Oklahoma summers often bring “high pressure domes” that cause 100+ degree days and no rain. The resulting heat stress can cause nitrate accumulation in summer annual forage crops. Producers are very cautious about cutting or grazing the drought-stressed forages and for good reason. However, when the first drought-easing thunderstorm comes along, cattlemen are anxious to cut the forage or turn in the cattle on the field that has just received rain.
This practice can lead to a potentially dangerous situation. As the plant starts to grow and turn green once again, the nitrate uptake is accelerated. Plant enzymes (such as nitrate reductase) are still not present in great enough quantities or active enough to convert the nitrate to plant proteins. Therefore the plant nitrate concentrations become even greater in the first few days after the first rain.
Producers should exercise caution and test forages before cutting or grazing shortly after a drought-easing shower. Some of the greatest concentrations of nitrate in forages will be recorded at this time. Usually by 7 – 10 days after a “good” rain, plant metabolism returns to normal and nitrate accumulations begin to decrease. Be sure to test the forage before cutting and storing a large quantity of potentially poisonous hay.
Source: Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist