Helpful Tips for Grazing Stressed Forage
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Written By: Lauren Langley, Livestock Extension Agent, Alamance County
Sources: Purdue Cooperative Extension, King’s Agriseeds, University of Minnesota Extension
*Edited to add, with the abnormally dry weather Extension is receiving a lot of questions regarding grazing and/or harvesting stressed forages*
Prussic Acid & Nitrate Poisoning
Every fall I get questions related to prussic acid and nitrate poisoning in sorghum, sorghum-sudan, and sudangrass fields. These summer annuals when stressed due to drought or frost can pose a potential threat to the livestock consuming them. Often times these annuals are planted for summer pasture, hay, or silage.
Prussic acid poisoning can occur when the plant tissue is damaged by frost (can also occur from hail and herbicide damage). Prussic acid formation is also a threat when heavy rates of nitrogen are applied and soils are deficient in P and K. Grazing poses the greatest risk because the highest amount of prussic acid is found in the leaves, and that is where the animals like to graze first.
Here are a few tips to help prevent prussic acid poisoning:
- Never turn hungry animals out onto questionable forage. If feed is questionable, feed good quality hay or silage first.
- Wait until plants are at least 18 inches to graze.
- After frost, wait at least 7-14 days to graze or cut, or until leaves are dead or dried out.
- Make sure hay is properly cured before baling, since cyanide escapes from the plant tissue. Curing decreases prussic acid content by up to 75%.
- Wait at least 3 weeks after harvesting and storing new silage.
- If high N is applied to soil that is low in phosphorus and potassium, plants may be at greater risk.
If the plant becomes stressed due to drought or herbicide injury, it may accumulate nitrates, which can lead to nitrate poisoning. Here are a few tips to help prevent nitrate poisoning:
- Split your N applications over the course of the growing season, so the plant has less opportunity to take up excess.
- Delay harvest 7 days after a drought-ending rain. Nitrate levels will be highest just after the plant resumes growth, and it will take several days of active growth for levels to go down again. Nitrates are non-volatile, and will remain in non-ensiled plants after cutting and baling.
- Raise cutting height. This can mean 6-8 inches or higher. Nitrate concentration is higher at the plant base, and lower in the leaves.
- Test all suspect forages at a qualified lab. Take representative samples from the field or core samples from the bale.
- Segregate all forages high in nitrates. Depending on the nitrate level, these can be fed if diluted with a certain proportion of normal feed, especially by feeding hay to hungry animals before giving them access to the contaminated feed. Generally horses and other monogastrics tend to be more tolerant than ruminants.
Frost & Nonstructural Carbohydrates (Sugars & Starch Levels) in Grass
Quality of both legumes (i.e. alfalfa) and grasses begin to decline after a hard “killing” frost. Legume quality deteriorates more rapidly than grass quality because legumes will lose their leaves and grasses do not (leaves contribute significantly to the overall quality of both grasses and legumes).
Grasses often become more palatable (preferred) because of the elevated nonstructural carbohydrates (NSC) values. Legumes and grasses tend to have elevated NSC values (an indicator of starch and sugar levels) after a frost, and it is recommend that horse owners wait up to a week before resuming grazing after a killing frost in an effort to avoid some health problems such as foundering. (NSC values will eventually decrease over time) Also, forage protein, equine total digestible nutrients (TDN), and equine digestible energy decrease gradually after a hard frost. This decline is due to a combination of the forage plants leaching nutrients and continued plant respiration.
Bottom line: grass species tend to retain their nutritional value longer after a killing frost (compared to legumes), and horse owner should wait up to a week before resuming grazing after a killing frost.