*Agriculture Canada Research Station, 107 Science Crescent, Saskatoon, Saskatchewan, Canada S7N 0X2. Contribution No. 741.
Communication presented at the Conference on Planned Animal Health and Production in Dairy and Beef Cattle. Western College of Veterinary
Medicine, University of Saskatchewan, Saskatoon, Saskatchewan. June 11-15, 1979.
Sweetclover has a notorious reputation for causing "sweetclover disease" when improperly cured. In spite of this, however, sweetclover remains a useful forage legume with valuable agronomic traits. It is droughtresistant and well adapted to Western Canada. Sweetclover is the highest yielding legume forage in this region and is valuable in soil improvement, silage, hay and pasture production and a prized crop for the honey producer. It is the most saline-tolerant of the legumes and is particularly useful on saline "white alkali" soils where cereals and other crops cannot grow.
Special precautions are necessary to avoid spoilage and concomitant dicoumarol formation in preserving sweetclover hay and silage. Feeding recommendations are suggested for the safe utilization of spoiled forage. Low coumarin cultivars of sweetclover are completely safe and will not result in sweetclover disease despite spoilage. The breeding program at Saskatoon is expected to produce a new low coumarin (yellow flowered) sweetclover cultivar within the next two years.
Can. vet. J. 21: 149-151 (May 1980)
In the 1923 Annual Report of the Ontario Veterinary College, Schofield (9) first reported and demonstrated a causal relationship between moldy sweetclover and a mysterious bleeding disease in cattle simulating hemorrhagic septicaemia and blackleg. This causal relationship was later corroborated by Roderick and Schalk (8). It was subsequently found that the causative agent was dicoumarol (3, 11), an anticoagulant formed from coumarin, a naturally occurring plant metabolite found in large quantities in sweetclover (10). With such demonstrated problems associated with the use of this crop as a preserved forage, the question may well be asked - why grow sweetclover? The purpose of this paper is to explain why sweetclover continues to remain a valuable forage crop in Western Canada, to recommend precautions for its safe use and report on breeding programs which will eliminate the problem of sweetclover disease.
Sweetclover, a native of Asia Minor, was introduced into North America in the 1700's. It was originally considered a troublesome weed. However, it was eventually recognized as a valuable forage crop. This legume is a sweet-scented (vanilla-like aromatic coumarins), upright, broad-leaved, tall-growing legume. It is biennial, i.e. living through only one winter and then dying at the end of the second year of growth. It may attain a height of two to three feet in the seedling year, while in the second year it may grow to a height of four to six feet. In the second year it flowers and sets seed with the aid of honey bees or wild bees. There are two common types of sweetclover, white-flowered (Melilotus alba) and yellow-flowered (M. officinalis). The yellow-flowered sweetclover is shorter in growth, has finer stems and leaves and flowers approximately seven to ten days earlier than the white-flowered type. Farmers and ranchers have a strong preference for yellow-flowered sweetclover.
Sweetclover is a fast-growing and productive legume which produces more forage per acre than other legumes grown in Western Canada, including alfalfa (4). It is valuable for soil improvement, silage production, hay and pasture. Since it is biennial, it is particularly useful in short rotations and adds maximum nitrogen to the soil compared to other legumes such as red clover, white clover and alfalfa. It is also highly prized as a source of nectar and pollen by the honey producer. Sweetclover is also very important for its seed production as seen from the following data on retail sales (Annual Reports, Sask. Dept. Agric.):
Sweetclover is drought-resistant and is particularly well adapted to the drier areas of western Canada (6). It is most productive on fertile, welldrained clay and clay loam soils. However, it can also be successfully grown on sandy loams, heavy clay soils and Gray Luvisol soils. It frequently thrives along roadside ditches where there is a good moisture supply and reseeds itself when left uncut or ungrazed.
Soil improvement. Sweetclover is one of the best legumes for soil improvement (2). The widely branched, deeply penetrating tap roots open up the subsoil. The roots use nutrients not available to plants with shallow roots. When sweetclover plants decay, the nutrients in the roots are released for use by other crops.
When properly inoculated, sweetclover contributes considerable organic matter and nitrogen to the soil. When plowed under in the year after seeding, the succulent top growth and the fleshy, heavy roots decay rapidly. This improves aeration drainage and the general physical condition of the soil.
Sweetclover finds special use on saline "white alkali" soils which are unsuitable for cereal crops. With the reported increasing acreage of saline soils in the prairies and the lack of saline-tolerant crops, particularly legumes, sweetclover thus fulfils a special need for seeding on such soils. It is a common observation that growing sweetclover on such areas often permits the growth of subsequent crops which previously could not be grown. Sweetclover, with its deeply penetrating tap root, opens up the subsoil and when the roots decay numerous channels are formned which improve the circulation of air and the percolation of water. It probably also serves to lessen the salinity of the top layer of the soil by lowering the water table through acting as a "biological pump" to supply its profuse vegetative top growth.
Hay. Sweetclover is not considered a good hay crop. When cut just before blossoms appear and if properly cured, it makes a fairly good hay, but at this time the stems are succulent, high in water and hard to dry sufficiently for safe storage. The mowed or swathed hay requires treatment with a conditioner attachment. When hay is left for the stems to dry out to a safe moisture level, the leaves dry, become brittle and shatter badly. If cutting is delayed, the proportion of coarse stems to leaves increases and feeding value decreases. Such plants are tought, woody and rather unpalatable to livestock. It has been my impression (personal communications) that the large round bales may not shed water as well as square bales and also being of variable density do not dry out uniformly. Experience in the use of these bales should eventually tell the story. However, it is very clear that special effort should be made to put up wellcured sweetclover hay because of the danger from "sweetclover disease".
Silage. Sweetclover is an excellent silage crop because of its high forage yield and it is usually harvested as silage rather than hay. Sweetclover should be cut for silage at the 10 to 20% bloom stage of flowering. Moisture content should be from 65 to 72%. Sweetclover should be conditioned and lie in the swath for two or three hours on a sunny day, or longer on a dull day, before cutting with a forage harvester. When the silage pieces are 1.3 cm (12 in.) long and a squeezed handful slowly breaks apart when hand pressure is released, it is ready for ensiling. If the squeezed handful falls apart when pressure is released, it is too dry and if it stays in a firm ball, it is too wet. Silage needs firm packing as soon as it is placed in the silo to eliminate air, to assure rapid fermentation and to avoid spoilage. The silo should be filled as quickly as possible to limit spoilage. The surface layer should be of another forage that is less susceptible to mold than sweetclover (12).
Place a plastic cover over the silo as soon as it is filled. Spoilage of sweetclover silage is dangerous because of the potential problem of sweetclover disease.
In poorly preserved sweetclover hay or silage, coumarin may change to dicoumarol, a potent anticoagulant (cf. figure below). When eaten by farm livestock, dicoumarol in spoiled sweetclover forage interferes with normal blood clotting and causes an often fatal condition called sweetclover disease (7). The blood of affected animals loses its normal ability to clot and livestock may bleed to death from the slightest injury, either internally or externally.
This conversion of coumarin to dicoumarol is apparently accomplished by common molds found in the soil, such as Penicillium nigricans and P. jensi (1). The chemical process involves condensation of two 4- hydroxycoumarin molecules through formation of a methylene bridge.
Most sweetclover forage is safe for feed; however, improperly cured and moldy hay or silage must be used with caution. Widespread outbreaks of sweetclover disease in the winter period are invariably associated with poor harvesting and curing conditions the previous summer (personal communications).
Isolated reports of sweetclover disease usually, though not always, are related to slight to severely molded forage. Unfortunately, it is not possible to judge the sweetclover hay or silage from visual appearance. Such forage may appear well-cured and normal but it can occasionally contain the toxic agent dicoumarol. On the other hand, not all spoiled and moldy sweetclover is toxic. Forage which is suspect should be thoroughly sampled and the samples forwarded to a Feed Testing Laboratory which can provide a chemical analysis for dicoumarol.
Moldy and improperly cured sweetclover hay and silage should be used with caution. Severely spoiled sweetclover forage should be removed and disposed of in a safe place away from farm animals. The remaining sweetclover may be fed for two weeks, followed by two weeks on another goodquality forage. An alternative suggestion is to feed it daily, provided that it does not constitute more than 25 to 30% of the daily forage intake. Research (8, 12) has indicated that such feeding procedures avoid elevated prothrombin times.
The ultimate solution to avoid sweetclover disease is the use of lowcoumarin sweetclover cultivars. Such an example is the white-flowered cultivar, Polara (5). This cultivar, released from Saskatoon, contains only trace amounts of coumarin, the precursor to the powerful anticoagulant dicoumarol. Hence no degree of spoiling or improper curing can cause sweetclover disease from the use of such low coumarin forage. As mentioned previously, however, farmers have a strong preference for yellowflowered sweetclover. The sweetclover breeding program at Saskatoon is in the final phases of testing an advanced strain of a low coumarin yellowflowered cultivar. Preliminary results indicate satisfactory agronomic performance. It is hoped that this new low coumarin strain will be licensed within the next two years.
1. BELLIS, D.M. Metabolism of coumarin and related compounds in cultures of Penicillium species. Nature 182: 806-807. 1958.
2. BOWREN, K.E., D.A. COOKE and R.K. DOWNEY. Yield of dry matter and nitrogen from tops and roots of sweetclover, alfalfa and red clover at five stages of growth. Can. J. Plant Sci. 49: 61-68. 1969.
3. CAMPBELL, H.A. and K.P. LINK. Studies on haemorrhagic sweet clover disease. IV. The isolation and crystallization of the haemorrhagic agent. J. biol. Chem. 138: 21-33. 1941.
4. GOPLEN, B.P. Legume breeding. Proc. Canadian Forage Crops Symposium, pp. 225- 259. 1969.
5. GOPLEN, B.P. Polara, a low coumarin cultivar of sweetclover. Can. J. Plant Sci. 51: 249-251. 1971.
6. GOPLEN, B.P. and A.T.H. GROSS. Sweetclover production in Western Canada. Agriculture Canada. Publ. 1613. 1977.
7. LINK. K.P. The anticoagulant from spoiled sweetclover hay. Harvey Lecture Ser. 34: 162-216. 1943-44.
8. RODERICK, L.M. and A.F. SCHALK. Studies on sweetclover disease. N. Dak. agric. Exp. Stn Bull. 250. 1931.
9. SCHOFIELD, F.H. Damaged sweetclover: the cause of a new disease in cattle simulating haemorrhagic septicaemia and blackleg. Report Ontario Veterinary College. pp. 21- 34. 1923.
10. SMITH, W.K. and R.A. BRINK. Relation of bitterness to the toxic principle in sweetclover. J. agric. Res. 57: 145-154. 1938.
11. STAHMANN, M.A., C.F. HUEBNER and K.P. LINK. Studies on haemorrhagic sweetclover disease. V. Identification and synthesis of the haemorrhagic agent. J. biol. Chem. 138: 513-527. 1941.
12. WHITE, W.J., J.E.R. GREENSHIELDS and w. CHUBATY. The effect of feeding sweetclover silage on prothrombin time of blood of cattle. Can J. agric. Sci. 34: 601-606. 1954.