Almost as predictable as the coming of the winter season, will be the quickly spread horror story of the death of several cows from a herd that was fed "the good hay" for the first time after snow storm. Ranchers that have harvested and stored potentially high nitrate forages such as forage sorghums, millets, sudangrass hybrids, and/or Johnsongrass, need to be aware of the increased possibility of nitrate toxicity. Nitrate toxicity is most likely if the cows are fed this hay for the first time after a severe winter storm. Cattle can adapt (to a limited amount) to nitrate intake over time. However, cattlemen often wait and feed the higher quality forage sorghum type hays during a stressful cold wet winter storm. Cows may be especially hungry, because they have not gone out in the pasture grazing during the storm. When fed the hay, the cows eat a larger than normal meal. They may be stressed and slightly weakened by the cold, wet conditions. This combination of events make them even more vulnerable to nitrate toxicity.
The rancher is correct in trying to make available a higher quality forage during severe winter weather in an effort to lessen the loss of body weight and body condition due to the effect of the wind chill. But if the forage he provides to the cows is potentially toxic, his best intentions can back-fire.
The best approach would be to know ahead of time the concentration of nitrate in the hay. Send a sample to a testing laboratory and that will quantify the amount of nitrate in the sample. If the producer is confident that the hay is very low in nitrate content then use of the hay should be safe. If the nitrate content is unknown or high in nitrate, then precautions should be taken. Feeding small amounts of the hay along with other grass hays during the fall and early winter days can help to "adapt" the cattle to the potential of nitrate. This is not a fool-proof concept. If the hay is quite high in nitrate, it can still be quite dangerous. Diluting the high nitrate feed with other feeds can reduce the likelihood of problems. Feeding a high starch grain (such as corn or milo) at a rate of about 7 pounds per 1000 pounds of body weight per head per day will reduce the risk of nitrate toxicity.
The grain should be fed about 1 to 2 hours prior to feeding the high nitrate hay. Be sure to acclimate the cows to the grain gradually as well, so as to not create digestive disorders due to grain overload.
To learn more about nitrate toxicity and management of “high nitrate feeds” read OSU Fact Sheet No. F-2903 "Nitrate Toxicity in Livestock". http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-1996/F-2903web.pdf
Source: Glenn Selk, OSU Extension Animal Reproduction Specialist