The following is discussion to better understand the difference between prussic acid and nitrate accumulation in forage crops.

PRUSSIC ACID — Animal toxicity from "hydrocyanosis" or "prussic acid poisoning" is a concern when using sorghums and sudans as a feed source. Cattle, sheep and goats grazing on green, growing sorghum crops should be watched closely for any signs of prussic acid poisoning.

Young sorghum plants, especially the leaves, contain dhurrin. The digestive process breaks dhurrin into glucose (sugar) and (HCN) hydrogen cyanide, which goes into solutions immediately, creating lethal hydrocyanic acid.

Prussic acid poisoning is most often associated with sorghums and sudangrass. It can also occur in a number of other plants: Johnsongrass, flax, arrowgrass, birdsfoot trefoil, chokecherry, wild cherry, black elderberry, white clover, velvetgrass, and Christmasberry.

Danger of prussic acid poisoning is greatest at immature stages of growth and decreases with maturity. Plants harvested at immature stages of growth may by higher in prussic acid than when left to maturity.

Regrowth or second growth, is often harvested or grazed at immature stages and may also be higher in prussic acid.

Generally speaking, the short, dark green plants have the greatest potential for prussic acid poisoning. Except at very immature stages, prussic acid accumulates largely in the leaf portion of the plant.

Frost or freezing causes plant cells to rupture, thus allowing prussic acid to be released. If the potential for the poisoning is great before freezing, then the danger of poisoning is very great while the plant is frozen and for a week or two following. It is during this time that prussic acid is being released by the plant. The potential for prussic acid poisoning does not increase if the potential did not exist before freezing.

INTERPRETATIONS

Following are general guidelines to use when interpreting laboratory results for prussic acid in feedstuffs. These guidelines are for mature, healthy livestock in good condition. Younger animals or those in poor conditions may show toxicity symptoms at lower HCN levels.

LOW: Considered safe to graze or green chop.

MEDIUM: Use caution when grazing or green chopping. Harvesting by baling as sun-cured hay or by ensiling are better choices because they reduce the HCN toxicity hazard. Allowing plants to mature will help lower HCN levels. Do not graze or green chop for two weeks after a non-killing frost.

HIGH: Doubtful to graze or green chop. Harvesting by baling as sun-cured hay or by ensiling are better choices because they reduce the HCN toxicity hazard. Allowing plants to mature will help lower HCN levels. Do not graze or green chop for two weeks after a non-killing frost.

VERY HIGH: Dangerous to graze or green chop. Do not harvest by these methods. Harvesting by baling as sun-cured hay or by ensiling are better choices because they reduce the HCN toxicity hazard. Allowing plants to mature will help lower HCN levels.

NITRATE POISONING — Many producers grow summer annuals such as Sudan for dry forage. Nitrate poisoning is many times confused with prussic acid poisoning, but is the result of the accumulation of nitrate-nitrogen (NO3) induced by drought, chemicals, or lack of sunlight. Many types of forage can accumulate nitrate, including: pigweed, wheat, corn, oats, alfalfa, sorghums and some native grasses. In a drougth-stricken year, dryland corn harvested for silage would be a concern.

The nitrate-nitrogen is converted to nitrite (NO2) which can be extremely harmful to livestock. In cows and sheep this takes place in the rumen, in horses in the caecum. Swine, goats, poultry and dogs are also susceptible.

Nitrite that is absorbed into the bloodstream combines with hemoglobin to form a compound called "methemo-globin". The function of hemoglobin in the body is to carry oxygen from the lungs. Methemoglobin cannot carry oxygen.

Toxicity occurs when enough methemoglobin is produced to lower the oxygen-carrying capacity of the blood below a critical level. This causes oxygen starvation of the body and asphyxiation. In severe cases the animal actually smothers to death.

Management tips to help reduce nitrate poisoning concern:

• Do not harvest during or immediately following stress growing conditions.
• Do not harvest immature plants.
• Avoid excessive nitrogen fertilizer applications.
• Nitrates may be reduced during the drying process of making hay and further reduced when storage moisture of 15% is reached.
• Nitrate tends to accumulate in the lower portion of the stalk. Leave 10-14 inches of stalk when chopping forages suspected to have toxic levels of nitrate.
• Allow the ensiling process to decrease nitrate content. Over time, "silo gas", nitrogen oxide, is formed. Accumulation of this gas may be lethal to man or animals. Animals should not be allowed access to liquids draining from silos, as they can have high nitrates.
• Always analyze samples of any crop suspected of having high nitrates and develop feeding plans accordingly.

As with any testing procedure, the test is only as accurate as the sample. The plant sample must be representative of the way the forage will be fed or grazed. For example, if the forage will be grazed, livestock generally do not graze the lower stalk. Collect a sample that does not contain the lower stalk.

In research conducted at KSU, the nitrate (NO3) content of 23 bales of forage Sudan raised under dryland conditions on an upland crete silt loam soil averaged 2,764 ppm, but varied from 1,525 to 6,250 ppm on an as-fed basis.

As this study demonstrates, a perplexing variation in the nitrate content and possible toxicity of different forage bales off the same field can occur. What causes this inconsistency? Certainly, changes in soil type and topography, and improvements such as terrace channels can contribute to diverse fertility and moisture conditions across the same field. In addition, such effects as fertilizer spreader overlap and herbicide drift can cause variations in plant physiology, and they may operate parallel to plant rows. Thus, at harvest time, a single large round bale made parallel to the crop rows could contain a much higher nitrate content than a bale produced a few feet to the left or right. You may have tested a sample of the bales in the field for excessive nitrate levels, but you may not have tested the one or two that will do the damage!

Nitrate--Livestock Hazard Rates:

Very Low: Considered safe to feed.

Low: Considered safe feed for non-pregnant animals. Suggest limiting intake to less than 66 percent of the total dry matter intake for pregnant animals if nitrate level is at the upper end of this range.

Medium: Suggest limiting intake to less than 33 percent of total dry matter intake for pregnant animals and less than 66 percent of the total dry matter intake of non-pregnant animals.

High: Suggest limiting intake about 33 percent of the total dry matter in rations for non-pregnant animals. Not advised for use with pregnant animals.

Very High: Suggest limiting use of this feedstuff to less than 33 percent of the total dry matter in rations. Not advised for use with pregnant animals.

Extremely High: This feedstuff not advised for use in a free-choice feeding program. Suggest limiting intake to less than 10 percent of the total dry matter intake in rations for non-pregnant animals. Not advised for use with pregnant animals.

* As with any testing procedure, the test is only as accurate as the sample. The plant sample must be representative of the way the forage will be fed or grazed. For example, if the forage will be grazed, livestock generally do not graze the lower stalk. Collect a sample that does not contain the lower stalk.

Remember that even if your forage tests high for either prussic acid or nitrates - not all is lost. By knowing what potential risks your forage contains, you can still use your forage safely.