Plant roots absorb nitrogen from the soil, much of it in the form of nitrate, and convert it into proteins in the leaves for plant growth. Under normal conditions, proteins are formed at the same rate nitrates are absorbed. In late 2008, we received less than average rainfall, raising the potential for nitrate poisoning in cattle.
Protein synthesis is inhibited and nitrates may accumulate when plants are drought-stressed. Overfertilization can add to the problem, but nitrate poisoning can occur even if no N fertilizer is applied. Herbicides also have the potential to temporarily increase nitrate levels in plants. Nitrates typically accumulate in the lower one-third of the plant, mainly in the stems, and accumulate more at night and on cloudy days. Rapidly growing parts of plants tend to have lower levels of nitrates.
Nitrate poisoning can occur in a wide variety of plant species, such as barley, bromegrass, corn, fescue, pearl millet, oat, rye, wheat, sorghum, sorghum-sudan, forage sorghum, sudangrass, johnsongrass, turnips, ryegrass and other forage species. Sometimes during drought, the preferred forage becomes limited so the cattle will feed on whatever plants are available. Some weeds like pigweed, horsenettle, sicklepod, silver leaf nightshade, dock, lambsquarter, morning glory and Russian thistle may also cause nitrate poisoning.
Nitrate poisoning may occur when cattle consume forages containing excessive amounts of nitrates. Nitrates are converted to nitrites by microbes in the rumen. Nitrite is readily absorbed by the blood, where it oxidizes hemoglobin (which is the oxygen carrier in the body) into methemoglobin. Oxygen is not transported to other parts of the body by methemoglobin, thus causing oxygen deprivation to body tissues.
Nitrate poisoning can be acute, but other symptoms include staggering, gasping, salivation and trembling. Rapid and labored breathing may also be observed.
Nitrate poisoning can be differentiated from prussic acid poisoning by distinct blood and mucous membrane colors. Chocolate brown blood and muddy brown mucus membranes indicate nitrate poisoning, whereas prussic acid poisoning is indicated by bright red blood and bright pink mucous membrane color.
Following are some precautions and solutions to implement when faced with the possibility of feeding high-nitrate forages to your cattle.
- Acid soils and phosphorus-deficient soils increase plant nitrate accumulation. Apply lime and phosphorus according to soil test recommendations.
- Follow a good soil fertility management program to apply nitrogen fertilizer at proper rates.
- During drought, test forages before feeding to know the nitrate levels.
- Dilute forages high in nitrates with other, low-nitrate feeds.
- Do not overstock or overgraze pastures to reduce the consumption of stems.
- Do not turn hungry cattle onto potentially high-nitrate pasture.
- Reduce nitrate levels by ensiling forages.
- Treat nitrate poisoning by intravenous injection of methylene blue.
- Don't graze drought-stricken forages for at least one to two weeks after rainfall.
- Don't cut drought-stricken forages that have high levels of nitrates for forage to feed as the only feed. When cutting high-nitrate forages, set the mower at a higher level to avoid stems and stalks.
- Don't graze sick, hungry, stressed or pregnant animals on high-nitrate forages as they are more prone to nitrate toxicity than healthy animals.
- Limit-graze animals on high-nitrate forages. Slowly increase the amount of time cattle graze on these as they become acclimated.