Members of the sorghum family have been reported to cause �cystitis� syndrome in horses. Cystitis (inflamed urinary bladder) is characterized by uncontrolled urination, abortion or stillbirths in mares, and by the inability to coordinate rear quarters. Cystitis affects only horses and is of no significance in ruminants. Horses are also susceptible to the prussic acid and nitrate poisoning problems more commonly associated with the sorghum family. For these reasons, it is best to avoid sorghum forages in the feeding of horses.
 |
Prussic Acid Poisoning |
Prussic acid (HCN) or hydrogen cyanide has long been recognized as a potentially devastating problem in the pasturing of livestock on sorghum forage. Symptoms of prussic acid poisoning include: nervousness, increased rate of respiration, trembling, blue coloration of mucous membranes, spasms, convulsions, and death. Prussic acid is concentrated in new growth of leaves and shoots. As the plants mature, prussic acid content declines due to the relative increase in stem, leaf, sheath, and midrib. The regrowth after cutting, intensive grazing, or after a mild frost can be very high in prussic acid because it is essentially all new leaves and shoots. Likewise, the growth that occurs immediately after a period of drought may have dangerously high levels of prussic acid. Generally, as long as growth is continuous the danger of prussic acid poisoning is relatively slight, however, should growth be interrupted and then begin again, extreme caution must be exercised.
The risk of prussic acid poisoning can be reduced by sound management practices. Do not begin grazing of these products until 18 to 20 inches of forage accumulation has occurred. If the crop is stunted by drought or frost do not graze or chop until the crop has recovered (several days after the first flush of growth). Do not pasture following a closely cut hay or greenchop harvest until 18 to 20 inches of regrowth has occurred. Avoid pasturing these crops after a killing frost for three to five days or until all growth has turned down. Apply phosphorous and potassium fertilizers according to soil tests and avoid excessive application of nitrogen fertilizer. High rates of nitrogen fertilizer combined with low soil levels of phosphorous can double the prussic acid content of young leaves. If in doubt, harvest the crop as hay or silage. Prussic acid dissipates as the forage lies in the windrow and studies have shown that the ensiling process reduces the prussic acid level by 50% to 80%.
While all sorghum forages have the potential to produce dangerously high levels of prussic acid, there are differences among species and cultivars. A ranking of these products from lowest potential to greatest is as follows; (1) varietal sudangrass, (2) hybrid sudangrass, (3) hybrid sorghum/sudangrass and (4) forage sorghums.
|
Nitrate Poisoning |
The nitrate content of sorghum and pearl millet forages can be high, depending upon growing conditions and the source and amount of nitrogen fertilizer. Excessive rates of nitrogen fertilizer from any source can result in a problem, however, nitrate accumulation by these forage crops is usually greater when nitrate fertilizers are used rather than ammonium sulfate or urea. Low light intensity, frost, hail, low temperature, disease infection, insect feeding, etc., can result in high nitrate content in these forages.
High nitrate forage consumed by livestock results in poor growth, reproduction problems and poor milk production. If nitrate content of forage is very high, rapid death can and does occur. Nitrate is converted by the animal into nitrite. Nitrite absorbed into the blood converts normal hemoglobin into methemoglobin, which is incapable of transporting oxygen to body tissues.
Treatment for animals suffering from nitrate toxicity usually consists of an intravenous injection of methylene blue solution, which brings about the reconversion of methemoglobin to hemoglobin. If nitrate problems are suspected, a representative sample of the forage should be sent to a laboratory for analysis. If the suspect forage is confirmed to be dangerously high in nitrate, it should be mixed with other roughage before feeding or discarded.
The risk of nitrate poisoning can be substantially reduced by sound management practices. Avoid grazing or harvesting these crops immediately after periods of drought, hail or frost. Do not make excessive nitrogen fertilizer applications. It is best to apply a portion of nitrogen fertilizer preplant (40 to 50 lbs.) and then after each grazing period or harvest make relatively small applications (20 to 30 lbs.). Avoid grazing these products after harvest as hay or greenchop until sufficient regrowth has occurred (nitrate accumulates in lower parts of the stalk). Harvest in the afternoon or evening, after the crop has been exposed to several hours of sunlight (avoid harvesting on cloudy, overcast days). If forage is suspect, harvest as silage. The ensiling process reduces nitrate concentration by 40% to 60%. Finally, never allow hungry animals to pasture potentially high nitrate crops without first feeding roughage to reduce initial intake.
While it is difficult to define the level of nitrate that will produce toxicity symptoms or adversely affect animal performance, when the line is crossed the results may be devastating. Generally, the sorghum sudangrass hybrids and pearl millet accumulate higher levels of nitrate than does sudangrass.