Pasture management, including the incorporation of a sound rotational grazing system, can significantly reduce the number of infective parasite larvae in pastures.  (Photo courtesy USDA Natural Resources Conservation Service)

Use of dewormers in a cattle herd can significantly improve the average level of production; however, care must be taken to avoid a buildup of resistant parasite populations.

Important differences between drugs include the overall effect on the larval stages of internal parasites, the effect on external parasites and the route of administration. General recommendations regarding these drugs should be discussed with your herd veterinarian. Always use dewormers in compliance with the label claims of your product.

The most important factor in using these products is to ensure that each animal receives the correct dose based on its body weight, regardless of the route of administration. Applying the correct dosage based on body weight will give the best efficacy. Under-dosing will not eliminate all the parasites and promotes resistance, while overdosing may be harmful to the animals and is an unnecessary expense.

Resistant parasite populations develop over time from repeated use of the same deworming products. The more frequently a dewormer is used, the quicker that resistance may develop. Monitoring effectiveness of treatment can help you determine if and when switching products is necessary.

Cattle build up resistance to parasite infestations slowly, and younger animals are more at risk of clinical disease.

Weaned calves, heifers and second-calf cows are the most important populations to manage for parasites. Older cows have had the opportunity to develop resistance and should not require annual or semiannual treatment in the absence of clinical signs.

Pasture management through rotation, alternate species grazing, haying and rotational tillage can significantly reduce the number of infective larvae on a pasture.

Focused deworming of individuals showing clinical signs of parasitism (diarrhea, anorexia, high fecal egg counts, etc.) rather than mass treatment of groups can be effective in promoting overall herd performance and reducing the development of resistant parasites—all while reducing the level of contamination in pastures.

By only treating animals that are most affected we leave a population of parasites in the less affected animals that have not been exposed to the anti-parasitic drug.

If mass treating next spring, the use of a persistent product for parasite control should be utilized to decrease new fecal shedding and pasture contamination (check labels for duration of residual effect).

This treatment should be done as late as possible in the spring and up to 4–6 weeks after grass turnout to help limit infestation rates in calves. Maintaining appropriate stocking densities to prevent overgrazing can also help limit parasite exposure.

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