Plant-based Livestock Disorders

Several of the plant species used as cover crops can contain toxic substances when consumed by livestock. These substances can impact productivity or even cause death. Other forages can create physiological problems for livestock under certain conditions. The four primary issues to be aware of are nitrate poisoning, prussic acid poisoning, hypomagnesemia, and bloat.

Nitrate Poisoning

High concentrations of nitrate can occur in both warm season grasses such as sorghums, sorghum-sudangrass, and millets, and some cool season grasses such as barley, oats, wheat, cereal rye, and ryegrass. Brassicas are often used in cover crop mixtures and can also accumulate high levels of nitrates. High nitrate concentrations occur as a result of drought conditions and/or cool, cloudy conditions. Young plants in the vegetative stage can have greater nitrate levels than more mature plants. Nitrates accumulate in stalks and lower parts of the leaves closer to the soil surface. If growing conditions are favorable for nitrate accumulation, forage should be tested to determine if nitrates are present at toxic levels before allowing grazing by any ruminant animal.   Delay grazing of the crop until conditions improve to allow nitrate content to drop to a safe level. Consider making silage/baleage of drought-damaged forage to allow some nitrates to dissipate, and for better control of the forage in the animal’s diet.

In general, feeds with less than 4,000 ppm nitrates are considered safe for all livestock.  Nitrates over 9,000 to 10,000 ppm are potentially toxic. Concentrations between 4,000 and 9,000 ppm are considered risky for pregnant animals. These guideline concentrations are reported as “nitrates.” Some labs report “nitrate-nitrogen”.  Results reported as nitrate-nitrogen are nearly 4 times greater than those reported as nitrate; consequently, animals are at risk at much lower concentrations. Always verify which reporting method is used when reading nitrates testing results.

Prussic Poisoning

Prussic acid (hydrocyanic acid) can build up to toxic levels in the leaves of sorghums and sorghum-sudangrass, but not in pearl millet. Prussic acid causes death by interfering with the oxygen-transferring capacity of red blood cells.  Stress conditions like drought or regrowth after a frost can create circumstances where the prussic acid in plants concentrates to toxic levels.

Hypomagnesemia (Grass tetany)

Low levels of magnesium in the blood of livestock (hypomagnesemia) can result from grazing ryegrass or small grains late in the winter, especially during periods of cool, cloudy, and rainy weather. Early lactation cows and ewes are particularly predisposed to this condition. The condition occurs when plants are deficient in magnesium due to the low soil content of this mineral.  Feeding a mineral with high magnesium can help prevent the condition.

Bloat

Pastures that have a high content of certain legumes or lush grass growth can cause bloat in cattle and sheep. Bloat occurs when foam forms in the rumen that prevents burping of the gases produced during fermentation (mainly methane and carbon dioxide). In severe cases, bloat can result in the death of the animal. Most of the annual winter legumes used as cover crops are known to have a low potential to produce bloat. However, animals grazing lush pastures of ryegrass or small gains in spring are susceptible to bloat. To help prevent bloat, do not turn hungry animals out onto lush pastures in the early morning or when there is a heavy dew.

Additional Resources
Contributors:

Dr. Matias Aguerre, Assistant Professor, Clemson University; Dr. Lisa Baxter, Assistant Professor and Forage Extension Specialist, University of Georgia; Philip Brown, Grazinglands Specialist, Georgia Natural Resource Conservation Service, Julia Gaskin, Sustainable Agriculture Coordinator – retired, University of Georgia