As instances of internal parasite resistance to deworming treatments are becoming more numerous among the country's cattle herds, it becomes apparent that herd managers must pay closer attention to the use of these valuable herd-health tools. Controlling parasites is a proven practice to improve performance in all stages of cattle production; however, both overuse and underuse of anthelmintics can lead to a resistance problem.

Overuse is the result of producers deworming too often, and underuse occurs when too little product is administered, according to Mac Devin, DVM, Boehringer Ingelheim Vetmedica, Inc. "In a closed population, if you treat too frequently, the remaining worms are the only ones left to breed," he says. "Resistance increases with each generation. And if less than the labeled dose is given, again, this leaves too many survivors to pass on their resistant genes to the next generation."

To establish a resistance management program in your herd, Devin recommends, with the help of your herd veterinarian, first conducting a fecal egg count reduction test to detect if you have parasite resistance in your herd. Two tests are taken; after the first test, treat the entire herd, then come back in 14 days to take another count and then calculate the loss.

"Do this test on 20% of your herd, or 20 animals, whichever is less," he says. "We want to see a 90% fecal egg production reduction." Less than that would suggest a resistant population is present.

Secondly, herd managers should do a coproculture. Samples are incubated and then evaluated by a parasitologist to determine the composition of the nematode population.

With this information, cattle producers can determine the most effective program that will minimize the risk of resistance. To accomplish this, Devin recommends these steps:

  • Keep your veterinarian involved in your parasite management program.
  • Choose a dewormer with the greatest efficacy. Look at the active ingredient in the parasite control product because there are differences in persistence and efficacy. For example, CYDECTIN® is the only product in the moxidectin family, and it has label claims for longer persistence than some other endectocide anthelmintics for some nematode species.
  • Never under-dose—follow label directions.
  • Quarantine, test and deworm new animals before turning them out with the rest of the herd.
  • Periodically rotate product chemistry according to your veterinarian's advice.
  • Use strategic management practices such as good hygiene and rotation of pastures and/or animal species to reduce pasture contamination.

Of these steps, administering the full label dose to each individual animal is the most important, Devin says. "If you underdose, it allows more resistant worms to survive, and each generation gets more and more resistant."