With the end of the growing season approaching, many producers may consider grazing their final harvest of alfalfa, but they need to pay careful attention to nitrate toxicity.
Frost will not typically cause nitrate toxicity in alfalfa, but it is always recommended to test the crop, especially if background nitrogen
levels are high.
Nitrate concerns can occur in annual species. In the case of sorghum species, poor growing conditions including frost can increase prussic acid concentrations. Of greater concern for alfalfa is a possible increase in bloat potential.
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Several factors are known to affect the bloat potential in any legume: the amount of soluble protein in the legume and possibly the type of protein; the presence or absence of condensed tannins and the release rate of the soluble protein.
Condensed tannins are responsible for precipitating some of the soluble protein and for the non-bloating legumes such as birdsfoot trefoil and crownvetch.
Cicer milkvetch is the only known non-bloating legume that does not have tannins and is why the release rate of the soluble proteins is considered a third factor affecting bloat.
According to the traditional theory of legume pasture bloat, froth was attributed to soluble proteins in the rumen fluid, which were produced by legume forages.
Current theories place more emphasis on the involvement of small particles and microbial activity.
Alfalfa, which is rapidly digested, provides for bacterial blooms that produce large quantities of gas and slime. The rumen bacteria attached to these particles have an abundance of carbohydrates, both internal, in the form of storage granules, and external, in the form of slime.
The occurrence of bloat is consistently associated with increased levels of these particles in the rumen fluid.
Alfalfa has a reputation of being bloat safe after a killing frost. However, as long as the alfalfa remains green and succulent, there is a risk of bloat.
The first frost ruptures plant cells, producing small plant cell wall fragments and increasing the amount of potassium, calcium and magnesium ions, all of which can increase the risk of bloat. Not until the standing crop dries substantially does bloat incidence decline.
At least one week is usually required to dehydrate or dry down frost-killed alfalfa before bloat risk is reduced. Bloat is not a concern in alfalfa that has been field cured for baling.
Research has substantiated the claim that the risk of bloat may be reduced by waiting until the dew is off the alfalfa before allowing cattle to graze.
However, the claim that creeping rooted
alfalfa is bloat safe is unfounded.
The stage of alfalfa development or maturity is an important factor in preventing pasture bloat. Bloat potency is highest at the vegetative or prebud stage and decreases progressively as plants grow to full bloom.
Moving to new pasture in the afternoon reduces the predisposition of cattle to bloat. Pasture management systems that promote continuous and rapid ruminal clearance, with more bypass and less gas production, are most likely to reduce the incidence of bloat. Grazing alfalfa plants that have been swathed and wilted is another way to reduce bloat.
As a result, the potential for frothy bloat can be increased by frost, but lessened if alfalfa has begun to flower, cattle are moved into a new pasture in the afternoon, grazing is continuous and not interrupted, bloat reducing supplementary products are used and as the alfalfa plant dries.
