Hot, dry summer weather brings about heat and drought stress on summer forage crops. Stressed plants such as the forage sorghums can occasionally accumulate dangerous concentrations of nitrates, says Glenn Selk, Oklahoma State University Extension animal scientist. These high-nitrate plants, either standing in the field or fed as hay, can cause abortion in cattle or death if consumed in great enough quantities.

Nitrates don’t dissipate from sun-cured hay (in contrast to prussic acid), so once the hay is cut, the nitrate levels remain constant. Therefore, producers should test hay fields before cutting for hay, Selk says. Some management techniques to reduce the risk of nitrate toxicity (Note: the risk of this poisoning cannot be totally eliminated), include:

  1. Test the crop before harvest. If it has an elevated concentration of nitrates, you still have the option of waiting for normal plant metabolism to bring the concentration back to a safe level.

  2. Raise the cutter bar when harvesting. Nitrates are in greatest concentration in the lower stem. Raising the cutter bar may reduce the tonnage, but cutting more tons of a toxic material has no particular value.

  3. Know the extent of nitrate accumulation in the hay. If you still doubt the hay’s quality, send a sample to a reputable lab for analysis, to get an estimate of the nitrate concentration. This will give some guidelines as to the extent of dilution that may be necessary to more safely feed the hay.

  4. Allow cattle to adapt to nitrate in the hay. By feeding small amounts of the forage sorghum along with other feeds such as grass hay or grains, cattle begin to adapt to the nitrates in the feed and develop a capability to "digest" the nitrate with less danger. Avoid the temptation of feeding the high-nitrate forage for the first time after a snow or ice storm. Cattle will be stressed, hungry and unadapted to the nitrates.
For more info, pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-1996/PSS-2903web.pdf
-- Ron Hays, Radio Oklahoma Network