Editor’s note: The following was written by Eldon Cole for the University of Missouri Extension website.
The dry conditions have prompted some to check summer annual forages such as sudan, sorghum sudan, millet and even johnsongrass for elevated nitrate levels.
Under certain conditions, high nitrates may also be seen in corn, fescue and Bermuda grass. Even weeds such as lambsquarter, pigweed and smartweed are capable of being at risk from nitrate poisoning when high levels of nitrogen or animal manures in combination with dry, stressful weather strikes.
Samples checked with the diphenylamine-sulfuric acid spot test have ranged from no detectable nitrates to some fields that could cause problems if cattle were forced to eat the high-nitrate forage exclusively. Stalk samples that turn dark blue immediately should be submitted to a forage testing lab for a quantitative test to determine specifically the risk and how to manage it with grazing animals or those being fed hay. Laboratories typically report the percent nitrate in the forage.
The highest nitrate levels are found in the lower portion of the stem. When grazing, don’t force cattle to eat it down to the ground. A week or so after a good rain, the nitrate levels typically are lower. High nitrate hay does not reduce levels in storage but may be processed and blended with feeds not containing nitrates for safe feeding.
Prussic acid may also be a concern in sorghum sudan and johnsongrass forages, but Extension does not test for it. It usually is less of a concern if the plants are 20 to 24 inches tall or more when livestock are turned out on it.