Central Plains and Midwestern 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. (Myth number 1).
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.
For many years, producers thought that the time of day of cutting would affect the nitrate concentration in the summer annuals that were harvested. (Myth number 2). This harvesting practice was based on the assumption that the plant continues soil nitrate uptake during nighttime hours, followed by accelerated conversion of the nitrate to protein during daylight hours.
To evaluate the significance of the change in nitrate concentration in forage sorghums during the day, Oklahoma State University Extension Educators collected samples at two hour intervals from 8 AM to 6 PM. Five cooperator’s fields (“farm”) were divided into quadrants. Three random samples, consisting of ten stems each, were taken from each quadrant at the specified interval. The samples were analyzed at the Oklahoma State University Soil, Water, and Forage Analytical Laboratory to determine the level of nitrates, in parts per million (ppm).
As expected, differences between “farms” were substantial and significant. The mean concentration of nitrate for individual farms varied from only 412 ppm to 8935 ppm. The mean nitrate concentrations across all farms were 3857, 3768, 4962, 4140, 4560, and 4077 ppm for samples at 8 AM, 10 AM, noon, 2 PM, 4 PM, and 6 PM, respectively. Remember, most laboratories consider nitrate concentrations at, or above 10,000 ppm potentially lethal.
There was much more variation between farms than between harvest times. Time of day of harvest did NOT impact nitrate concentration or proportion of dangerous samples of forage sorghum hay. Don’t be led into a false sense of security by thinking that forages cut in the afternoon or evening are safer. Source: Levalley and co-workers. 2008 OSU Animal Science Research Report.
To learn more about nitrate toxicity download and read OSU Fact Sheet PSS-2903 “Nitrate Toxicity in Livestock”