Hot, dry summer weather brings about heat and drought stress on summer annuals. Stressed plants such as forage sorghums can occasionally accumulate dangerous concentrations of nitrates. These high-nitrate plants, either standing in the field or fed as hay, can cause abortion in pregnant cattle or death if consumed in great enough quantities. 

Nitrates do not dissipate from sun-cured hay (in contrast to prussic acid), so once the hay is cut the nitrate levels remain constant. Therefore, test hay fields before you cut them, advises Glenn Selk, Oklahoma State University emeritus extension animal scientist.

Testing the forage before cutting gives you an additional option of waiting and allowing for the nitrate to lower in concentration before harvesting the hay. Some major sources of nitrate toxicity include annual sorghum-type plants, including sudan hybrids, sorgo-sudans, sorghum-sudans, millets and Johnsongrass

A nitrate concentration of 10,000 parts per million (ppm) is considered potentially lethal to cattle.

The risk of nitrate poisoning cannot be totally eliminated, Selk says. However, here are some management techniques to reduce the risk of nitrate toxicity:

1) Test the crop before you harvest it. 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. You cannot estimate nitrate content just by looking at the field. 

2) Raise the cutter bar when harvesting the hay. 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 and the levels that are dangerous to different classes of cattle; i.e., pregnant cows, youngstock. If you still have doubts about the quality of the hay, send a forage sample to a reputable laboratory for analysis to get an estimate of the nitrate concentration and the extent of dilution that may be necessary to more safely feed the hay. 

For more information, read OSU Fact Sheet PSS-2903.

Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist