Precautions to take when grazing and feeding cover crops

As we approach fall and winter, several producers will start grazing their cover crops and annual forages. Others may have already harvested their annual forages and plan to start feeding them to their livestock. 

Although grazing cover crops can provide high quality feed, extend the grazing season and reduce the amount of hay needed to be fed, there are some precautions to reduce the losses to toxicity of nitrates and prussic acid. These precautions also apply to harvested annual forages to prevent nitrate toxicity. 

Nitrate toxicity

Species such as corn, millet, sorghums, Sudangrass, sunflower, turnip, radish, collards, kale, oats, cereal rye and wheat can be nitrate accumulators. However, there are several factors affecting their nitrate toxicity levels. Species grown in drought conditions, fields with high nitrogen availability, stunted plant growth due to herbicides or plant diseases and young plants (toxicity decreases in mature plants) can have higher nitrate levels. 

To ensure cover crops intended for grazing and/or for harvested annual forages are safe to be fed, testing for nitrates is recommended. 

It is important to note some labs report nitrate content as “nitrates (NO3)” while others report it as “nitrate nitrogen (NO3-N).”

This is important to understand, as nitrate nitrogen is 4.43 times as risky as straight nitrate. If results are reported in “nitrate (NO3),” multiply the number by 4.43 to convert it to “nitrate nitrogen.” If the lab reports it as a percentage, multiply it by 10,000 to convert to parts per million (ppm).

Management strategies

Careful management can reduce livestock losses when there is a chance of nitrate toxicity. Additionally, there are strategies to help mitigate the risk of nitrate poisoning.

Adapt animals slowly to cover crops and/or harvested feed with toxic nitrate levels; use livestock at lower risk (i.e. open cows and growing calves) rather than livestock at higher risk (i.e. pregnant cattle are at higher risk and can abort); make sure cattle are full prior to grazing cover crops with high nitrate levels. 

Additionally, do not feed harvested annual forages with high nitrate levels to very hungry animals. Graze pastures and fields with moderate to low stocking rates so they can be more selective and graze plant parts with lower nitrate concentration.

Other considerations include: supplement livestock with a probiotic, grain and/or a low nitrate feedstuff frequently to dilute the nitrate concentration; and wait five to seven days to graze after a non-killing frost (when new growth may occur). If new shoots or regrowth occurs, remove animals immediately.

However, when forages are at severe risk (more than 3,400 nitrate nitrogen ppm), it is best not to use them. Additionally, even when risk is low, it is a good management practice to fill cattle with good hay (safe hay in terms of nitrate toxicity) before exposing them to forages with any nitrate level. This allows a slow nitrate intake which allows the rumen in cattle to adjust to nitrate. 

Prussic acid

Prussic acid can result in sudden death in livestock. Symptoms include staggering, gasping, trembling muscles, convulsions, respiratory failure, mucous membranes in the mouth and eyes turning blue and cherry red blood at death. Species such as Sudangrass, sorghums, milo, some legumes (i.e. birdsfoot trefoil) can obtain prussic acid.

Prussic acid toxicity from cover crops can also lead to losses in a herd. Prussic acid is greater in crops grown in soils with high nitrogen availability and are deficient in phosphorus. Leaf blades and young plants have higher prussic acid. Therefore, following a frost, if new growth occurs, the new growth can have high levels of prussic acid. 

Additionally, forages grown under drought conditions may have higher prussic acid content. The reasons for this is, those forages are unable to grow out of the high prussic acid stage, and it reduces the availability of phosphorus to plants. 

There’re management strategies to reduce toxicity of prussic acid. Several recommendations include: Select varieties low in prussic acid content; obtain a soil sample and apply phosphorus if needed to ensure adequate phosphorus levels; and analyze forages for prussic acid content. 

Hay does not need to be tested unless there is a concern for high prussic content. Don’t graze until forages have reached a mature height (i.e. at least 18 inches in Sudangrass). Wait five to seven days to graze after a non-killing frost (when new growth may occur). If new shoots or regrowth occurs, remove animals immediately. 

In addition, it’s important to not allow hungry livestock to graze cover crops containing high prussic acid, and forages containing high prussic acid can be converted into hay, silage or green chop as those processes reduce the prussic acid significantly.

Summary

Grazing cover crops in late fall and winter can be beneficial for many producers, as it provides high quality forage and reduces the amount of hay needed to feed. However, taking precautions such as testing forages for nitrate and prussic acid content is extremely important. Doing so can help reduce the amount of losses to a herd. 

Additionally, other management practices such as grazing at the proper time, feeding livestock with safe hay so they are full prior to exposure of cover crops with nitrate or prussic acid content, recognizing symptoms of toxicity, etc. can help mitigate the risk and losses due to toxicity. If it’s suspected there is a toxicity issue, reach out to a veterinarian immediately and remove animals from the feed. 

Alex Orozco-Lopez is a University of Wyoming Agriculture and Natural Resources Extension educator. He can be reached at aorozcol@uwyo.edu.