As we approach fall and winter, several producers will start grazing their cover crops/ 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 be made to reduce the losses to toxicity of nitrates as prussic acid. These precautions also apply to harvested annual forages to prevent nitrate toxicity.

Nitrate toxicity

Species such as corn, millet, sorghums, sudan grass, sunflower, turnip, radish, collards, kale, oats, cereal rye, and wheat can be nitrate accumulators (Drewnoski, 2015). However, there are several factors that affect 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 that your cover crops intended for grazing and/or your harvested annual forages are safe to be fed, testing for nitrates is recommended. Drewnoski (2015) reported the specific levels of nitrate nitrogen content in which they become risky to un-adapted cattle could be found below in figure 1. Un-adapted livestock whom are pregnant, very hungry. It is important to note that some labs report nitrate content as “nitrates (NO3)” while other names report it as “nitrate nitrogen (NO3-N)” (Anderson and Redfearn, 2016). This is important to understand, as nitrate nitrogen is 4.43 times as risky as straight nitrate. If your results are reported in “nitrate (NO3)”, multiply that number by 4.43 to convert it to “nitrate nitrogen” (Drewnoski, 2015). If the lab reports it as % multiply it by 10,000 to convert to ppm.

Nitrate nitrogen levels (ppm) and risk to un-adapted cattle
0 – 1100 = safe
1100 – 2100 = moderate risk
2100 – 3400 = high risk
>3400 = severe risk

Figure 1: Levels of nitrate nitrogen content in which they become risky to un-adapted cattle (Drewnoski, 2015).

Careful management can reduce livestock losses when there a chance of nitrate toxicity occurring. Additionally, there are strategies that help mitigate the risk of nitrate poisoning (Anderson and Redfearn, 2016).

• 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).
• Do not allow animals that are very hungry livestock (make sure cattle are full prior) to graze cover crops with high nitrate levels. Additionally, do not feed harvested annual forages with high nitrate levels to very hungry animals.
• Graze pastures/fields with moderate to low stocking rate so that they can be more selective and graze plant parts with lower nitrate concentration.
• Supplement your livestock with a probiotic, grain, and/or a low nitrate feedstuff frequent to dilute the nitrate concentration.
• Waiting 5-7 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 a severe risk (>3400 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 (Andrews, 2016). Species such as sudan grass, sorghums, milo, some legumes (i.e. birdsfoot trefoil) obtain prussic acid (Duppstadht, 2020; Andrews, 2016; Beckman, 2020).

Prussic acid toxicity from cover crops can also lead to losses in your herd. Prussic acid is greater in crops grown in soils that have high nitrogen availability but 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 that those forages are unable to grow out of the high prussic acid stage and because it reduces the availability of phosphorus to the plants.

Management strategies to reduce toxicity of prussic acid.

• Select varieties that are low in prussic acid content.
• Obtain a soil sample and apply phosphorus if needed to ensure adequate phosphorus levels.
• Analyze your forage for prussic acid content. Hay does not to be tested unless you are concern there is high prussic content.
• Do not grass until your forages have reached at a mature height (i.e. at least 18 inches in sudangrass).
• Waiting 5-7 days to graze after a non-killing frost (when new growth may occur). If new shoots or regrowth occurs, remove animals immediately.
• Do not allow animals that are very hungry livestock (make sure cattle are full prior) to graze cover crops that contain high prussic acid.
• Forages that contain high prussic acid can be converted into hay, silage, or green chop as those process reduces the prussic acid significantly.

Summary

Grazing cover crops in late fall/winter can be beneficial for many producers as it provides high quality forage and reduces the amount of hay they need to feed. However, taking precautions such as testing your forages for nitrate and prussic acid content is extremely important. Doing so can help reduce the amount of loses to your herd. Additionally, other management practices such as grazing at the proper time, feeding your 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 be help mitigate the risk and loses to toxicity. If you suspect s toxicity issue, reach out to your veterinarian immediately and remove animals from the feed.

By: Alex Orozco, Extension Educator, Crook County, University of Wyoming