Cattle deaths from larkspur poisoning have plagued the industry for well over a century. This year appears to be no different, particularly in British Columbia where there has been a marked increase in larkspur-related death losses. Recently, I was asked by a colleague if a strategic mineral supplementation program would provide any relief from larkspur poisoning. Not only did I not have a good answer for him but I also had to admit that I did not know a great deal about larkspur poisoning. Looking into the issue, I found the topic quite fascinating and thought I would relate some of my findings in this column.
Larkspur is a member of the Delphinium genus with over 60 wild species. Commonly, they are grouped based on height at maturity and preferred growing location into low, tall and plains categories. Tall larkspur species are characterized by their blue/purple flowers with a characteristic spur at the back. Their hollow stems can reach up to six feet in height. Tall larkspur species are generally found at higher elevations. In Canada, according to the 2008 publication Stock-Poisoning Plants of Western Canada, tall larkspur can be found in the western boreal forest, the foothills of the Rocky Mountains and in the interior of British Columbia. Low larkspur species, also with blue flowers, reach a height of one to two feet and can be found in southern Saskatchewan from the Wood Mountains west to the foothills of the Rocky Mountains. Plains larkspur are intermediate in height and are typically found in short grass prairie country.
Both tall and low larkspur species contain a number of alkaloid compounds, the most significant being methyllycaconitine. This toxin can cause muscular paralysis leading to respiratory failure and death. Affected cattle can show a variety of symptoms including muscular weakness, staggering, laboured breathing, bloat and inability to stand. In contrast to cattle, sheep are relatively unaffected by larkspur consumption.
Larkspur toxicity is dependent on a number of factors including plant species, stage of plant maturity, plant part (i.e. stems vs. flowers vs. pods) and environmental conditions. Much of our understanding of the toxicity of tall larkspur can be attributed to the research of Dr. James Pfister and associates at the USDA. Their research shows that toxicity is greatest in immature plants (i.e. prior to flowering) and typically declines with maturity with the exception of the seed pods. In terms of palatability, they report that cattle tend to ignore tall larkspur plants prior to flowering, particularly if there is an abundance of other plant species available for grazing. However, once flowering begins and as the plants mature through pod development, cattle will readily consume the plant leading to a dangerous grazing situation. Toxicity of the plant drops significantly following pod shattering.
As indicated above, larkspur poisoning has plagued the industry in western North America since its earliest beginnings. To date, few reliable solutions have been developed. One strategy, employed with some success is to graze sheep ahead of cattle on infected pastures, as they are relatively immune to the plant. Herbicide use on concentrated areas of larkspur infestation can reduce plant density, however spraying is not always practical or selective. The most consistent protection from larkspur poisoning is deferred grazing. Plant toxicity declines significantly after pod shattering and as a result larkspur-infected pastures can be safely grazed late in the season. However, many consider this a waste of pasture resources.
Scientists from the USDA have looked at the interaction between plant maturity and toxicity and animal grazing preferences and have developed the concept of the “Window of Toxicity” with respect to cattle grazing tall larkspur-infected pastures. According to this concept, despite the fact immature plants are highly toxic, cattle can graze tall larkspur-infested ranges “relatively safely” prior to flowering, as palatability is low, particularly if there is an abundant supply of alternative plants. Similarly, larkspur-infected ranges can be safely grazed after pod shattering as plant toxicity is low. Cattle should not be allowed access to larkspur-infected pastures during flowering or pod development as the combination of plant toxicity and cattle consumption patterns can lead to poisoning. For Canadian producers looking to follow this strategy, a word of caution. Much of the American research was carried out on a tall larkspur species (D. barbeyi) that differs from that typically found in Western Canada (D. glaucum). I have found no evidence suggesting that this same early window of grazing exists in Western Canada.
In contrast to tall larkspur, low larkspur species mature earlier and reach grazing heights early in the spring. As such these immature plants pose a danger to cattle. As with tall larkspur, following pod shattering, toxicity is low.
Before closing, I should come back to the question on mineral supplementation. Reading the popular press you come across producer claims that cattle deficient in salt or specific minerals will selectively graze forbs such as larkspur to overcome the deficiency and that mineral supplementation reduces grazing pressure on the plant. Unfortunately, I have not come across any research that backs up this claim. If pastures are deficient in specific minerals, strategic supplementation is always a good idea; however, relying on a mineral supplement to reduce the risk of poisoning is in my opinion, a gamble.