Farm Progress

Some forages could be toxic due to drought

Jeffrey Stapper

July 23, 2009

4 Min Read

Drought has caused some uncommon crops, including corn and sorghum, to be baled, grazed or used for livestock forage. With this practice come some environmental conditions that could cause the plants to contain toxic levels of nitrate or prussic acid.

Nitrate is present to some degree in all forages; however when plants are stressed, normal plant growth does not occur, which may result in plants accumulating too much nitrate, or toxic levels. Nitrate poisoning can occur if the forage consumed contains high levels of nitrate, if a sudden diet change occurs, if conditions causing anemia exist, or if livestock consume supplements of urea or high-protein feeds along with forage containing moderate levels of nitrate.

Many kinds of plants can accumulate nitrate, including plants in the sorghum family, like johnsongrass, sudangrasses, sorghum hybrids, corn, small grains, careless weed or pigweed. Under dry conditions, plant roots continue to absorb small amounts of nitrogen, but the plant has too little water to keep growing, thus nitrate accumulates and is stored in the lower leaves and stems.

To help prevent losses from nitrate poisoning producers can take the following steps:

• Never turn hungry animals into possibly high nitrate forages. Turning cattle into holding pens or corrals full of manure with pigweed or nightshade species can result in immediate poisoning.

• Turning one old cow into a field to observe is not an effective test for nitrates, because cattle tend to graze the tops of plants first where the concentration is lowest. As they move to the lower plant parts poisoning could occur.

• Have hay tested before feeding, as nitrate levels remain constant in hay.

• If hay is high in nitrate, feed carefully with an energy supplement or in combination with

low-protein forages or other hay low in nitrates. Never feed high nitrate hay free choice.

Excessive nitrate consumption can be fatal to cattle. Nitrate concentrations in excess of 1 percent in the dry matter are considered toxic. However, lower concentrations also can cause health and reproductive problems and impede growth. Nitrate concentrations less than 0.3 percent are regarded as safe for pregnant cattle and 0.3 percent to 0.5 percent are safe for other cattle. As levels surpass 0.5 percent, the risk of reproductive failure, health problems and reduced performance increases.

Another concern with drought-stressed crops is prussic acid, which accumulates in stressed sorghum, sudangrass, johnsongrass and sorghum-sudangrass hybrids. Unlike nitrate, prussic acid may be present for a while and then dissipate from plants properly cured for hay.

Prussic acid accumulation can happen when:

• There are poor growing conditions that prevent stems from developing properly.

• Recent hay harvest or grazing causes slow and stunted growth of new plant tissue.

• Nitrogen fertilizers are over-used or there are other soil fertility or nutrient imbalances.

• Plants develop new growth after a prolonged drought.

• Plants are injured by herbicides, frost, hail or other events.

As forage cures after cutting, however, prussic acid levels will dissipate so proper harvesting and baling practices can alleviate some potential problem. Prussic acid accumulates mainly in leaves, with highest concentrations in new growth.

Test sorghum forages to determine the level of prussic acid. Levels of less than 500 parts per million on a dry matter basis are usually considered safe, while levels greater than 1,000 parts per million are considered hazardous and can be fatal to livestock.

Since nitrate accumulation and prussic acid formation occur in different parts of the plant, one sample will not adequately address the potential threats. When sampling for nitrates, select multiple plants and segregate the lower stalks (ground level to 8 inches) and then 8 to 16 inches above ground level samples. These cutting heights could be used to determine where plants should be cut if making hay following analysis. Nitrate levels are usually higher in the lower stalk. Samples collected for prussic acid should be composed of the newest leaves and damaged leaves.

The Texas Veterinary Medical Diagnostic Laboratory can test grass and hay for nitrates for $6 per sample: 1 pound dry hay or 5-pound green grass mailed in a paper sack and box. Prussic acid affected forages can also be tested (cyanide test) for $6 per sample if collected and shipped immediately in an air tight bag or container. For sampling instructions see their website: or call for more information toll free (888) 646-5623.

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