Drought has made it a very rough year for many herds in the U.S. In the midst of this scramble for forage in drought areas, we need to be careful about where feeds originate and the growth conditions in those areas. I’ve talked with veterinarians whose clients are experiencing some very high nitrate concentrations in drought-stressed feed crops.
Of most concern are cereal grasses – oats, wheat, barley and rye; crop residues such as milo and corn stalks; and also feed crops such as sudangrass and sorghum-sudan hybrids. Some weeds also can accumulate very high levels of nitrates, including Canada and Russian thistle, lamb’s quarter, pigweed, fireweed, dock, kochia, smartweed and johnsongrass.
If you’ve purchased or baled forage containing these weeds, be sure to check nitrate concentrations before feeding them. These weeds don’t have to be baled to cause problems; in drought-stressed, weedy pastures, these weeds may become the primary forage and lead to nitrate toxicity cases, especially if hungry cattle are turned into a new pasture where these weeds predominate.
Nitrate accumulation in plants is a multi-factorial process. Soil type has an effect; with low-pH, well-aerated soils providing greater potential for nitrate accumulation. Weather conditions also are a primary driver of the potential for excessive nitrate accumulation.
Very cool, cloudy weather may increase nitrate accumulation in plants because the low photosynthesis results in nitrate not being converted to plant proteins as efficiently. Meanwhile, very rapid growth during warm weather may lead to accumulation of nitrates in immature plants. Nitrates are generally lower in mature plants, but these mature plants may still contain toxic concentrations. Nitrates typically accumulate in the lower portion of the plant.
Drought affecting the plant in its early growth stages will also tend to cause nitrate accumulation. Drought is not an automatic indicator of high nitrates, though, and forages should be tested to confirm the concentration.
A ruminant converts nitrates to ammonia in the rumen, with the intermediate product being nitrite, which is 10 times more toxic than nitrate. Rumen bacteria usually use the ammonia to form bacterial protein. But, if overwhelmed, the excess nitrate and nitrite are absorbed through the rumen wall in excess amounts, causing the toxicity.
Nitrate and nitrite cause disease by oxidizing the iron in red blood cells, forming methemoglobin from hemoglobin. Methemoglobin is incapable of carrying oxygen to the tissues, which causes the animal to show signs of lack of oxygen. Abortions can also occur due to nitrates, as well as poor performance when high nitrate levels are combined with other nutritional and environmental factors.
The good news is that ruminants can adapt to higher, but not acutely toxic nitrate concentrations, if slowly introduced. Blending higher-nitrate feeds with lower-nitrate feeds is an alternative.
It’s important to remember that nitrate toxicity can result from a combination of feed and water contributions. Fermentation of feeds reduces the nitrate concentration, with reductions of 50% possible. You’ll want to test the ensiled product of high-nitrate feeds to confirm the reduction prior to feeding, however.
It’s important to also know the contribution of nitrates from your water source, especially in shallow and/or poorly cased wells in crop-production areas. Both water and feed nitrate contributions should be considered when deciding whether to utilize a forage for your cattle.
The potential for nitrate toxicity is an unwelcome complication to an already complicated year in some areas of our country. But, getting together with your herd-health veterinarian to evaluate your feeding and pasturing plans in relation to potential toxicities can turn this complication into a management factor rather than a recipe for disaster.