Nitrates in Forages, Should We Worry?

There has been a lot of worry on the part of livestock producers afraid that their forages may be high in nitrates. A few people have actually tested their hay but most just go on what their neighbor is saying and before you know it everybody has a nitrate problem when maybe only a few really do.
Warm-season annual grasses, such as forage sorghums, sorghum-sudan hybrids (haygrazer types), and the various millets can also accumulate nitrates to a level that is toxic to cattle during periods of dry weather. This list can also include weeds such as careless weed, lambsquarter, nightshades, and johnsongrass but really does not include bermudagrasses. Bermudagrasses are different because the nitrate accumulation would happen in the roots not the stems or leaves. I have never seen bermudagrasses with high nitrate problems. We can possibly also add rolled up corn and sorghum stalks that didn’t produce grain to the list this year.
Dr. Larry Redmon, Extension Forage Specialist sent me this article that I thought might help understand what is going on. Typical nitrate accumulation occurs with excessive N fertilization followed by a period of drought, although toxic levels of nitrates have been observed in warm-season annual grasses with as little as 50 lbs of N/ac under drought conditions. While aboveground plant growth is reduced, nitrate uptake continues to occur and concentrates in the forage tissue. Ruminants are affected because microbes in the rumen are able to convert nitrate to nitrite. Nitrite is then absorbed into the bloodstream where it converts hemoglobin, which carries oxygen throughout the bloodstream, into methemoglobin, which does not carry oxygen. Cattle death is due to asphyxiation.
The total level of nitrate in forage will determine whether or not the forage is safe to feed. Remember: Nitrate levels in hay do not diminish with time! Nitrate levels in silage may be reduced by 50% or more by the ensiling process but may still be excessive for safe feeding. Only a forage analysis for NITRATE (currently $5.00 at the Texas A&M University Soil Testing Lab) will determine whether or not the fresh forage, hay, or silage is safe to feed to livestock. Nitrate levels of 0.5% or 5000 ppm or greater may be dangerous to feed to animals and greater than 1.5% or 15,000 ppm are toxic to most classes of livestock. The official Texas A&M University advisory is to not feed forages that contain greater than 1% or 10,000 ppm nitrate. The more conservative number of 5,000 ppm, however, may be a much safer number to use in actual practice.
Producers using warm-season annual forages or johnsongrass should have their hay crops tested prior to harvesting. Look at the forages carefully. If the forage to be harvested for hay has been under drought stress, there is a good likelihood that it is high in nitrates. If a good precipitation event occurs and plant growth is reinitiated (good green color, no droopy leaves), then the forage may be safe to feed, but a forage analysis for nitrate would still be advisable. DO NOT HARVEST the forage and then test! To do so could wind up costing you time, effort, and money and result in a hay crop that you will not be able to feed. Likewise, cattle should not be pastured on warm-season annual grasses or johnsongrass if conditions are such that nitrate levels could be elevated to a toxic level. Again, only a forage analysis can determine if the forage is safe to graze.
Prussic Acid Can Be A Problem As Well
Certainly prussic acid can be a problem in our sorghum type plants but with the severity of the drought I haven’t really worried about prussic acid causing much of a problem. Most of the sorghums and johnsongrass were basically dead and without moisture they don’t produce prussic acid. After any rains produce new growth then this new growth will be the lushest of the plant and will contain the highest concentrations of prussic acid. I would caution producers to be careful of fields with a lot of johnsongrass or sudan in them after any rains. These will probably contain too much prussic acid to graze but after growing a couple of weeks the doses should come down and be fine.