Drought-Related Toxins
Drought continues in Rockbridge. The recent thunderstorms have been welcome but, as is typical of summer weather patterns like these, the rainfall totals are extremely spotty with some fields getting a significant rain of ½-inch or more but other fields in very close proximity (often less than a mile) getting little or no rain.
Pasture, hay, and summer annuals grown for livestock feed that are drought-stressed can accumulate two particular toxins. One is nitrate that can occur in hay, pasture, corn, sorghum, or sudan grasses that were heavily fertilized in anticipation of spring or summer rain that never materialized. The second is prussic acid, or hydrogen cyanide which is unique to certain plant species including sorghum, sorghumsudan hybrids, and johnsongrass.
For nitrates, testing the plant material is recommended if medium to high rates of nitrogen fertilizer (50 pounds or more) were applied to the crop and the plants have put on little to modest growth. When plant growth slows due to drought, the plant may still take up nitrate but not convert it into plant tissue so the nitrate just accumulates. Nitrate is converted to nitrite in the animal’s digestive system and nitrite destroys the ability of red blood cells to transport oxygen in the animal’s body resulting in death.
Nitrates in plants harvested as hay are preserved and remain toxic. The same crop harvested and preserved as silage or haylage may have 50% to 75% less nitrate because the microbial activity associated with the fermentation process of ensiling can metabolize nitrate.
The highest concentration of nitrates is typically found in the bottom stem of the plant, so cutting the crop a little higher off the ground can reduce nitrate risk. Dilution of the toxic effects of nitrate can be achieved by ensuring the animals consume feed or forage known to be nitrate-free along with the suspect forage.
Prussic acid is a toxin unique to certain plant species that include sorghum, sorghum-sudangrass, sudangrass, shattercane, and johnsongrass. These plants accumulate prussic acid only when drought-stressed. New growth and young plants generally have a higher level of prussic acid so waiting for plants to reach greater than 24 inches height greatly reduces any risk. Unlike nitrate, which tends to concentrate in the lower stem, prussic acid accumulates in the leaves. The hay making or silage-making process will break down as much as 50-75% of any prussic acid in a livestock forage.
For more information about testing hay or silage for nitrate or prussic acid or other farmrelated issues due to drought, contact me, Tom Stanley, through the Rockbridge Extension Office at (540) 463-4734 or [email protected].
Special thanks to Kristen Ulmer, Extension agent in Augusta County, for her contributions to the content of this article.
