Deworming decisions are farm-specific and depend on the age of the animal, how much exposure they had to infective larvae while grazing, when they were last dewormed, and what products were used.

1. Who should be dewormed? We do not deworm every animal in the herd. If you want t get the best return on deworming investment, then do so to only cattle under 4 years of age. Natural tolerance to internal parasites develops as the animal matures; the worm’s presence is less likely to harm an older animal’s production. However, bulls, regardless of age, are more susceptible to parasitism.

Along with decreasing cost, not exposing older female cattle to de-wormers provides an anthelmintic-susceptible population of internal parasites known as refugia. Refugia pass along their susceptible genetics when mating with anthelmintic-resistant worms. Maintaining refugia is the only way to slow the progression of anthelmintic resistance. The heaviest 2- and 3-year-old replacement females may also be used as a source of refugia by not deworming them. Performing fecal egg counts may help determine if deworming is warranted.

2. When do we deworm the calves? This depends on their previous larval exposure from grazing and when the last de-wormer was administered. Larvae can survive for months in pasture topsoil (up to 18 months for some species); therefore, from year to year, each farm needs to map its pastures and paddocks and potential worm populations. The metabolism of internal parasites slows as the temperature drops, during a process known as inhibition.

3. Where should youngstock be dewormed? We place the dewormed youngstock onto a dry lot or a pasture that has refugia present. As the dewormed youngstock graze, they will ingest refugia to mate with any internal worms that survived treatment, including inhibited or sexually active worms that are likely anthelmintic-resistant.

4. What deworming product do we use? On one day, administer a macrocyclic-lactone (i.e., ivermectin, epinomectin, doramectin) and a benzimidazole (albendazole, fenbendazole) product. The Veterinary Doctor advised that the combination treatment is recommended because using different modes of action simultaneously increases the killing success over using one product alone or switching products between treatments. Use each one as the label directs: at the correct dose for the correct weight of the animal. Use a fecal egg count reduction test to determine the efficacy of the dewormers used.

It is important to know that deworming decisions in one season impact future deworming decisions. Maintaining refugia, using combination therapy, and monitoring fecal egg counts and fecal egg count reduction test results will help preserve future anthelmintic effectiveness on your farm. Gone are the days when you could simply administer one product without thinking about the product’s effectiveness and its contribution to increasing worm resistance.