Cattle Deaths While Grazing Canola (Nutrition Update, Volume 9)

Last fall, cattle deaths while grazing canola stubble in Saskatchewan were initially attributed to prussic acid poisoning. Subsequent investigations showed that the deaths were due to consumption of treated canola seed which had been dumped in a nearby slough. A similar incidence in Manitoba also caused numerous cattle deaths at one farm when Counter treated canola seed was consumed by cattle on pasture.

Prior to the final diagnosis in Saskatchewan, the concern regarding prussic acid poisoning was raised. A passing reference in a popular nutrition text "Feeds and Feeding" by Ensminger indicated that crops belonging to the Brassica genus were susceptible to prussic acid accumulation. Although the cause of the deaths was eventually resolved the question remained - Does canola stubble represent a health risk to grazing cattle?.

In the aftermath of the Saskatchewan experience, a meeting was held with University of Saskatchewan plant and animal researchers, Western College of Veterinary Medicine and Saskatchewan Agriculture and Food representatives. The consensus of this group was that while the cyanogenic glycoside and the biochemical mechanism necessary for the release of prussic acid are present, the risk of cattle poisoning due to prussic acid is low. The scientists concluded that the combination of low levels of glycoside in canola forage and an inherently inefficient biochemical pathway, make it unlikely that high enough concentrations of prussic acid woud be produced to cause a health risk. While the risk is not zero, it should be considered very low.

Counter has a high acute mammalian toxicity and is highly toxic to birds, fish and other wildlife. A cattle producer in the Interlake recently lost over 100 feeder calves when a small amount of treated canola seed was inadvertently mixed into the feed. Excess treated canola seed should be used up by double planting - not by dumping it in the field or by storing for future use. Producers wanting to graze canola stubble this year are advised to check the pasture for spills of treated seed prior to releasing cattle.

Nutrition Update
Volume 9 No.2, August 1998