (and revised in 2001), fulfilling a similar purpose and facilitating trade globally as the EU Regulation does in Europe (CEC , 1991; FAO, 2001).
By virtue of such instruments, organic farming ('ecological' or 'biological' agriculture in continental Europe) is a clearly defined system of production that has food quality, human health, environmental, animal welfare and socio-economic aims – aims that derive more from a consumer perspective than from a producer perspective. The result is that organic food has a very strong brand image in the eyes of consumers and thus can command higher prices for retailers and farmers than for conventionally produced food.
Grassland, and in particular forage legumes, plays a major role in almost all successful organic systems, because nitrogen (N) is the most important nutrient for most crops, and organic farming principles and standards place strong emphasis on building soil fertility with minimal use of non-renewable resources.
Principles of organic farming
The detailed husbandry standards of organic farming are primarily based on the principles of enhancement and exploitation of the natural biological cycles in soil (e.g. N2-fixation, nutrient cycling), in crops (e.g. manipulation of competitive ability of crops and populations of natural predators of crop pests) and in livestock (e.g. rumen digestion in ruminants, development of natural immunity in young animals, interruption of host-pathogen relationships). In addition, there is strong emphasis on optimizing animal welfare, avoidance of pollution and improvement of wildlife habitats on the farm.
The aim is to work with natural processes, rather than seek to dominate them, as is often the case in intensive systems, and to minimize the use of non-renewable natural resources, such as the fossil fuel used for manufacture of fertilizers and pesticides (IFOAM, 1998).
EU Regulation 2092/91 (CEC, 1991) sets out the minimum standards for organic crop production in Europe and defines how certification procedures must operate. The important elements in this in relation to grassland are as follows:
It can be seen from the above that the central feature of organic farming is an ecological approach to the management of the system. Soil and manure management, and grassland and livestock husbandry, are modified in order to maximize biological N2 fixation and soil N mineralization, to minimize nutrient losses from the farm, and to minimize disease and parasite problems in livestock. Ultimately the objective is to achieve moderate levels of output from healthy and stress-free animals living in harmony with their environment, with little or no veterinary drug input, little or no fertilizer input, and little or no pesticide input.
In contrast, conventional systems have at their disposal a wide range of fertilizers, crop protection chemicals and veterinary drugs, which enables the farmer to overcome to a large extent the natural ecological constraints of soil, crop and livestock ecosystems. The objective in conventional systems has tended to be to maximize output per unit area, whereas the organic farmer has deliberately chosen to restrict output per unit area in order to preserve fossil energy, minimize chemical input, and enhance biodiversity and animal welfare.
Nitrogen is the single most important nutrient required for herbage growth. In organic systems, nitrogen is also supplied from mineralized soil organic N and from application of organic manures, but the ultimate source of N input to the system is atmospherically-derived N2 fixed by legumes.
In organic swards managed for production, white clover (Trifolium repens) is by far the most appropriate and widely used forage legume in temperate maritime climates, because of its adaptability to a range of management and soil fertility conditions. It is persistent, can thrive at a soil pH of 5.8 (mineral) or 5.5 (peat) as well as in more alkaline soils, and can be used for management regimes ranging from continuous sheep grazing (for which small-leaved varieties are most suitable) to lax defoliation, including cutting (for which larger leaved varieties are most suitable). Red clover (T. pratense) can also grow in moderately acid soils, like white clover, and is highly productive, but is not persistent and should only be sown in mixtures intended for short-term leys (up to three years), primarily for cutting. It is probably the best species for use as a one- or two-year green manure. Alfalfa (Medicago sativa) and sainfoin (Onobrychis sativa) also have considerable potential as crops for conservation, but are suitable only for soils with a high pH and, in the case of alfalfa, good drainage. In very acid soils, of pH 5 and less, bird’s foot trefoil (Lotus corniculatus) and alsike clover (T. hybridum) are alternatives to white clover and red clover respectively.
Although perennial ryegrass (Lolium perenne) has been criticised
as being more appropriate for intensive high-N systems than for organic
or low-input systems, it is undoubtedly the most suitable species for
ley farming in temperate maritime conditions, given its ease of establishment,
yield potential, persistence and quality characteristics. Tetraploid varieties
are known to promote a higher clover content in the sward than diploid
varieties, because of their more open growth habit, and should constitute
at least 75 percent of the perennial ryegrass component in organic
seed mixtures. The main weakness with perennial ryegrass is its lack of
winterhardiness in situations with severe winters, and so in Scandinavia,
continental Europe and
As indicated elsewhere in this chapter, maximizing production per hectare is not always the primary goal in organic farming, and this is true also in terms of choice of species for organic pastures. Apart from productive legumes and grass species, there may be merit, particularly in terms of animal nutrition and health, in offering a wider range of herbage species to livestock. In discussions on botanical composition in grassland, three different sward types can be identified: (i) short-term leys; (ii) permanent swards with a production goal; and (iii) permanent swards managed specifically for their nature conservation value (floral and faunal diversity objectives). Evidence for a division of permanent grassland into the two categories listed above can be seen in the survey of 91 organic grassland sites in Schleswig-Holstein by Worner and Taube (1995), who found that the index of stand value (for agriculture) was closely related to the presence of perennial ryegrass and white clover. The percentage of legume in the sward increased with intensity of utilization, and the content of T. repens tended to be negatively correlated with species diversity. Similarly, Younie and Armstrong (1996) observed relatively little species diversity in an intensively managed and highly productive organic sward over a period of nine years from sowing. The swards were sown initially with a mixture of L. perenne, P. pratense and T. repens. After nine years, over 95 percent of ground cover in this sward comprised L. perenne and T. repens.
Sown pastures, therefore, are usually poor in species diversity, but
have a major advantage in terms of yield and quality. Tables 9.1
and 9.2 compare three pasture types in the
Table 9.1 Production parameters in three different types of organic pasture in Friesland province(1).
Table 9.2 Number of cuts per year and species number in three different types of organic pasture(1).
Sown pastures had a higher clover yield and a higher N yield than old swards. Total yield and the protein content of natural pastures were lower still. This herbage could not be used as the main fodder for high yielding dairy cows, mainly because of the low protein content. The potential number of cuts per year was greatest with grass–clover swards, but the number of species in the old organic pastures was higher (Table 9.2). The input of N must be limited to between 50 and 150 kg N ha-1 yr-1 in order to maintain the species diversity of this old Lolio-Cynosuretum community. Below 50 kg N ha-1 yr-1, and especially under wet conditions and a cutting regime, the natural grasslands were even more botanically diverse. Phosphate status was also correlated with species diversity (negatively) and primary production (positively). Oomes, Korevaar and Altena (1980) showed that a low level of soil potassium was also an important prerequisite for high species diversity.
Although it is clear that there is a correlation between sward composition and sward productivity in terms of livestock output per hectare, farmers often decide to keep their old pastures. The cost of reseeding is high and there is a risk of damaging soil structure in heavy clay soils and peat areas, but farmers also have personal interests, for example in the maintenance of meadow birds in their grasslands. In addition, the maintenance of a high health status in grazing animals and the enhancement of nutrient cycling are regarded by some farmers as important priorities and equal to maximizing output per hectare. Species-rich permanent pastures have an important role to play in achieving these objectives. As long-term biotopes, they are well adapted to the local environment and can offer the advantages associated with a well-developed genotype–environment interaction (compared with the short-term ley, which inevitably employs foreign genotypes). Species-rich permanent pastures constitute a bank of genetic resources as well as a resource for wildlife (insects, butterflies, birds). De Boer (1990) concluded that the potential for meadow birds is highest in extensive grasslands (receiving less than 50 kg N ha-1 yr-1), in combination with long rest periods (longer than 60 days).
A number of herb species are deep rooting and have a better mineral status than “improved” sown species (Jones, 1990; Swift et al., 1993; Fisher and Baker, 1996), and therefore potentially contribute to the health resource of farm animals and to an enhanced nutrient cycle within the farm. They also tend to be more drought resistant than grasses and their tap root systems may also lead to a more open soil structure.
Wilman and Derrick (1994) reported higher calcium and magnesium status in lambs fed a range of herbs, including ribwort (Plantago lanceolata) and dandelion (Taraxacum officinale), compared with perennial ryegrass. Similarly, again in comparison with perennial ryegrass, Younie et al. (2001) demonstrated significant improvements in calcium, sodium, magnesium, copper and vitamin B12 (cobalt) status of lambs from feeding a 100 percent chicory diet (Cichorium intybus). Several trace elements (selenium, zinc and copper) are important in relation to the immune reaction (Van der Brug, 1996). Baars and Opdam (1998) found a relationship between trace element supplementation and somatic cell counts in organic dairy heifers fed a grass–red clover diet. In addition, some herbs, particularly those with a high content of condensed tannins (CT), have been shown to exhibit anti-worm properties when fed to lambs (see below). A number of authors have reviewed the potential of a range of forage herbs, including Elliot (1914), Foster (1988) and Frame, Fisher and Tiley (1994). The herbs with the greatest agronomic, nutritive and medicinal potential are chicory, ribwort, birdsfoot trefoil and, to a lesser extent, burnet and yarrow.
Despite the potential advantages of herbs, there has been little
effort to integrate them into intensively managed sown pastures (Foster, 1988).
This is changing to some extent, at least in
A major challenge for the future, therefore, is to develop
management techniques that can ensure that herbs can make a predictable contribution
in a mixed, intensively managed sward. Research in
Thus, while production and biodiversity tend to be incompatible, opportunities for segregation or integration of different sward types can be considered. This can be approached at landscape level, farm level or field level. Within a farm or within a landscape, it is possible to maintain more extensive permanent grasslands (with or without animal manure) alongside more intensively used sown grasslands. In many situations, of course, there is no dispute about which is the more appropriate choice. On the one hand, land of inherently low nutritional status (e.g. peats or pure sands) may not have the potential for high levels of production. On the other hand, short-term grass–clover leys that are part of a crop rotation will have little opportunity to develop into species-rich pastures, although, as outlined above, it may also be possible to develop productive sown pastures that incorporate a different range of herbage species from that used at present, for specific purposes, such as de-worming paddocks. Similarly, at field level, the two approaches can be married by the management of grassland strips or field margins and boundaries for nature conservation, improved mineral nutrition or anti-worming areas.
Control of perennial weeds in organic grassland
While a diverse range of herbage species is desirable in some contexts, pernicious weeds such as dock (Rumex obtusifolius), ragwort (Senecio jacobea) or creeping thistle (Cirsium arvense) can rarely be seen as desirable sward components. Control of these perennial weeds presents a major agronomic challenge in long-term organic grassland, since herbicide use is prohibited. As in conventional grassland, the best method of weed control is to maintain a dense, vigorous sward that offers little opportunity for weed seeds to become established. In practice this is often not possible, particularly where cutting for hay or silage is necessary, since this opens up the sward and allows an opportunity for weeds to flower and set seed.
Where the site is ploughable, adoption of a regular programme of grass renewal (i.e. a ley system, using 3–5-year leys) will minimize the risk of the problem developing in the first place. This ley policy also has other major advantages in the organic farming context, such as the annual provision of swards uncontaminated with stomach worm larvae, and the opportunity to exploit fertility built up in the soil by growing arable cash crops or annual forage crops for finishing lambs. However, there is an expense involved in reseeding and there will be situations where site conditions prevent regular ploughing and reseeding.
Ragwort is effectively controlled in long-term grassland by sheep grazing in winter and early spring, but this is not effective for dock and creeping thistle. With these weeds, other approaches are necessary. A dock infestation becomes worse much more rapidly under a cutting than under a grazing management regime (Courtney, 1985), and so continuous cutting for hay or silage should be avoided in permanent grassland, if possible. Regular topping of docks in grazed swards, and soil aeration to improve grass growth and cause physical damage to dock roots, may also limit the increase in dock density (Hopkins et al., 1997). At low levels of dock infestation, hand digging should be undertaken in order to prevent dock numbers from increasing. When dock-infested grassland is destroyed prior to establishment of the following crop, rotovating the turf to a depth of 8–10 cm in early to mid-summer, followed by tine cultivation and desiccation of the surface material for 8–10 weeks before ploughing, has been successful in farm practice (M. Measures, pers. comm.). In addition to killing the crowns of mature dock plants, this practice essentially creates a stale seedbed, encouraging germination of dock seeds that are subsequently killed by the further cultivation. Cropping with a whole-crop silage mixture of cereals before reseeding with grass provides a further opportunity to reduce the dock seedbank through the allelopathic and competitive characteristics of the cereal crop. Ideally, this should be combined with harvesting of the mature dock tap roots during seedbed preparation for the cereal crop, e.g. by ridging and use of a potato stone separator or potato elevator harvester to bring the roots to the surface. Creeping thistle can be controlled by the competition created by the dense canopy of a specialist red clover mixture managed for early-cut silage. The creeping thistle plants will be depleted within one or two seasons under this regime. Topping at an early growth stage (15–20 cm) is reported as being more effective than topping later, immediately prior to flowering (M. Dock Gustaffson, pers. comm.). Inevitably, a weed control strategy involving a combination of these approaches is likely to be required for long-term control of these weeds.
Organic farmers in maritime NW Europe rarely express concern about pests and diseases in grassland, although clearly stem eelworm (Ditylenchus dipsaci) (in red clover) and Sclerotinia clover rot (Sclerotinia trifoliorum) could potentially be major problems. Slug damage can be a problem where clover varieties are used that contain low levels of cyanogenic glucosides, and in oversowing situations (Baars, 2002). There is little that can be done to control these problems within the constraints of the organic standards, except by selection of resistant varieties and by rotation, i.e. alternating between legume species where appropriate rather than basing the rotation entirely on one species. Pests and diseases are greater threats with alfalfa than with red and white clovers. Verticillium wilt (Verticillium alba-atrum) and alfalfa weevil (Hypera spp.) are probably the most important, but Frame, Charlton and Laidlaw (1998) list 22 diseases and 20 pests of alfalfa. Varietal resistance and rotation are again the main approaches in controlling these problems in organic farming.
Maintaining livestock health at grass
Stomach worm infection in lambs is the most common health problem in organic lambs (Halliday et al., 1991; Roderick, Short and Hovi, 1996). Grazing management has a major role to play in minimizing worm infection (Younie et al., 2003). It should be the main component of worm control strategy, and management decisions must be designed primarily to minimize parasite infection. Essentially, this involves restricting the stocking rate of susceptible animals, particularly young stock, and minimizing the contamination of pasture with parasite larvae and the parasite challenge to these animals, for example, by:
Parasite challenge to susceptible animals is likely to be least on lowland, mixed farms with a mix of cattle and sheep enterprises, where opportunities exist to utilize crop stubbles, crop residues and annual forage crops as well as grassland; where young, larvae-free reseeds are available on an annual basis; where all grass fields can be mown for conservation and included in alternating clean grazing systems; and where cattle can provide an effective dilution of the parasite challenge to lambs. Conversely, parasite and disease problems are likely to be greatest on the sheep-only upland farm, where opportunities to take forage conservation cuts are limited and a clean grazing system is difficult to establish.
Even where it is possible to establish an alternating clean grazing system, it is not always effective in controlling parasitic gastroenteritis in lambs, primarily because of the pasture contamination caused by the ewes themselves at lambing (the peri-parturient egg rise) (Halliday, Gray and Younie, 1994). There is a need to consider other approaches to reducing the problem, e.g. breeding for resistance to parasites in the sheep flock, exploiting the differential parasite burdens of different pasture species (Keatinge et al., 2002; Niezen et al., 1998a, b; Thamsborg, Roepstorff and Larsen, 1999). For example, a number of herbage species, primarily high-condensed-tannin species such as Lotus pedunculatus and L. corniculatus, but also chicory, have been shown to reduce the effects of worm infection in young animals (Barry and McNabb, 1999; Molan et al., 2000). The suggested mechanisms for this include (i) a direct toxic effect of high-condensed-tannin on the parasite; (ii) an improvement in protein or mineral status of the animal, or both; and (iii) an adverse affect of these herbage species on the survival of the parasite in the sward. Further parasitological and agronomic research is necessary to determine how to utilize and integrate these species into the grassland system to best effect (e.g. as separate anti-worming paddocks for use at critical times, or as an integral component of the main seed mixture).
Grassland and ruminant livestock management on organic farms should be planned to make the maximum use of grazing. Apart from considerations of the cost of conserving grass (and the concomitant nutrient removal from the soil, see below) and the cost of organic concentrates, health problems tend to be fewer in animals kept outdoors, particularly where calving and lambing are undertaken outdoors (provided weather and soil conditions permit). Stress, which can contribute to metabolic disorders (e.g. hypomagnesaemia) or breakdown in natural immunity, should be minimized at all times in the grazing situation, for example by appropriate construction of gateways and feeding stances, so as to minimize poaching (and hence potential foot problems), by provision of access to shelter (e.g. by hedging along field boundaries), and by careful grouping, feeding and handling of animals.
Bloat is often mentioned as a potential risk in legume-based grassland systems. While the risk exists (in the grazing situation, but not with silage, and with cattle, but much less so with sheep), it is a relatively infrequent occurrence and can be predicted (and therefore avoided) to a large extent. The risk is greatest in the second half of the growing season (in NW maritime Europe), if animals are offered relatively rank herbage with a high clover content and low dry matter content (e.g. silage aftermaths or around dung pats, on wet or dewy mornings). Hungry animals should never be introduced to such herbage in these conditions. Animals should be introduced to aftermaths soon after the crop has been removed, when the regrowth is very short, so that the clover grows up to the animals rather than vice versa. In high-risk situations, a straw buffer feed reduces the risk. Dairy cows should be fed a buffer feed of forage after milking before returning to pasture. However, poloxalene bloat preventative, fed with a carrier feed such as barley or brewer's grains, is the most effective insurance. The risk of bloat is also substantially reduced in legumes with a high tannin content, e.g. sainfoin, and so considerable effort is now under way to breed anti-bloating clovers with a high tannin content, although varieties produced by transgenic procedures (genetic engineering) would not be permitted in organic farming.
Organic farming systems and nitrogen fixation
As indicated earlier, nitrogen is the single most important nutrient
required for herbage growth. In organic systems, N is supplied from mineralized
soil organic N and from applications of organic manures, but the ultimate
source of N input to the system is atmospherically-derived N fixed by
herbage legumes. The N resource in organic farming is, therefore, a renewable
resource in comparison with fossil-fuel derived artificial N fertilizers.
Whitehead (1995), quoting seven series of trials in the United Kingdom,
reported amounts of fixed N ranging from 0 to 445 kg N ha-1 yr-1,
with an overall average of 152 kg N ha-1 yr-1.
Other workers have reported N fixation input to grassland as a fertilizer-N
equivalent value, ranging between 100 and 250 kg N ha-1 yr-1
for temperate European conditions. The lower end of this range is associated
with soils of high potential for N mineralization, continuous grazing
management and smaller-leaved white clovers, while the upper end of the
range is associated with soils of low potential for N mineralization,
cutting or rotational grazing management, large-leaved white clovers and
red clover or alfalfa (Reid, 1970; Morrison, 1981; Frame, 1990; Davies
and Hopkins, 1996). Halberg, Kristensen and Kristensen (1995) estimated
atmospherically derived N inputs on six organic mixed farms in
The level of N fixation is notoriously difficult to measure accurately (Wood, 1996) but is accepted to be primarily dependent on the clover content of the sward, particularly in the early years of a ley (Frame and Newbould, 1986; Van der Meer and Baan Hofman, 1989). However, this is not a linear relationship. In addition to contributing to herbage growth, a proportion of the atmospherically fixed N is stored in roots and stubble, or is immobilized in soil organic matter, i.e. it contributes to the longer-term build up of soil organic matter and soil N status. Simultaneously, a proportion of the N in the soil organic pool is mineralized and becomes available for uptake by pasture plants. As the supply of N from mineralized soil organic matter increases (e.g. in the later years of a ley, or in swards with a very high clover content), the relative contribution from N fixation to the overall supply of N to the sward declines (Younie, 1992; Davies and Hopkins, 1996). Baars (2002) has reported that, in the Netherlands, between 40 and 65 kg N is fixed per tonne of white clover DM, but as clover content in the sward increases above 50–60 percent in DM, N fixation decreases to 7 kg N per tonne of white clover DM because of the increased amount of N in the soil and the lack of companion grass to act as a sink for this soil N.
Halberg, Kristensen and Kristensen (1995) estimated that, on Danish organic farms, the amount of surplus N available annually for immobilization in the soil and for loss to the environment was between 18 and 231 kg N ha-1 on grass-clover leys and between 3 and 59 kg N ha-1 on permanent grass. The equivalent figure calculated by Watson and Younie (1995a), for an organic beef farmlet, was 103 kg N ha-1, compared with 216 kg N ha-1 for a conventional high-N-fertilizer system. On a whole-farm basis, the average N surplus for 14 commercial organic farms calculated by Halberg, Kristensen and Kristensen (1995) was 124 kg N ha-1 yr-1, while for the comparable sixteen conventional farms the N surplus was 240 kg N ha-1 yr-1.
Thus, herbage legumes, normally in the form of grass-clover leys, play a vitally important role in organic systems through the capture of the atmospheric N resource, enabling its subsequent exploitation by ruminant livestock and arable crops. In a self-contained organic farming system, the proportion of grass-clover ley or herbage legume crop in the rotation will determine the total N input, which will in turn determine arable crop yields (Younie et al., 1995). The ideal proportion of fertility-building crop (i.e. legume) in the rotation has never been definitively determined. A minimum of 50 percent is often suggested, but good crop yields have been obtained in good soils, at least over the short term, with a stockless rotation containing only one year in five of red clover green manure, plus a grain legume (Cormack, 1997). Nitrogen fixation rates tend to be higher in the early years of a grass–clover ley (e.g. Kristensen, Høgh-Jensen and Kristensen, 1995), which would suggest that short-term (two- to three-year) leys are a more efficient way, compared with longer-term leys, of utilizing the atmospheric N resource within the overall context of fertility building and exploitation. Johnson et al. (1994) have also suggested that the optimum length of ley in terms of its residual value to following crops is three years.
A well aerated soil is an essential aim in organic farming because of the beneficial effect this has on soil N mineralization and the reduced risk of N loss through denitrification. The positive influence of grassland on soil structure is well established (Tisdall and Oades, 1980; Eder and Harrod, 1996) and it has been suggested that legumes in particular enhance the soil aggregation process (Angers and Carter, 1996). Reganold et al. (1993), in a paired-farm comparison between biodynamic and conventional farms in New Zealand, found that pasture soils on biodynamic farms had significantly lower bulk density and lower penetration resistance than pastures on conventional farms, indicating better soil structure and less compaction. These authors suggested no reasons for these differences, but a higher clover content in the swards may have contributed.
Although a grass-clover ley will undoubtedly improve soil structure relative to the arable cropping phase, soil compaction does occur in grassland situations, including organic farming, and indeed is of particular importance in the organic context, given the heavy reliance on mineralized N in organic farming. Heavy silage-making equipment, high stocking densities, particularly when soil conditions are wet (e.g. around feeding troughs in winter), will all cause compaction and reduce soil N availability, both to the grass–clover ley itself and possibly to the following arable crop. The effect of compaction on N offtake in unfertilized grass (reflecting N mineralization), caused by a typical sequence of silage harvesting operations, is shown in Table 9.3.
Table 9.3 Effect of compaction on annual N off-take (kg ha-1) from three silage cuts (zero N fertilizer input).
Outdoor pig production can also cause serious soil compaction, which can have an adverse residual effect on the subsequent arable crop (Edwards and Watson, 1997). As an indication of development of anaerobic zones in the soil, Petersen and Eriksen (1999) also measured a high level of denitrification from grazing areas with outdoor sows. The strategy for moving huts and siting feeding areas must aim to avoid compaction (Larsen and Kongsted, 1999). In the further development of organic pig farms (as opposed to conventional farms), it is very important that grazing systems for the pigs are developed so that the pigs actually utilise the grass sward as a source of nutrients. This may at the same time lead to outdoor pig production systems with much smaller negative effects on soil compaction and nutrient losses. There is need for further research, within the context of organic farming, to quantify these effects and to examine practical methods of ameliorating soil compaction in organic grassland.
Soil biological activity
A high degree of soil biological activity is associated with good soil structure and soil N mineralization. Manipulation and enhancement of this biological resource is a major aim of organic farming. As a result of the permanent green cover and vigorous root growth afforded by grassland, by the absence of soil disturbance, and by the consequent increase in soil organic matter, soil biological activity is greater under grassland than under arable cropping. This is one of the primary reasons for the inclusion of grass–clover leys in organic rotations. Earthworm populations increase during the ley phase and decline under cropping as a result of increased frequency of cultivation, as shown in organic crop rotation experiments in NE Scotland (G. Armstrong, unpublished information).
Likewise, from the same set of experiments, Watson et al. (1996) showed that soil microbial biomass and activity were greater under grass–clover than under arable crops. These results are not unique and have been mirrored elsewhere (e.g. Follett and Schimel, 1989; Weil, Lowell and Shade, 1993).
Because of the higher investment costs and lower profitability of livestock
systems relative to cropping systems, stockless organic rotations are
an attractive proposition economically. However, an unanswered question
remains about the level of nitrogen supply (and therefore crop yield)
that such systems can sustain with a reduced proportion of herbage legume
and grass–clover ley in the rotation. Part of this question relates to
the maintenance of soil biological activity in a system with high cultivation
and cropping frequency. It would seem logical that the nearer to a continuous
cropping system we approach, and the shorter the ley or green manure phase,
the lower the level of soil biological activity, the lower the soil organic
matter content and the lower the potential for soil N mineralization.
Indeed, the long-term Rodale Farming Systems Trial in the
Higher soil biological activity (e.g. earthworm populations) has sometimes been reported in organic grassland (Figure 9.1) compared with conventional grassland (Reganold and Palmer, 1995). The addition of more organic matter in the form of animal manures will improve the food supply for earthworms and conversely the use of biocides in conventional agriculture may reduce earthworm populations, but it is conceivable that the application of mineral fertilizer in conventional agriculture, by virtue of increasing the food supply, will enhance earthworm populations (Younie and Armstrong, 1996). However, Baars (2002) has shown that, compared with slurry and farmyard manure (FYM), superphosphate and muriate of potash do reduce earthworm populations, possibly as a result of lower soil pH. In these studies, application of FYM resulted in the highest density of earthworms, particularly those worm species that move vertically into deeper soil layers.
Maintaining soil nutrient status
One of the major aims of organic farming is to minimize the use of non-renewable resources. It follows, therefore, that soil nutrient status must be maintained as far as possible through management means and by efficient nutrient cycling within the farm. In grassland, the main priority concerns the removal of nutrients from fields cut for hay or silage. With a herbage yield of 10 tonne DM ha-1 yr-1, nutrient offtake in the herbage will be in the region of 80 kg P2O5 ha-1 yr-1 and 260 kg K2O ha-1 yr-1 (Fowler, Watson and Wilman, 1993; Younie et al., 1998). This can obviously have a major effect on soil K and P content and consequently on herbage and crop yield.
In a systems comparison in
Figure 9.2 shows the effect of first-cut silage yield on exchangeable soil K content nine months later, in grass–white clover swards in an organic crop rotations trial on a sandy loam soil (10 percent clay) in NE Scotland. The relationship is significant at the P<0.001 level.
Similarly, on a sandy soil in the
Clovers are poor competitors for soil P and K compared with grass. The reductions in K content in the experiments of Baars (2002) described above caused a significant loss of clover from the swards. This was also shown in an experimental situation on a sandy soil by Van der Meer and Baan Hofman (1999). Fothergill, Davies and Morgan (1995) have also shown the significant effect of P and K on white clover content in upland pasture. Low clover content resulting from restricted K input has also been observed in swards in a long-term trial on sandy loam soil in NE Scotland (Younie, unpublished information). This adverse effect of low soil P and K content on clover content will restrict the input of atmospherically fixed N in the system and have a knock-on effect on herbage yield. Newton and Stopes (1995) reported that soil P status was the best predictor of herbage production in a survey of organic farms.
It is essential, therefore, that organic grass–clover swards intended for conservation be given high priority in the distribution of manures, particularly on sandy soils. In a field trial in which a range of manure and silage effluent treatments were applied to an organic grass–clover sward on loamy sand soil (5 percent clay) for two cuts of silage, it was estimated that, in order to maintain a zero change in soil K content, an annual application of 233 kg K ha-1 (282 kg K2O ha-1) was necessary (Younie et al., 1998). The same trial indicated that where two cuts were being harvested, a single spring application of 20 t FYM ha-1 was less effective in maintaining soil K content than two applications of 10 t ha-1, one for each cut. Baars (2002) showed that K yield and K concentration per kilogram of herbage DM were affected by the total K application (Table 9.4). Phosphorus yield and P concentrations, however, were much less affected by P inputs.
At the same time, careful consideration needs to be given to K application strategy in order to minimize the risk of luxury uptake. Luxury uptake of K can result from large spring applications of manure, and therefore manure and effluent applications should be spread evenly throughout the season, perhaps even with an emphasis towards late-season application, although this increases the risk of leaching loss of N and K.
Because of the labile nature of K in manure and to a lesser extent in soils, the risk of loss during the nutrient cycling process itself is substantial, in contrast to the situation with P. Berner (1986) estimated losses of 50–60 percent of total K from FYM heaps on commercial organic farms. Nolte and Werner (1994) estimated that 78 percent of whole-farm K losses were lost during the internal nutrient cycling process, but only 25 percent of P losses, and suggested that the K balance could be improved substantially by improving manure handling and storage procedures, such as by covering manure heaps and avoiding application of manures in autumn and winter.
Apart from the specific issue of manure management, practical issues of management and land type often limit the efficiency with which nutrients can be recycled within the farm. Ideally, in order to prevent excessive nutrient offtake from individual fields, management of any one field should alternate annually between cutting and grazing, rather than continuous cutting. This alternating management also has benefits in terms of weed control (see earlier). However, in practice this is difficult to achieve on many farms, e.g. dairy farms, where the cow grazing pastures must be located within walking distance of the milking parlour, or on upland farms where a restricted choice of flat fields for cutting results in some fields being cut every year. On some farms, all stock are fed and wintered outside, e.g. on rough ground. While this may be healthy for the livestock, it means that nutrients are being transferred from silage cutting fields to the wintering land. On farms with arable crops as well as grassland and livestock, inevitably there will be transfer of nutrients, of K in particular, from grassland to arable cropping land, given that FYM is one of the major sources of nutrient for arable and vegetable crops in organic systems. However, in a mixed ley–arable system the rotation of crops means that these nutrients will ultimately be uniformly distributed around the farm.
Table 9.4. Relationship between mineral yield (Y) or mineral concentration (C) and mineral input (I) (for each equation, n = 9).
Nevertheless, even if 100 percent efficiency in nutrient cycling were possible, there would still be a considerable loss of P and K in crop and livestock products sold off the farm and in leaching losses in the field (on biodynamic farms, Nolte and Werner (1994) estimated 14 percent of K losses and 73 percent of P losses were in products sold off the farm). The inference is, therefore, that soil P and K in grassland will be depleted over time, or that supplies of P and K must be brought into the system. In the Netherlands, where neighbouring specialist organic arable farms and organic dairy farms have developed partnerships for transfer of feed and manure, there is a view that soil P and K content on grass–clover swards on dairy farms should be maintained by mineral fertilizer (see below) and that the manure is better targeted towards arable land (Baars, 2002).
The rate and extent of depletion of soil P and K, and therefore the need for inputs, will depend on the level of crop yield and nutrient offtake and the level of soil nutrient reserves. Some clay soils have large reserves of potassium ions and have the capacity to release K into the exchangeable fraction, and hence support good crop growth over a long period of time. Sandy or coarse-textured soils with a low clay content, in contrast, have a low reserve of non-exchangeable K and will be unable to sustain high production levels without K inputs.
Rock phosphate is permitted as a nutrient input in organic farming and is a relatively effective source of P, but unfortunately there are few effective sources of K input that are acceptable in organic systems. Simpson and Stopes (1991) assessed a range of potassium fertilizers for organic farming and found that rock potash was a relatively ineffective source of potash, whereas lime kiln dust gave useful responses. Potassium sulphate, potassium salts (kainit, sylvinite) and sugar beet by-product (Cummulus K) are effective, but under EU Regulation 2092/91, before these products can be used the producer must prove to the certification body that there is a need. A further potential source of potash is in straw bought in for bedding, but, assuming a stocking rate of one Livestock Unit per forage hectare, this will supply only about 13 kg K ha-1 if one tonne of barley straw is purchased annually per Livestock Unit. Further research is necessary to assess the effectiveness of approved external sources of these nutrients, potassium in particular.
The impact of organic grassland on the environment
Care for the environment, including wildlife habitats, and minimal use of non-renewable resources are enshrined as principles of organic farming. One of the strategic objectives at farm level is to minimize leaching of nutrients into surface and ground waters. For the organic farmer in particular, this is important from a production, as well as from an environmental, point of view. The extent to which organic farming can be regarded as a sustainable system can also be assessed by the potential for reduced use of fossil fuel resources and for enhanced biodiversity. Grassland plays an important role in these elements of organic philosophy. Aspects of botanical composition and biodiversity in organic grassland were discussed earlier in this chapter, and so the discussion below focuses on nutrient loss to the environment and on energy use.
Nutrient emissions to the aquatic environment
It is now accepted that the risk of nitrate-N leaching from grassland is influenced more by the total quantity of N circulating in the system, rather than by the source of the N. Swards with similar stock carrying capacity will result in similar environmental losses of N, whether based on N derived from N-fixation by legumes or supplied as fertilizer N (Titchen and Philipps, 1996). A number of comparative studies have shown that the N surplus, and hence the potential for nitrate-N leaching, is less from clover-based systems, including organic grassland, than from intensively fertilized grassland (Halberg, Kristensen and Kristensen, 1995; Watson and Younie, 1995a; Tyson et al., 1996). However, Schils (2002) showed an increased loss of nitrogen as white clover content in the sward increased, especially where the sward was grazed after September and in combination with high proportions of Poa annua and Taraxacum officinale (dandelion) in the sward.
Maximizing green cover of soil over winter is encouraged in organic farming, not only to ensure uptake of surplus mineral N from the soil and so minimize leaching losses, but also to minimize the risk of soil erosion. The inclusion of a ley in the crop rotation obviously goes some way towards achieving this objective, but other forage species are often also sown specifically as winter covers during the arable cropping phase of the rotation. Species such as mustard (Sinapis alba) and Phacelia are effective scavengers of soil N and are commonly used as cover crops, but graminaceous species such as grazing rye (Secale cereale) and Italian ryegrass (Lolium multiflorum) can also be used. At farm level, however, the farmer must balance the perceived benefits in terms of the amount of N saved, or prevented from leaking to the environment, against the establishment costs, including seed costs. The extent of any benefit will depend not only on the species used as a cover crop, but also on how well it establishes, and on how much mineral N is present in the soil initially. In this latter regard, the winter following the first arable crop in the rotation is normally when soil N is at its highest level in the cropping phase of the rotation and therefore when winter cover crops offer the greatest potential benefit. A further consideration in relation to establishment of winter covers is the length of the growing season available for establishment, following the harvest of the main crop. Effective establishment of winter covers can be achieved from direct sowing following early harvested crops such as early potatoes or vegetables, or in locations with a long growing season; however, in areas with a short growing season, or following a late-harvested cereal, for example, direct sowing of winter cover crops can often result in poor establishment (Watson and Younie, 1995b). In these situations, undersowing the winter cover with the previous cereal crop will give better establishment.
In the context of organic mixed ley-arable systems, the management of
the transition phase from ley to arable cropping, the most critical point
for N leaching in organic rotations, is of crucial importance. It is clear
that autumn ploughing (usually for establishment of a winter cereal) leads
to more leaching of N than spring ploughing. Cobb et al. (1997)
measured leaching losses of between 119 and 132 kg N ha-1
in organic winter wheat in
Date of ploughing of the ley (or indeed of a grass winter cover crop) relative to the sowing date of the following crop is important not only in relation to minimizing leaching loss, but also in controlling N immobilization by the incorporated ley vegetation. If a ley (or any other vegetation type with a high carbon:nitrogen ratio) is ploughed too close to the sowing date of the crop, N can be immobilized and this can significantly reduce crop N uptake and yield. Younie and Watson (1995) measured a 1.3 t ha-1 reduction in grain yield of spring barley following a March ploughing of an organic grass–clover ley, compared with a January ploughing date. A delay of at least one month is necessary between the ploughing of a ley or a grass winter cover crop and the sowing of the subsequent arable crop.
Nutrient emissions to the atmosphere
Ammonia volatilization from livestock systems makes a major contribution
to acid rainfall. For example, in the
In addition to ammonia volatilization, grassland and ruminant livestock
systems represent a major source of the greenhouse gases methane (CH4)
and nitrous oxide (N2O), and carbon dioxide (CO2)
is released in the manufacture of inputs such as fertilizers and the burning
of fossil fuels on farms. Bakken et al. (1994) compared the emissions
of greenhouse gases from high intensity and low intensity (organic) dairy
They calculated that, on a per-hectare basis, the organic system produced less CO2 equivalents; but because of the lower output per hectare of the organic system, the amount of CO2 equivalent per unit of milk produced was almost identical for the two types of farm. However, the energy consumption on the organic farm used by Bakken et al. (1994) for their study may be unusually high because thermal weed control was used in the fodder beet crop. N2O emissions from grassland on the organic farm were only one third that from the conventional farm, as a result of lower fluxes from the soil (due to lower N levels) and the absence of emissions from the industrial production of nitrate. The largest contributor of greenhouse gases was CH4, originating primarily from rumen digestion (contributing approximately 60 percent of CO2 equivalents), and the organic farm produced only 17 percent less per hectare than the conventional farm, simply mirroring the lower stocking rate.
Table 9.5 Effect of grassland farming intensity on global warming gas emissions (expressed as t CO2equivalent ha-1 per annum).
Thus, organic grassland systems do appear, on a per hectare basis, to produce less pollution of the aquatic and atmospheric environments, although to some extent this is simply a reflection of lower stocking rates. Since the main sources of pollution from organic livestock farming are derived from integral components within ruminant livestock systems (e.g. volatilization of ammonia, methane from rumen digestion), rather than from excessive nutrient or energy inputs, the main approach towards reducing emission of nutrients and greenhouse gases must be the further improvement of internal nutrient cycling (e.g. slurry injection in early spring rather than surface spreading in early winter).
Energy use in organic grassland
Energy consumption is discussed in the previous section in the context
of CO2 release and the contribution of organic grassland to
global warming. It is also pertinent to discuss the influence of organic
grassland systems in terms of consumption of non-renewable fossil fuel
resources. Refsgaard et al. (1998), in a modelling exercise based
on case studies, compared the energy utilization of the crop enterprises
on fourteen organic and seventeen conventional dairy farms in
Silage harvesting had the biggest demand for direct energy (i.e. diesel fuel) in both organic and conventional systems (51–53 percent), but organic farms consumed more direct energy for manure handling, particularly for dry manure (17 percent and 6 percent for organic and conventional systems, respectively). However, the main difference in energy consumption between the two farm types was in indirect energy use, in which conventional grassland exceeded organic grassland by a factor of eight, primarily because of the energy required in the manufacture of fertilizer N. Grass+clover was more energy productive than fodder beet or whole-crop silage, and increasing the forage proportion grazed rather than ensiled improved the productivity still further.
Organic grassland appears to be relatively energy efficient, principally as a result of the non-use of N fertilizer. Scope for further improvements in energy utilization appears to lie primarily in modifications to the silage making process and manure handling and application.
Table 9.6 Model of energy productivity in grass-clover swards in organic and conventional dairy farms; non-irrigated sand, Denmark
Adoption of high dry matter silage systems (to minimize the amount of water that has to be moved) and big bales (to minimize the amount of chopping involved) may offer opportunities for reducing the energy costs associated with silage making. There is a need to model the energy costs of the manure handling process; composting will increase costs but subsequent spreading may be easier and incur lower energy costs.
Livestock systems for organic grassland
Given satisfactory levels of soil P, K and pH status (and soil moisture supply), the level of herbage production and livestock output from organic grassland is largely dependent on its legume content. In this regard it equates closely with conventional low-external input grass+clover swards. A considerable body of research literature now exists on legume-based grassland systems and most of this is relevant to organic farming (e.g. Frame, Charlton and Laidlaw, 1998).
Published data on herbage production measured under organic farming conditions is presented in Table 9.7. These generally reflect the conditions under which the swards were monitored, namely site class, legume species, cutting date or regrowth interval. The data of Halberg and Kristensen (1997) included a comparison with conventional farms. Corrected for differences in climate, the “organic” yield of grass+clover swards was 12–14 percent lower. In the comparison of Younie and Wightman (1992), the organic system yielded 16–18 percent less than the comparable conventional system, which received 270 kg N ha-1 yr-1.
Table 9.7 Production per hectare from organic grassland
Factors relating to livestock nutrient supply (from grassland and supplementary feeds) have a major influence on the choice of livestock system on organic farms. Organic standards specify that ruminant animals should obtain most of their nutrient supply from forage. Calving in spring or early summer rather than in autumn is an ideal system since the peak period for nutrient requirement in the animal matches the grass growing season. With sheep, early lambing in January is not an appropriate system, given the level of concentrate feeding involved. In contrast, lambing later than normal, in late April or May, to coincide with the later spring growth of clover-based organic swards, is well suited to organic farming and reduces the input of concentrates and the overall cost of production. However, in practice the marketing implications (i.e. when the animal is likely to be ready for slaughter) must also be taken into account together with these technical factors when selecting a date for calving and lambing.
Milk yield is very important for farm profitability in organic dairying. It is often difficult to satisfy expected nutritional requirements in winter diets, particularly the need of high yielding dairy cows for absorbable protein. In this respect, it is interesting that Mogensen and Kristensen (1999) found no effect of protein supplementation in organic dairy herds fed a diet with a very high proportion of grass+clover silage. This underlines the importance of highly digestible, high quality grass+clover silage for winter feeding. However, in summer, pastures with a high white clover content can give an excessively high protein content in the diet, resulting in an energy:protein imbalance. Milk yield and nitrogen efficiency are improved if supplementary fodder crops with high energy content, such as maize silage or cereal whole-crop silage, are fed during this period, which usually occurs after June in NW Europe. Measurement of the amount of urea in milk is an effective means of determining the level of protein excess in the diet (Van Eekeren, 2000).
Profitability of livestock enterprises is a function of physical output multiplied by unit sale price, minus input costs. Physical output per head from organic beef and sheep enterprises compares well with equivalent conventional grass-based enterprises (Younie and Mackie, 1996; Younie, 1997), with beef animals reaching slaughter condition in 17 to 21 months. In contrast, the average milk yield per cow from organic dairy herds, at between 5 000 and 6 000 kg-1 cow-1 yr-1, is normally lower than from conventional units (Houghton and Poole, 1990; Redman, 1991; Lampkin, 1994; Weller, Cooper and Padel, 1996; Krutzinna, Boehncke and Herrmann, 1996), reflecting the generally lower levels of concentrate input in organic systems, although milk yields of 7 000 kg-1 cow-1 yr-1 are achievable. The high cost of purchased organic concentrates can result in relatively high feed costs per head, particularly in dairy and finishing beef enterprises, and although this is offset by reduced forage costs, total variable costs per head are generally only slightly lower than in conventional enterprises.
Fluctuations in unit sale price of both organic and conventional products make detailed financial comparisons difficult and valid only in the short term. Where organic premiums can be achieved, gross margin per head for organic livestock enterprises is normally substantially higher than the conventional equivalent. On a per hectare basis, organic systems that are clover-based will normally support a stocking rate and livestock output equivalent to a conventional system receiving up to about 180–200 kg N ha-1 yr-1 and should therefore achieve similar or better gross margins compared with conventional enterprises, even without the benefit of organic premium prices. With the added bonus of higher prices, organic systems have the potential to be substantially more profitable on a per hectare basis than conventional enterprises (e.g. Keatinge, 1997; Younie, 1997). At the same time, compared with intensively fertilized conventional enterprises receiving more than 200 kg N ha-1 yr-1, organic premiums are probably essential if organic systems are to match equivalent conventional systems on gross margin per hectare.
For example, under Danish conditions, financial analysis showed a considerably higher profitability from organic milk production in the period 1993–1997 compared with conventional production. During this period, the organic milk price at the farmgate was 30–40 percent higher than the conventional milk price (Danish Central Advisory Service, 1998). It is estimated that an organic premium of 15–20 percent is necessary for organic milk production to maintain a level of profitability equivalent to conventional milk production.
In fulfilling the principles of organic farming, the two major strategic objectives of farmers are to maximize the efficiency of internal nutrient cycling, thus minimizing the need for nutrient inputs, and to minimize problems by modifying the management system. These objectives are much easier to achieve in the sphere of grassland-based livestock systems than in the arable farming sphere: a wide range of herbage legumes is available to ensure an adequate N supply; there is built-in nutrient cycling by animals eating the herbage; and management opportunities exist for minimizing weed and pest problems.
However, this is not to say that further challenges do not exist for organic grassland farmers. These challenges exist not only at the farm system level, but also derive from external pressures. The organic movement in general is faced with pressure to dilute its standards because of global competition as organic becomes more accepted as a mainstream system. Pressure on the standards also comes from the increasing reliance on genetic manipulation techniques in breeding programmes and on the targeting of the organic sector by agricultural ancillary industries with new input products.
There are other pressures that require discussion within the organic movement, such as the issue of farm specialization and nutrient inputs. The ideal organic farm is a mixed grass+livestock+arable farm in which nutrients are cycled efficiently via herbage, crop by-products and livestock. The need for imported nutrients in such a system depends primarily on the proportion of arable crop in the rotation (and hence the proportion of nutrients that leave the farm). However, as in conventional agriculture, there is a trend towards specialization in organic farming, and there is a need to consider how best to integrate livestock farms with arable farms. Given the importance of soil organic matter, there may be a case for transfer of manure from livestock farms to neighbouring partner arable farms, with P and K removal from the livestock farms compensated for by the importation of permitted mineral P and K fertilizers. A discussion needs to be initiated within the organic movement to discuss the acceptability of approaches such as this. Linked with this is the issue of use of human waste in organic farming, a practice that is currently prohibited but that in theory would close the nutrient cycle.
At the farm system level, organic livestock should be in harmony with their environment. This includes relationships with other stock, with the nutritional and physical environments to which they are exposed, and the interaction they have with humans. In this context, there is a need for further development and adoption of grassland management techniques for parasite control in livestock. A better understanding is required of the complex links between parasite epidemiology, weather conditions, botanical composition of the sward, sward structure, stocking rate and stock handling. Another area where grassland management has a role in animal health is in mineral nutrition, particularly the balance of minerals in the herbage. In conventional farming, mineral imbalances are sometimes caused through fertilizer applications. There should be a lower risk in organic farming, where external nutrient inputs are minimized, but nevertheless greater knowledge and understanding is required. This may involve greater acceptance and use of unimproved natural pastures or, in sown swards, the use of seed mixtures with a wider range of species. Such swards and mixtures would also offer an opportunity for self-medication by animals. This is a concept that is claimed to apply to animals in the wild, but it may also apply in domesticated animals if the opportunity existed.
While the agronomy of the major legumes is now sufficiently well understood to provide reliable herbage production guidelines for most farmers, there is still a need for more reliable forage conservation techniques in marginal or extreme areas (e.g. regions at high altitude or high latitude; regions with high or low rainfall), for example by the introduction of new varieties or species and better soil management, in particular through improved soil aeration. Control of perennial weeds, particularly docks, is still one of the major problem areas in organic grassland and simple and reliable control techniques have still to be developed. Given the complexity of vegetation dynamics in grass swards, it is likely that simple solutions are not possible and that a weed control strategy involving a number of approaches is necessary, but nevertheless we still need to find the most efficient control techniques possible.
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|1 SAC, Craibstone Estate,
Bucksburn, Aberdeen AB21 9YA, United Kingdom.
2Louis Bolk Instiitute, Hoofdstraat 24, 3972 LA Driebergen, The Netherlands