Check forages for nitrates

  • Published: Friday, July 17, 2020

Nitrate testThe dry conditions of July have prompted some to check summer annual forages such as sudan, sorghum sudans, 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.

The samples I’ve checked with the diphenylamine-sulfuric acid spot test have ranged from no detectable nitrates to some fields that could cause problem if cattle were forced to eat the high nitrate forage exclusively. The accompanying photo shows the range in color, nearly black on high risk forage to a clear or very pale blue on negative or very low nitrate levels.

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 following table gives the various risks and feeding guidelines.

% Nitrates (NO3)               Feeding safety
0.0  –  0.44                          Considered safe

0.44 – 0.66                          Safe for non-pregnant animals.  Limit to 50% of the total dry matter for pregnant animals

0.66 – 0.88                          Limit to 50% of dry matter in all rations

0.88 – 1.54                          Limit to 35-40% of dry matter intake

1.54 – 1.76                          Do not feed to pregnant animals and limit non-pregnants to 25% of their dry matter.

1.76 and Up                        Do not feed - toxic          

Ruminants are most affected by elevated nitrate levels. Horses and swine are less sensitive.

The highest nitrate levels are found in the lower portion of the stem. When grazing don’t force them 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 in on it.

For more information contact the nearest University of Missouri Extension field specialist in agronomy and livestock.

Writer: Eldon Cole

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