To Reduce The Risk of Prussic Acid Poisoning

On October 19, 2019

To Reduce The Risk of Prussic Acid Poisoning

Warm-season annuals are a widely-used, excellent way to provide forage for livestock in the summer months when cool-season perennials slow their growth – or during the “summer slump”.  In most cases, more than one grazing or harvest can be obtained from these warm-season annuals if nitrogen fertilizer is applied after harvest or grazing to stimulate regrowth, allowing the forage to grow well into the cooler days of fall.

Extension Agronomist Jessica Williamson explains a cyanogenic compound is normally found in sorghum species. This is in a bound, non-toxic form called dhurrin; however, after a killing frost or another source of damage to the plant, a compound also present in these forages called emulsion reacts with the dhurrin and “frees” it, causing a highly toxic, extremely poisonous cyanogenic compound within the plant.  A concentration of a mere 0.1 percent or greater of dry tissue is considered dangerous and could kill livestock.

All species of sorghum contain prussic acid within the vegetative portion of the plant.  Sorghum, johnsongrass, and shattercane contain the greatest levels of prussic acid.  Sudangrass contains approximately 40 percent less prussic acid than other sorghums; however, a sorghum x sudangrass hybrid contains a greater level of the toxic compound than sudangrass alone.  Improvements in genetic development of forages now allow options for planting varieties of sorghums that contain lower levels of prussic acid, helping to reduce the risk of prussic acid poisoning in livestock.

Another option for incorporating a summer annual pasture or hay crop while reducing the risk of prussic acid poisoning is the use of pearl millet and foxtail millet, which do not contain toxic levels of prussic acid, even after a killing frost.  Therefore, it can be grazed any time.

Generally, the greatest levels of prussic acid can be found in the leafier areas of the plant.  After a killing frost, toxic prussic acid does not begin to decline until after the leaves have died.  To be safe, wait at least 7-10 days after a killing frost to graze or green chop forage.  If forages regrow after a non-killing frost, do not graze or feed until the regrowth has reached a minimum of 2 feet in height or 2 weeks, as the regrowth will likely contain high, very toxic levels of prussic acid.

Heavy rates of fertilization and drought can also cause high levels of prussic acid accumulation in these forages even months before a killing frost; therefore, precaution should be taken during these conditions as well.  Ensiling these forages helps to reduce the risk of toxic levels of prussic acid, as some of the toxic components escape during the fermentation process as gas.  Sorghum silage should not be fed any earlier than 3-4 weeks after harvest as a precaution.

If chopping for silage is desired, a producer should wait 5-7 days after a frost before harvesting.  These forages with a risk of high cyanide levels at the time of chopping should be ensiled for a minimum of 8 weeks before feeding and should be analyzed before feeding to ensure the toxic compounds have been reduced to a safe level for consumption.

To Graze Corn Reside

Corn stover is often used as ground cover, baled for bedding for the winter season, but it can also be utilized as a forage in some situations where feed supplies are limited. In Pennsylvania, it is common for corn residue left after combining grain to be utilized as ground cover throughout the winter, serving to protect the ground and helping to eliminate runoff of nutrients and soil erosion.  Extension Agronomist Jessica Williamson explains another common practice is baling corn stover and utilizing it as bedding for livestock during the cold months of the year.  The absorbency helps to keep animals dry and warm during the harsh weather.

However, an often overlooked usage for corn residue is its feeding value – either baled or grazed.  Mature, non-lactating, mid-gestation beef cattle require approximately 50% total digestible nutrients (TDN) and 7.1% crude protein (CP) in their rations.  In a spring-calving cow herd, cows will not be lactating and will be in mid-gestation during the winter.  Corn stover has a 35-55% TDN and a 4-7% CP, and with a little supplementation, can be utilized effectively as a forage.

When the diet of a ruminant animal drops below 7% CP, rumen function begins to decline, causing a decrease in utilization of consumed forages.  Therefore, when feeding corn stover as a forage, it is important to supplement a nutritional protein source so ruminal motility remains optimal and animal production does not decline.  This is especially important during the winter when nutrient requirements increase slightly.

Supplementing with a high-quality dry hay with optimum TDN and CP will help fill the void in nutrients left by the corn stover, while the corn residue will provide essential fill and fiber for optimal ruminal fermentation.

Studies have shown that grazing crop residue has no negative impact on subsequent crop yields and can improve soil health through the reapplication of nutrients removed and hoof impact.  If corn stover is baled and removed, it is recommended that a winter annual be planted to minimize the risk of soil erosion, nutrient runoff, and to provide ground cover for the winter season, especially if manure will be applied to that field.

Additionally, farmers with a grain operation and no grazing livestock could lease their fields with corn residue to neighboring livestock operations for fall grazing and obtain an additional income source.

Quote Of The Week: “Autumn… the year’s last, loveliest smile.” – William Cullen Bryant