"Not All Hay Is Good Hay"
by: Dr. Mario Villarino
Hopkins County AgriLife Extension Agent
Earlier in the week, I had a visit from a beef producer in our county who had lost several head of cattle recently. He was concerned about the hay he was feeding and suspected toxins were involved.
There are toxic components affecting hay during drought, including nitrates and prussic acid. These toxic components create different problems in cattle, but both can be present during hay production, during drought, and as we all know, drought has been prevalent in the Southern U.S. for several months.
If forage contains too much nitrate, the animals cannot complete the conversion process and nitrite levels build up. Nitrite is absorbed directly into the bloodstream through the rumen wall, where it combines with hemoglobin to form methemoglobin. Hemoglobin carries oxygen in the blood, but methemoglobin does not. The formation of methemoglobin can cause an animal to die from asphyxiation, or lack of oxygen. The animal's blood turns brown instead of the normal bright red.
Signs of nitrite poisoning usually appear suddenly due to tissue hypoxia and low blood pressure as a consequence of vasodilation. Symptoms include rapid, weak heartbeat with subnormal body temperature, muscle tremors, and animals appear weak and cannot coordinate their legs. Under certain conditions, adverse effects may not be apparent until animals have been eating nitrate containing forages for days to weeks.
Prolonged exposure to excess nitrate coupled with cold stress and inadequate nutrition may lead to the alert downer cow syndrome in pregnant beef cattle: sudden collapse and death can result.
The other major toxin on hay harvested during drought is prussic acid. Drought stressed plants produce cyanogenic glucosides (prussic acid) as they grow. Glucosides are sugar compounds that break down in the rumen, freeing they cyanide from the sugar and forming hydrocyanic acid (HCN) -- which is commonly known as cyanide. The HCN combines with hemoglobin to form cyanoglobin which does not carry oxygen. Livestock poisoned by cyanide have respiratory stress similar to that caused by nitrate poisoning.
A blood test can quickly distinguish between nitrate and prussic acid poisoning. If prussic acid is the the toxic agent, the blood will be cherry red, unlike the chocolate brown color seen in nitrate poisoning.
If you are concerned about unexpected cattle mortalities and suspect toxic hay, there is an inexpensive test that can be used in the laboratory to identify nitrates and prussic acid in hay!