Sweet clover poisoning
Much of the Tri-State Livestock News area has seen ideal growing conditions over the last two years for yellow sweet clover, and it shows. There is abundant growth of sweet clover in pastures and hay fields throughout western South Dakota, and I’m sure it extends anywhere that wet conditions have existed last year and/or this year.
Sweet clover is a biennial plant. In its first year, there is very little top growth because it puts most of its effort into establishing a deep root system. It is likely that many people don’t even notice it in the pasture or field because it is only a couple of inches tall. However, in the second year of its life, it puts up tall stalks with abundant yellow flowers and provides a lot of forage. In areas that had good precipitation last year, a large crop of sweet clover established and had ideal conditions for root growth. For those of us having wet conditions again this year, there is prolific production of those plants that established so well last year.
Besides producing a large amount of forage under these growing conditions, sweet clover also provides very nutritious forage if managed properly. It is a legume, so like alfalfa and other clovers, it is high in protein. Additionally, unless it is allowed to get tall with heavy stems, it can be highly digestible and thus a good source of energy. However, it also has some problems that need to be dealt with.
First, as a grazing resource, one must remember that it is a legume and therefore there is a potential for bloat. Managing to prevent bloat includes feeding the livestock before turning them out to graze so that they are not hungry and thus are less likely rapidly eat a large amount of sweet clover (or any other legume with bloat potential). Ruminants can adapt to forage with high bloat potential over a period of several days, so a person may need to pull them off the pasture for part of each day for several days or feed hay on the pasture so intake of the sweet clover is limited until they have developed tolerance. Additionally, poloxalene (Bloatguard) and ionophores like Rumensin also help to prevent bloat. Be sure to start feeding these substances several days before turning the cattle out so they have adapted before they consume the sweet clover or other legumes.
The major problem, however, is sweet clover poisoning. Sweet clover contains a substance called coumarin, which is converted to dicoumarol by fungus in moldy hay. Dicoumarol is an anti-clotting agent that will cause them to hemorrhage (bleed) severely. This can be internal bleeding so there are no apparent signs of the problem looking at the cattle. One sign to look for is black, tarry manure caused by internal bleeding into the intestinal tract. As already indicated, this is a problem with sweet clover hay that is put up too wet so that it molds in the bale.
The conversion of coumarin to dicoumarol can occur even if there is only a little mold in the hay, so mold level is not a good indicator of the dicoumarol level. The best prevention of the poisoning is to be sure the hay is cured well before baling. This is a problem with a productive stand of sweet clover, though, because it forms heavy, strong stems that are difficult to dry adequately. One option is to cut it at a relatively immature state before thick stems are formed. Baling into large, dense bales can make the problem worse because mold formation in damp hay will be at the worst at the core of a large bale.
Sweet clover poisoning is not a problem when grazing pasture because the conversion to dicoumarol is not possible without the mold fungus growth that occurs in wet hay. It can also be prevented by making silage rather than hay as long as the ensiling fermentation process is rapid and complete so that the silage is stable and does not allow mold growth. An option in a wet year like this wherein drying hay might be a problem would be to plastic-wrap large round bales to make bale silage. Describing the details of that process is enough material for another column that will have to wait for another day, but it’s a great option for conserving forage when hay making is difficult.
There are reports of sweet clover poisoning causing reproductive problems in cows. This appears to also be related to the dicoumarol level. The dicoumarol can pass into the fetus and cause fetal death and abortion. Thus, this is more of a problem in later gestation during the winter when pregnant cows are consuming sweet clover hay. I recently checked with the USDA Poisonous Plants Research Laboratory in Utah and they are not aware of any information that suggests that cattle on pastures containing sweet clover during the breeding season will have any problems with fertility or conception.
Sweet clover is an opportunistic plant that is going to be abundant in pastures and hay fields when the growing conditions are favorable. Although it can cause problems as described, it is a source of nutritious forage if managed properly to control the problems.