As summer progresses, less rainfall and higher temperatures are common. These conditions are often associated with the potential for prussic acid and nitrate accumulation in harvested hay and pasture forages. Extended periods of cloudy weather can also create problems, as plants do not photosynthesize as efficiently. This slows the conversion of nitrates into amino acids and proteins in certain forages consumed by livestock.
Forages and weeds of concern are most commonly sorghum-family species, including forage sorghum, sorghum-Sudan grass hybrids, Sudan grass and Johnsongrass. These species are valued for their productivity during hot, dry conditions but can accumulate toxic compounds when exposed to environmental stress. Because of this, it is important to scout fields for signs of plant stress to help determine whether sorghum-family forages are safe to feed to livestock.
Prussic acid poisoning
Prussic acid, also known as hydrocyanic acid, is a fast-acting toxin that can kill livestock when consumed in large amounts over a short period. Livestock exposed to small amounts on a regular basis may develop limited tolerance to the compound, but this should not be relied upon as a management strategy.
A cyanogenic compound called dhurrin is naturally present in sorghum plants and remains nontoxic until the plant experiences a stress event. These stress events may result from drought, frost, prolonged cool and cloudy weather, or mechanical injury caused by trampling, chewing or harvesting. Once plant cells are damaged, enzymes come into contact with dhurrin and convert it into toxic prussic acid.
Leaves generally contain higher concentrations of prussic acid than stems, while reproductive structures contain little to no toxin. Young regrowth is especially toxic to livestock because it contains the highest concentrations of dhurrin. Since animals often prefer tender new shoots over mature growth, grazing should be delayed until plants reach about 18 to 24 inches in height.
Prussic acid levels begin to decline after sorghum-family crops are cut for hay. When harvested as hay or silage, wait at least two to three weeks after the forage has completely dried or fermented before feeding. If grazing is planned after a drought or other stress event, allow regrowth to reach 18 to 24 inches before turning livestock onto the pasture. Following a killing frost, leaves must completely die before prussic acid levels begin to dissipate. Under these conditions, waiting seven to 10 days before grazing is generally recommended.
Heavy nitrogen fertilization may also increase the potential for prussic acid accumulation when combined with environmental stress. Additional caution should be used before grazing or harvesting heavily fertilized sorghum fields after a stress event.
Nitrate toxicity
Although prussic acid is an important toxin to monitor, nitrate toxicity is often the greater concern for livestock producers. Many of the environmental conditions that favor prussic acid formation also contribute to nitrate accumulation. High nitrogen fertilizer rates can further increase the risk of nitrate toxicity, particularly in warm-season annual grasses such as Sudan grass. Applying nitrogen according to soil test recommendations and monitoring rainfall patterns can help reduce the likelihood of excessive nitrate accumulation.
Nitrate is the primary form of nitrogen absorbed by plants from the soil. During early growth, nitrate uptake is rapid while plants actively convert nitrate into amino acids and proteins. Under normal growing conditions, nitrate concentrations generally decline as plants mature because the absorbed nitrate continues to be incorporated into plant proteins while plant biomass increases.
However, when environmental stress such as drought, prolonged cloudy weather, frost or certain herbicide injuries disrupts photosynthesis or protein synthesis, nitrates may accumulate to toxic concentrations regardless of the plant’s stage of development.
The risk of nitrate toxicity is generally greatest while plants are in the vegetative stage rather than the reproductive stage. Unlike prussic acid, nitrates are typically most concentrated in the lower stems, while leaves contain lower concentrations and grain heads contain the least.
If rainfall follows a drought or other stress event, plants often begin using accumulated nitrates as growth resumes. Producers should wait about seven to 14 days before grazing or harvesting to allow nitrate concentrations to decline.
When livestock consume excessive amounts of nitrate-containing forage, nitrates are converted to nitrites in the rumen. Nitrites then convert hemoglobin into methemoglobin, which cannot effectively transport oxygen throughout the body. This can result in respiratory distress and, in severe cases, death.
Several management practices can reduce the risk of nitrate poisoning. Livestock can be gradually adapted to higher-nitrate forages by slowly increasing intake over two to three days. Healthy animals generally tolerate this process better than those in poor condition.
Feeding grain or other carbohydrate sources also promotes the conversion of nitrates into microbial protein within the rumen, improving the animal’s ability to tolerate higher nitrate levels. Even so, producers should continue following a gradual feeding program when introducing high-nitrate hay. Using a total mixed ration to blend forage with grain is another effective management strategy when feeding hay that may contain elevated nitrate concentrations.
Before feeding hay, a quantitative nitrate analysis conducted by an approved laboratory can provide valuable information about its safety. Nitrate concentrations between 5,000 and 15,000 parts per million should be fed with caution and only as part of a carefully managed feeding program. Forages containing more than 15,000 parts per million should not be fed. Forages testing below 5,000 parts per million are generally considered safe but should still be fed with consideration for the class, age and condition of the livestock.
For assistance with hay sampling or questions about prussic acid or nitrate toxicity, contact a local University of Missouri Extension agronomist.
