By Brad Gillmore
Colorado State University Extension
Area Livestock Agent
A few questions have come in this spring to the local Cooperative Extension and Natural Resources Conservation Service offices about poisonous plants, namely locoweed and larkspur.
The effects of poisonous plants on livestock grazing infested ranges can easily go unrecognized and may cost ranchers hundreds of dollars in lost production. This article covers locoweed identification, livestock poisoning symptoms, summary from one Colorado, research project and livestock management.
Locoweed is toxic to cattle, sheep and horses. Locoweed has been identified as the most common and costly cause of livestock poisoning in the western United States. Favorable growing conditions have resulted in a marked increase in the appearance of a particular species of white locoweed, Oxyfropis sericea, in Colorado and other parts of the Rocky Mountain region.
Locoweed is a perennial that grows in clumps, with mature locoweed plants ranging in height from six to 12 inches. Flowers are borne on leafless stalks emerging from the center of the plant. Flowers are pea-like and produce pea-like pods, with kidney-shaped seeds that retain vitality for 40 plus years. Depending on species, flowers are white, shades of pink or shades of purple. Leaves are pinnately compound, covered with fine hairs and emerge near ground level. If you have Internet access, the following addresses are just two of many good sites with pictures that come up when searching for information about locoweed, www.vth.colostate.edu/poisonous_plants/ and www.pprl,usu.edu/locoweedl.htm.
Cattle generally avoid eating locoweed, unless feed is scarce. However, locoweed has a relatively high nutrient value. Once animals develop a taste for the plant, they will graze it preferentially. Cows that selectively graze locoweed will in turn teach their calves to eat it.
Swainsonine is the toxic alkaloid found in locoweed and is responsible for locoweed poisoning in livestock. Swainsonine affects the central nervous system and various organs and tissues, including the liver, kidneys and lymph nodes. In some instances, locoweed can produce symptoms similar to those of bovine virus diarrhea (BVD). Swainsonine readily crosses the placenta and like BVD can cause fetal death, abortion, congenital defects and weak calves. Other signs typical of locoweed toxicity include depression, abnormal behavior, hydrops and poor gaining calves. At high elevations, locoweed can predispose cattle to brisket disease.
A study took place on a 450-head cow-calf ranch, in northern Colorado. In the fall of 1994, approximately 15% of the calves on this ranch were markedly underweight, appeared depressed and behaved abnormally when worked. Weaning weights of these calves were approximately 100 pounds less than those of their herd mates and herd averages from previous years.
Pregnancy rates in two of three separate cowherds had declined from an average of 96% in the fall of 1993, to approximately 80% in 1994. The pregnancy rate for the third cowherd was 94% in 1994. The breeding pasture for the third cowherd was geographically removed from the breeding pastures of the other two herds.
Initially, production declines were attributed to the effects of BVD. Four affected calves were presented to the CSU Diagnostic Laboratory for necropsy. Post-mortem examination showed no evidence of BVD, and no BVD virus was isolated. The calves did have microscopic lesions typical of locoweed poisoning.
After weaning, all calves from this ranch were moved to feedlots and placed on high concentrate rations. The calves affected by locoweed toxicity averaged only a 0.67 pound gain per day during the first two months on feed, compared to over 2.5 pounds per day for their unaffected herd mates. However, after 90 days in the feedlot, the affected calves exhibited compensatory gains that exceeded the anticipated 2.5 pounds per day based on ration formulation. (It is speculated that the compensatory gains observed in the affected calves resulted from the recovery of tissues from the effects of the swainsonine.)
Upon entering the feedlot, all calves were vaccinated with a modified-live IBR-BVD vaccine. The seroconversion response of the affected calves to the vaccine was markedly reduced, compared to that of the unaffected calves. The affected calves were revaccinated 35 days following the initial vaccination and response to the second vaccination produced the desired increase in antibodies.
Affected and unaffected calves were monitored through the first 120 to 150 days in the feedlot. The locoed calves required an additional 64 days on feed to reach market weight, compared to their unaffected herd mates. This extended feeding period cost the producer an additional $105.60 per locoed calf.
Chemical eradication of locoweed from all areas where livestock graze is neither economically nor environmentally feasible.
Therefore, it is necessary to incorporate management practices that will allow livestock to continue to utilize locoweed-infested ranges with minimal economic impact:
--Locoweed is a perennial and is among the first range plants to emerge in the spring. Cattle should not be turned onto spring ranges until desirable forages have made sufficient growth to support grazing.
--Hungry animals should never be turned onto ranges or pastures infested with poisonous plants. Livestock graze less selectively when hungry.
--The effects of the locoweed toxin are cumulative and beyond certain levels damages become irreversible. However, due to the short half-life of swainsonine, locoweed infested pastures can be utilized by rotating between "safe" and "loco" pastures in intervals of about 30 days--depending of course, on the condition of the pastures and the concentration of the locoweed plants. This type of rotation should provide sufficient time for the toxin to clear the animals systems without causing permanent damage.
--Overgrazing pastures will increase the risk of locoweed poisoning. Cattle should be observed regularly and moved off of locoweed-infested rangeland once they start eating locoweed.
--All parts of the locoweed plant are toxic, even when the plant is mature and dried. As a result, poisonings can occur at anytime throughout the year.
--Because the immune systems of locoed calves are compromised, it is recommended that vaccinations be delayed, or that locoed calves be revaccinated at least 30 days following removal from locoweed infested areas. This will help ensure that calves receive adequate protection against diseases, such as IBR and BVD.