Shepherd, Dr M. A. (2000) The environmental implications of manure use in organic farming systems (OF0161). ADAS.
This is the final report of Defra project OF0161.
A desk study was undertaken (a) to identify the important N flows in three types of organic farming systems (extensive upland, intensive lowland mixed and stockless vegetable systems) in relation to manure use and (b) to determine the effects of changed management practice on these flows. A study of the published literature has highlighted the key stages for nutrient management prior to land spreading.
Dietary inputs - amounts of N excreted and the partitioning between urine and faeces is important in determining the fate of N through the farming system. Diet affects this partition, but we were unable to find evidence of differences between organic and non-organic rations. Clearly, a crude N balance (N in feed - N in milk) provides a good guide to risk. Because N removal in milk is a small component, the more intensive systems are likely to generate more N as excreta.
Housing - it is estimated that, in the UK, NH3 losses from housing constitute c. 35% of the total NH3 emissions from cattle production systems (compared with 14% from manure storage) and about 20% of total N2O emissions. The limited available information suggests that losses by NH3 emission during housing are larger from slurry based systems than from cattle housed on straw. The converse is true of N2O losses.
Solid manure storage - composting offers advantages (namely sterilisation) but also can exacerbate loss of nitrogen as NH3. Losses of up to almost 80% of the total N have been reported. Our analysis of the numerous experiments suggests that the C:N ratio should be >30 at the start of composting to retain N (i.e. to reduce losses to <10%). Loose covering had little effect on retaining nitrogen. NPK are lost in leachate during manure storage (but only represent a true loss if the leachate is not collected for recycling).
Slurry storage - losses are predominantly as NH3 and typically 0.05% (winter) - 0.1% (summer) of the total N content per day. A crust would approximately halve losses. Stirring increases volatilisation. Aeration would have similar effects, but may also increase N2O emissions if it produces intermittent aerobic and anaerobic conditions. Covering stores would substantially decrease losses and options range from straw to concrete structures. Effectiveness increases with cost!
A simple N model was constructed to calculate the integrated effects of management practices during housing and storage. Our calculations suggested that an all-slurry system (though not permitted under organic regulations) would retain more N than a straw-based system: losses from slurry are greater during housing, less during storage (assuming the slurry lagoon is covered or has a crust and is not regularly stirred), and greater in the field (as ammonia and nitrate in particular): N losses from FYM after spreading are less, especially if composted.
The question that inevitably will be asked is ‘do conventionally managed systems provide more of a risk than organic systems?’ There is no straightforward answer. Many of the loss processes from manure will be the same between systems but they will be modified by management and by the intensity of the enterprise. It is this latter point - i.e. the nutrient balance - that will have most impact on any comparison; farms with a large nutrient excess will be more prone to large losses. Thus, generally, organic farms provide less emissions than conventional farming systems involving livestock. The move to more solid-based systems should result in smaller losses of N during housing, but practical measures to reduce losses during storage need to be investigated.
The outcome of all management processes is the production of manures that vary widely in composition between farms and often within farms. Our analyses of 14 cattle slurries and 45 cattle manures confirm the wide variations in nutrient contents as affected by many management factors. Generally, NPK values were c. 20-40% less than published values for ‘conventionally’ produced manures (although these mean values were also associated with a wide range of values). This probably reflects the lower intensity of organic production systems.
|Keywords:||manure, composting, slurry, aeration, nitrogen, phosphorus, potassium, nutrient flows, straw, volatilisation, air pollution, environment, OF0161|
|Subjects:|| Farming Systems > Farm nutrient management|
Environmental aspects > Air and water emissions
|Research affiliation:|| UK > Garden Organic (HDRA)|
UK > Institute of Grassland and Environmental Research (IGER)
UK > Organic Research Centre (ORC) - Elm Farm
UK > ADAS
UK > Department for Environment, Food and Rural Affairs (DEFRA)
|Deposited By:||Defra, R&D Organic Programme|
|Deposited On:||13 Apr 2006|
|Last Modified:||12 Apr 2010 07:33|
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