Forage sorghums are used by cattle producers for summer grazing or harvested for hay. Forage sorghums can be very productive and high quality, but can also accumulate toxic levels of nitrate when stressed. In the past, the assumption was made that the plant continues soil nitrate uptake during nighttime hours, followed by accelerated conversion of the nitrate to protein during daylight hours. Therefore, past recommendations have been to wait until afternoon to cut forage sorghum for hay if anticipated nitrate levels are marginally high. You have heard the old adage: “Never assume anything….”
To evaluate the significance of the change in nitrate concentration in forage sorghums during the day, Oklahoma State University Extension County and Area 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.
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