Internal parasites can affect every segment of dairy production when animals have been grazing even for short periods of time. Parasite infections have a costly effect on cattle health and performance including reproductive losses, hampered immune responses, reduced appetite and lower weight gain.

Newcomb harold
Technical Services Specialist / Merck Animal Health

Calves and replacement heifers developed on grass are at the greatest risk from the costly negative effects of parasites. Even low parasite burdens can affect production in the milking herd if cows are grazed at dry-off or during the milking period.

Deworm the pasture

Parasitologists have found that only 1 to 5 percent of worms on your farm are actually in the animal. Most are in the form of eggs and larvae on the pasture. So it is here that producers must think strategically to reduce the parasite population and prevent losses.

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When you consider a strategic deworming program, what you’re really trying to do is reduce the parasite burden on the pasture for as much of the grazing season as possible.


When the pasture is at its best, the environment for parasite transmission is ideal.Therefore, you should attack the worms when the pasture is growing rapidly.

Deworming at this strategic time will have the most impact by decreasing the number of eggs and larvae on the pasture and, as a result, worms in the animal.

Strategic deworming calls for a basic understanding of the life-cycle of parasites that normally infect the animal and an awareness of when they are present in the highest numbers on pasture. Grazing animals are exposed to parasite larva on a daily basis via the grass they consume.

Most parasites have the same basic life-cycle; however, they do not all appear at the same time of year, and weather can play a major role in the number of larva that are able to survive and infect the animal.

Warm wet weather, for example, is an ideal condition for parasites on pasture, while dry, hot weather is not.

These conditions make designing a blanket strategic deworming program to fit every producer impossible. Therefore, dairy producers should consult with their veterinarian or a parasitologist to help design a strategic deworming program for their operation.

Those variables aside, a basic strategic deworming program for the dairy producer may resemble the following:

  • Deworm grazing calves and replacement heifers at turnout, usually about 6 to 8 weeks after grass greens up in the spring.
  • Deworm again at days 28 and 56 following turnout.
  • Deworm animals at turnout on fall or winter grass. If the forage was planted on a prepared seedbed, parasite burdens should be low on the pasture and no further deworming may be needed until the spring.
  • Deworm your animals again in the fall after a killing frost occurs and cooler temperatures prevail.
  • Deworm cows at freshening that were grazed during the dry-off period. Use a dewormer with no milk withdrawal, and always read and follow label directions for all products.

Timing of cattle deworming is important, but not always convenient. If you are gathering dry cows or raising replacement heifers, deworming your animals can be accomplished with a chute-side oral drench treatment. But all too often, the ideal time to deworm these pasture-based groups does not overlap with the best time to gather and handle these animals individually.

Fortunately, with non-handling dewormers, producers can actually treat their animals at the ideal time without having to handle the cattle. Plus, studies show that it can cost up to $5 per head to handle and work your cattle, so there are economic as well as physical advantages to non-handling forms of treatment.

Dewormers are available in non-handling forms which can be fed to cattle in the form of minerals, pellets, range cubes, protein blocks or liquid feed.

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Is your dewormer working?

Timing and method of administration are just part of the deworming equation. What producers really need to know next is if their dewormer is working.

A growing body of research data indicates that avermectins are not as efficacious as they once were. Some studies are indicating that Cooperia is the main parasite left behind after treatment with avermectin compounds, which is indicative of tolerance or resistance.

Cooperia are especially detrimental to animals under 24 months old, so controlling this parasite is important in replacement heifers.

Some research studies are even showing reduced efficacy of the avermectin compounds to Ostertagia (the brown stomach worm) as well.

A useful tool for determining if your dewormer is still effective is the Fecal Egg Count Reduction Test (FECRT).Leading cattle parasitologists from around the United States developed a standardized protocol for the FECRT.

The protocol provides results that can be used to determine the efficacy of a given deworming program.

The FECRT is based on the fact that treatment with a dewormer should kill worms in an animal, eliminating the production and shedding of eggs. You should expect to have at least a 90 percent reduction in the average number of eggs by 14 days post-treatment.

If there’s less than a 90 percent reduction in egg shedding, the producer needs to go back and look at certain variables such as, “was the product used correctly, dosed correctly and applied properly?”

If all of these can be answered “yes,” then there is reason to be concerned about possible resistance problems and the producer should contact his veterinarian or parasitologist for further investigation.

The FECRT protocol is fairly simple for a producer to follow. Basically, the producer takes 20 fresh or observed dropping samples on the day he deworms his cattle. Then he repeats the sampling 14 days later.

The samples should be from animals in the same herd or management group but do not have to be from the same individual animals. They should also be from animals in the same age group.

The producer also needs to request that the lab use a Modified Wisconsin Sugar Float or Double Wisconsin Sugar Float procedure.

A solid foundation

A good parasite control program provides the foundation to nearly all other health and production management practices. That’s why it’s vitally important to make sure that you deworm your animals at the optimum time to have the greatest impact on the parasites in the animal as well as on the pasture.

Then work with your veterinarian or animal-health provider to ensure the product you are using is working by doing a fecal egg count reduction test. PD

Newcomb is a technical services veterinarian for Merck Animal Health. He can be contacted at