Using routine anthelmintics in beef cattle has long been a recommended and accepted management practice to improve animal performance and maintain animal health. For many beef producers anthelmintic drugs — dewormers — are used on a regular basis when cattle are vaccinated in the cow herd at spring and fall processing. Dewormers are used to control parasitic nematodes that attach to the gastrointestinal tract of cattle to feast on blood and its constituents. New findings indicate that drug resistance is accelerating in cattle parasites, which challenges the long-term practice of routine drug treatment.
Internal-parasite organisms have been shown to develop resistance to deworming drugs following repeated use of the same product throughout time. Frequently exposing the entire parasite population on a farm to the same drug will select for drug-resistant parasites. Throughout time those resistant organisms will dominant the parasite population, resulting in a population that is resistant to that drug. Once the majority of a parasite population becomes drug-resistant, the drug has little efficacy and may be no better than treating animals with water.
New parasite-control methods use selective and targeted use of drugs — not serial deworming of all animals at routine time points. That requires monitoring of parasite infection. An effective way of doing that is by performing fecal egg counts on a sample of cattle. Producers should collect fecal samples from the cattle, and then utilize labs or veterinarians to perform quantitative fecal egg counts. The results will be the number of fecal eggs per a 3-gram manure sample. Utilizing fecal egg counts to determine treatment thresholds will reduce the frequency of drug use. Once fecal egg counts have been determined producers can decide if treatment is required. It’s been determined that not all cattle need to be treated. Proper grazing management along with some cattle having developed immunity to internal parasites produces lesser numbers of internal parasites.
Many cow-calf producers process cattle in the spring and fall, with part of that processing to include routine deworming. Now producers are being encouraged to monitor herd infection using fecal egg count. Depressed egg counts indicate cows don’t require treatment, which allows producers to save dollars that would have been spent on dewormers. Reducing the use of dewormers not only saves money but also reduces the amount of chemical product used in beef production. It reduces the chances of parasites developing resistance. Protocols have been developed to collect fecal samples and compare egg counts to determine need for treatment. In instances calling for treatment, fecal samples can be collected two weeks post-treatment to determine product effectiveness. In cases that a given product is determined to be ineffective, switching product class usually is effective against internal parasites in beef cattle.
Mature cows are more likely to have developed immunity to internal parasites than younger animals. Each class of cattle should be sampled on a farm. Many farms have shown no need for treatment in mature cows. But calves and yearlings may have elevated fecal egg counts and require treatment. By treating just the susceptible calves infection can be controlled. The population not exposed to drugs is called the refugia population. It helps to dilute the resistance populations that inevitably grow on a farm with frequent drug use.
Adult nematodes living in the intestinal tract of cattle lay eggs that are excreted in the manure. First-stage larva hatch from those eggs, and then molt twice to become third-stage larva. The third-stage larva then migrate from manure onto forage. Cattle are infected with internal parasites by consuming those larvae that are found in the film of dew or moisture at inflated concentrations low in the pasture sward.
Grazing management is an important factor that influences internal-parasite infestation. Some have wrongly identified rotational grazing as a practice that can reduce infestation. Rotational grazing alone actually enhances the opportunity for cattle to become infected. Eggs released in the manure only require a few days to hatch and molt into larva that can infect the animal. Once third-stage larva are present in pasture they can survive many months waiting to be consumed by cattle. Rotating back into paddocks one or even two months later is ideal. A critical point is forage height. Larva attach to forage at about 4 inches of height. Grazing above that level may help reduce infection and provide better nutrition to maintain strong immunity against infection.
The cattle at the Michigan State University’s Upper Peninsula Research and Extension Center in Chatham, Michigan, are part of the Red Angus herd on a grass-finishing research program. Those cattle are intensively managed in a rotational system. The key factor is that they are nearly always consuming forages greater than 4 inches. Those cattle aren’t dewormed and have shown fecal egg counts that indicate treatment isn’t required. It’s not the rotational system that’s responsible. Rather it’s that the cattle seldom overgraze forages to the length that allow cattle to become infected by consuming larva. Another critical factor is to keep fecal egg counts at lesser numbers because cattle with lesser fecal egg counts aren’t contaminating pastures at nearly the same rate as cattle with greater numbers.
New recommendations continue to come forward as more information is obtained. Producers are now encouraged to collect manure samples to determine fecal egg count to determine if cattle should be dewormed — and to evaluate the effectiveness of deworming products. Visit www.canr.msu.edu for more information.
Frank Wardynski is a ruminant educator for the Michigan State University-Extension. He also specializes in business management, and food and animal systems.