Nitrates – A Toxic Problem!

Nitrates are a naturally occurring compound found in plants and used by plants in normal growth and development. However, when plant metabolism is altered, as may occur during drought conditions, then nitrates can become a hazard to livestock.
    Some common crop and pasture plants, as well as numerous weeds, have the potential to accumulate toxic levels of nitrate. In ruminants, the nitrate is reduced to nitrite, which is absorbed into the animal’s body and combines with the hemoglobin, which is the oxygen-carrying portion of the blood, thus forming methemoglobin. Methemoglobin cannot bind with or transport oxygen in the body, so anoxia occurs in the animal and it dies.
    Typical signs of nitrate toxicity include labored breathing, evidence of abdominal pain, cyanotic mucous membranes and probably most easily identified – dark brown or chocolate-colored blood.
    Nitrate accumulates in the vegetable tissue of plants, particularly in stalks/stems with less found in the leaves and seeds (grain).
    Plants that may accumulate nitrate include pigweed, nightshades, Johnson grass, sudan grass, sudan/sorghum hybrids, oats, rape, wheat, barley and corn, especially if corn damaged by drought is harvested for forage. Heavy fertilization of grasses and other crops used as forage for livestock, especially when combined with cool, cloudy weather, may result in toxic levels of nitrate in the harvested/grazed forage. Forage levels of 0.5 percent nitrate and above are potentially dangerous, with acute poisoning likely to occur if the nitrate levels exceed one percent. In water, levels of 200 ppm (parts per million) nitrate are potentially hazardous, with 1500 ppm causing acute toxicity.
    Chronic nitrate toxicity has been suggested to cause reduced growth, abortion, infertility, goiter and other non-specific problems.
    Table 1 presents data obtained from samples of Wyoming grown forages. In general the biggest problem is with the oat and sorghum/sudan hays. There were a few grain samples that contained potentially hazardous levels of nitrates also. The limited number of grain samples evaluated suggests there might be a problem with some oats and triticale. I would suspect these problem grains were immature and/or stressed in some way such as drought prior to harvest. Remember that nitrate will migrate downward in a stack of hay/straw/stalks; this can create potential problems with feeding these types of forages if they have been stacked longer than a year prior to feeding. The feed samples in this example were collected and analyzed form August to mid-December 1988.
    Table 2 illustrates forage nitrate levels and the expected animal response. If you have forage with nitrate levels above 0.5 percent, real care needs to be used during feeding.  Blending the forages with non-nitrate-containing forages, staggering or delayed feeding of nitrate forage and/or the use of grain in the diet will help reduce the toxicity. But each situation needs to be evaluated separately, and feeding programs designed accordingly.
    The problems many producers are facing with limited forage supplies available for the upcoming winter period may cause them to seek alternative forage sources and types to help fill this deficiency. Remember that when considering different feeds and feed sources operators need to explore such things as how the forages were grown, and harvested; were they stressed during growth and were they heavily fertilized with nitrogen.  Also it is a good idea to test all forages bought or raised to help identify the most efficient way to feed the forage resource.
    Jim Waggoner is the UW Extension Livestock Nutrition and Behavior Specialist .