Deworming Dairy Replacement Heifers Is A Balancing Act

Even a few parasites in a high-producing dairy cow have the potential to reduce herd performance. Parasite-free animals convert feed more efficiently, have fewer disease problems, enter the milking string sooner, produce more milk and have fewer reproductive difficulties.

Taking a proactive approach by treating for internal parasites stops worms from feeding on your bottom line. But, when done at the wrong time or without proper testing, deworming can lead to resistant parasites.

The more time your herd spends on pasture, the greater the likelihood they are exposed to parasites. Pastured animals are at greater risk of exposure to parasites than animals on dry lots or in total confinement.

Deworming replacement heifers to prevent parasite infestations provides dairy producers with a tool to boost herd health and ensure they don’t suffer from the production-limiting impacts of parasites when they join the milking string.

Testing is a crucial step

Fecal egg counts (FEC) can help determine if anthelmintic (dewormer) treatment is warranted (20 fecal samples from the same cohort group are needed to get a statistical look at the level of parasitism within that group). For more than a decade, our company has maintained the world’s largest fecal egg count test (FECRT) database. The purpose is to monitor field use efficacy of anthelmintics approved for use in U.S. cattle. FECRT are needed to ensure the treatment was effective.

The FECRT protocol consists of taking 20 samples the day of treatment and another 20 samples 14 days after treatment. Pre-treatment and post-treatment samples are sent to the same lab and tested to determine fecal worm egg counts. Results are entered into the national database and sent to the participating clinic.

An effective deworming program results in a 90% or greater reduction in fecal egg counts. The FECRT database results document that certain classes of dewormers are not as effective as they have been in the past. Therefore, it’s critical that animals are tested before and after they are treated and that the correct dose of dewormer is used and given or applied correctly.

Results from the database revealed that the most effective strategy to control internal parasites, as well as to ensure anthelmintic classes remain effective, is to use two or more classes of dewormers at the same time to treat animals.

When to treat

A fecal egg count (FEC) can be done prior to treatment to assess if animals need to be treated. And, if animals are treated, a FECRT should be done when animals are dewormed.

Depending on the location of your dairy, pastured cattle should be parasite-free during the winter months and treated in the spring once grass greens or turnout occurs. Deworming in the spring prevents parasite populations from rising during summer.

Replacement heifers should receive two successive treatments 30 days apart to ensure they are free from shedding parasite eggs back on the pasture for the first 90 days of the grazing period. This helps to reduce parasite contamination for the entire grazing season.

The economic benefits associated with deworming improve each year as parasite contamination is reduced. Several studies have been conducted to examine the effects of treating replacement dairy heifers strategically with fenbendazole. The results reveal the treated heifers gained more weight and reached puberty earlier than the control group.

Protecting product efficacy

In addition to testing and strategically treating animals, another measure to protect product efficacy is to leave some animals untreated. The practice, which is known as refugia, provides some parasites refuge from the anthelmintic or dewormer. The untreated parasites are a continual source of susceptible genes for the parasite population on that farm. When these parasites mate with resistant ones, they dilute the resistant gene pool in the parasite population, keeping the population susceptible to the anthelmintic longer.

After an animal is treated with a dewormer, the susceptible parasites die, and the resistant parasites survive and pass on resistance genes to their offspring. If there aren’t enough susceptible parasites left in the environment and in the animal, they cannot “thin out” the increase in resistant parasites that occurs after treatment.

Refugia is the share of the total parasite population not treated with a dewormer. By giving these worms refuge from the drug, there is no selection pressure to develop resistance, which keeps dewormers working effectively longer on the dairy.

Work with your veterinarian

Strategic deworming at timely intervals can help rid dairy animals of parasites and keep milk flowing on your farm. Now is a good time to start a conversation with your veterinarian to discuss diagnostic testing and to design a control program that involves treatment and prevention of internal parasites.  

Article First Published on 10 June 2020 by Progressive Dairy

Written by Harold Newcomb, Technical Services Veterinarian, Merck Animal Health