By Jessica A. Williamson
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 following harvest or grazing to stimulate regrowth, allowing the forage to grow well into the cooler days of fall.
During times of water stress or drought, or as a result of excessive nitrogen fertilization, nitrates and nitrites accumulate in the lower portion of the forage at a rate that is greater than what is normally present in the forage and can be toxic if consumed excessively by ruminant livestock. The only way to determine in excessive nitrate levels are present in forages is with a nitrate analysis.
Chart shows varying levels of nitrates and the toxicity potential to cattle.
|Nitrate Nitrogen (NO3-N) (ppm dry matter basis)||Recommendations|
|<1,000||Safe to feed under most conditions|
|1,000-1,700||Gradually introduce to ration. Feed some concentrate. Test all feeds and water. Dilute to 900 ppm NO3-N in total ration dry matter. Restrict single meal size.|
|1,700-2,300||Possible acute toxicity. Feed in a balanced ration with concentrate included. Dilute to 900 ppm NO3-N in total ration dry matter. Restrict single meal size.|
For forages with known high levels of nitrates, there are a few management practices that will help to prevent toxicity. Delaying harvest until stress conditions have passed will help to lower nitrate levels within the crop. Excessive nitrates typically accumulate at the base of the plant; therefore, chopping or mowing the forage much higher than usual will help to reduce the amount of nitrate harvested. Also, ensiling the forage has been shown to reduce the nitrate levels by half at the time of completion of the fermentation process. Wrapping baled forage or packing in a silo and eliminating oxygen so fermentation can occur is an excellent way to reduce nitrate levels.
It is always recommended to not feed toxic forage to livestock; however, unfortunately at times a significant portion of a feed supply will have high nitrate accumulation, deeming it necessary to feed due to a shortage of non-toxic feed on the farm. If forage is known to have higher than ideal nitrate levels, diluting the forage by incorporating a low-nitrate forage into the diet will reduce the overall nitrate consumption by the animal. Introducing the toxic forage slowly will help to get animals adapted to nitrate levels, as well as feeding small amounts frequently rather than at one large feeding.
Prussic Acid Poisoning
A cyanogenic compound is normally found in sorghum species 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.