Farm Progress

Drought may cause forage toxicityDrought may cause forage toxicity

Roy Roberson 2

August 7, 2007

6 Min Read

Nitrate poisoning is cropping up in pastures across the Southeast, and is putting an even more severe financial pinch on cattle operations trying to extend summer grazing to decrease the use of high priced corn for cattle feed.

Widespread drought in the lower Southeast and more sporadic, but often severe dry weather in the upper Southeast, is causing well-documented problems for row crop farmers, but perhaps as hard hit are cattle operations. Sorghum, sudangrass, sorghum-sudangrass hybrids, fescue, bermudagrass, pearl millet and naturally occurring johnsongrass are frequently utilized for hay, silage, or grazing in the Southeast. These popular grass species are generally safe to use as forages, but under drought conditions they can contain toxic levels of nitrate and prussic acid (cyanide).

Being aware of the factors that can result in accumulation of nitrates or the formation of prussic acid and using alternative forages during these periods will reduce chances of livestock losses, according to Chris Teutsch, assistant professor of forages at the Southern Piedmont Area Research and Extension Center in Blackstone, Va. Nitrate toxicity most often occurs when heavy nitrogen fertilization is followed by drought. Nitrates are taken up by the plant, but not utilized since plant growth is restricted by the drought.

Any factor that slows plant growth in combination with heavy nitrogen fertilization can result in nitrate accumulation. Drought stressed plants should not be grazed until growth has resumed after rainfall (usually 4-5 days). Auburn University Forage Specialist Don Ball says it is important to note drying or ensiling of forage does little to degrade nitrates, consequently, toxicity may occur months after harvest.

Cattle producers in Alabama, which is one of the Southeastern states hardest hit by drought, need to be particularly aware of nitrate toxicity, Ball says. Symptoms of nitrate poisoning include trembling, staggering, rapid and labored breathing, rapid pulse, frequent urination followed by collapse, coma, and death. The onset of symptoms and death is rapid and usually occurs within one to two hours.

In animals affected by nitrate poisoning, the blood will take on a brownish chocolate color, giving the non-pigmented skin and mucus membranes a muddy brown color. Teutsch says in some cases it may be advisable to wait to make hay until the drought is over and plant growth has resumed. In other cases, it may be advisable to raise the cutter bar 3-6 inches to decrease nitrates as a greater percentage of nitrates are stored in the lower stems of these plants.

When high nitrate forage is harvested as silage, nitrates can be reduced by 40 percent to 60 percent during the ensiling process.

In addition to excess nitrogen, an imbalance of other soil nutrients can affect forage nitrate levels. Plants growing in soils deficient in phosphorus, potassium, and some trace elements have high nitrate concentrations.

Herbicides such as 2, 4-D tend to disrupt normal growth processes in a plant and may cause a temporary increase in nitrate levels.

The nitrate content of certain harvested forages may be lowered by herbicide spraying since nitrate accumulating weeds are eliminated.

Nitrate concentration is usually the highest in young plants and tends to decline as the plant matures. However, mature plants can be high in nitrates depending upon soil nitrate levels and other conditions.

Plant parts closest to the ground contain the most nitrate. Nitrates are normally utilized in the leaves of plants. Unfavorable conditions cause nitrate accumulation in the stalks and stems of plants.

The Virginia Tech scientist urges cattle producers in drought stricken areas to test forage for nitrate levels. Anytime nitrate concentrations reach 2,500 parts per million there is reason for concern.

Any forage over 5,000 ppm is dangerous and should be limited to 25 percent of the total ration and should be supplemented with energy, minerals and Vitamin A. Forage samples that test over 15,000 are toxic and should not be fed to animals.

Animals under physiological stress (sick, hungry, lactating, or pregnant) are more susceptible to nitrate toxicity than healthy animals. Toxicity is related to the total amount of forage consumed and how quickly it is eaten, but, generally, if forages contain more than 6,000 ppm nitrate, they should be considered dangerous.

Diagnosis and treatment of nitrate toxicity should be performed by a veterinarian. However, in acute cases where time is limited, an antidote of methylene blue can be injected to convert the methemoglobin back to hemoglobin. Teutsch says another potential problem with some of these summer forages is prussic acid or cyanide poisoning. Under normal conditions these forages contain little free cyanide.

However, when freezing, drought stress, wilting, or mechanical injury damages plant tissue, an enzymatic reaction occurs and free cyanide is produced.

If forage is ingested while forage is under any of these stress situations, especially drought, cyanide is readily absorbed into the bloodstream where it interferes with normal cellular respiration.

Symptoms of cyanide poisoning in cattle are similar to nitrate poisoning and include labored breathing, excitement, gasping, convulsions, weakness, prostration and death.

Unlike nitrate poisoning, the onset of symptoms and death with cyanide poisoning is very rapid, occurring in minutes to several hours. In contrast to nitrate poisoning, the blood of animals affected by cyanide poisoning is fully oxygenated and bright cherry red in color.

Tuetsch says in most situations, sorghum species (including johnsongrass) pose little danger to grazing animals when properly managed. Young plants or regrowth after grazing contain higher concentrations of prussic acid and should not be grazed until plants have reached a height of 20-30 inches. Drought stressed plants should not be grazed until growth has resumed after rainfall (usually 4-5 days), he adds.

If large quantities of forage high in prussic acid are consumed rapidly, death can occur within a few minutes. However, the usual situation is that the animals consume smaller quantities of the forage over a longer period, causing first salivation, then a gradual increase in respiratory rate, followed by staggering, falling, severe convulsions and finally death within 45 minutes. Generally, animals that survive two hours after the onset of symptoms will recover.

Obviously, immediate treatment by a veterinarian is necessary to save poisoned animals. Treatment includes administering sodium nitrite and sodium thiosulfate.

Cyanide does escape from plant tissue. Therefore, hay that has been properly cured is safe to feed. Properly ensiled forage is also safe to feed. When forage is being utilized as green chop, it is important to feed the green chop in a timely manner. If the green chop is allowed to wilt or heat, cyanide is released and the forage becomes toxic.

If questionable forage must be grazed or utilized as green chop, feed dry hay along with the fresh plant material. The best option for cattlemen or dairy producers grazing cattle under drought conditions is to have forage tested for nitrates. Though other stressors can cause or contribute to nitrate or prussic acid poisoning, in the Southeast in 2007 drought is main reason for concern.

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