Cattle graze on a well-managed pasture, which can help reduce the parasite burden. (Photo by Lynn Betts, USDA Natural Resources Conservation Service)

The need to control internal parasites will exist as long as cattle are grazing pastures; however, parasite levels are not the same on all pastures or in all cattle.

Pastures that are heavily stocked generally have a higher parasite burden than lightly stocked ones. Meanwhile, cattle in a drylot are less likely to have heavy worm infections than those on pastures.

Young cattle will typically have more internal parasites than older cattle; therefore, the methods of controlling internal parasites should be developed to fit individual production situations.

Strategic deworming starts with understanding the life cycle of problem parasites, identifying seasonal changes in parasite burdens and implementing cost-effective control.

A successful deworming program, along with good overall herd management, will increase milk production in cows and thereby increase weaning weights of calves.

Anthelmintics provide an excellent tool for controlling parasites. Application of dewormers should not be aimed at treating infected cattle showing signs of parasitism. Instead, deworm cattle in a timely manner to reduce infection before symptoms of disease occur.

Treatment should also be aimed at interrupting the life cycle of the parasite in an effort to minimize pasture contamination.

Many anthelmintic products are on the market. Most are either avermectins (ivermectin, dormectin and eprinomectin), milbemycins (moxidectin) or benzimidazoles (oxfendazole, albendazole and fenbendazole).

Along with internal parasite control, avermectins and milbemycins provide the additional benefit of external parasite control as well as persistent protection for days to weeks after treatment.

Since cows, bulls and young stock are affected differently by internal parasites, corresponding treatment programs should also differ. Mature cows should be treated at least one time per year. The best time to treat the mature cow is near freshening. The mature cow’s susceptibility to parasite detriment increases during this time due to stress of production and a suppressed immune system.

In situations where parasite levels may be high, such as overstocked pastures, treating twice a year may be necessary.

In other situations, mature cows and bulls develop some degree of immunity to internal parasites, so levels may be low enough that they do not need any treatment. These conditions can only be determined by treatment followed by critical observation and the use of fecal egg counts.

Bulls, unlike cows, tend to be more susceptible to parasites and should be treated twice a year—in spring and fall.

Treatment of calves should begin when they reach 3 to 4 months of age and be repeated every three to four months until they become yearlings to optimize parasite control. Animals purchased as replacements or stockers should follow a similar program. The avermectin- or milbemycin-type products will provide the best means of controlling parasites in these animals.

Yearlings can be treated on a seasonal basis, spring and fall, until they are mature cows (a mature cow is generally recognized as an animal pregnant with her second calf).

If calves are backgrounded in a drylot, one initial treatment should be sufficient.

Visit with your veterinarian about setting up a herd health program that includes a strategic deworming program.

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Photo by Lynn Betts, USDA Natural Resources Conservation Service