Sign in
Learn / Care /

Cattle Deworming and Parasite Control: A Practical Schedule

Author: Elliott Garber, DVM

Parasites Cost More Than You Think

Internal parasites cost the U.S. cattle industry an estimated $2 billion annually in lost production, treatment costs, and death loss. The economic impact on individual animals ranges from subclinical production losses (reduced weight gain, lower milk production, poor feed conversion) to clinical disease and death in heavily parasitized young stock.

For decades, the standard approach was simple: deworm everything, twice a year, with whatever product was on the shelf. That approach is now creating a crisis. Anthelmintic resistance (parasites that survive deworming treatments) has been documented on over 90 percent of cattle operations tested in some regions. With only three drug classes available for cattle and no new classes in the development pipeline, the parasites are winning the arms race.

This guide covers the parasites that matter in cattle, the dewormers available and how they work, resistance management strategies based on current veterinary recommendations, and a practical deworming schedule that balances parasite control with long-term drug efficacy. If your current parasite program consists of blanket-treating the entire herd on the same dates every year, this article will challenge that approach with the science behind modern parasite management.

The Parasites That Matter

Major Internal Parasites

Cattle host dozens of internal parasite species, but a handful cause the majority of economic damage:

Who Is Most Vulnerable

Young cattle (under 2 years) are the most susceptible to parasitic disease because they have not yet developed immunity. Calves in their first and second grazing seasons carry the heaviest parasite burdens and suffer the greatest production losses.

Mature cows develop partial immunity through repeated exposure, which keeps parasite burdens low but does not eliminate them. However, immunity is suppressed during periods of stress: calving, peak lactation, and nutritional deficiency. This is why periparturient cows (around calving) often show elevated parasite egg counts and can contaminate pastures that calves will graze.

Available Dewormers

The Three Drug Classes

Only three classes of anthelmintic drugs are approved for use in cattle. Understanding which class a product belongs to is essential for resistance management, because resistance develops to the entire class, not just one brand.

1. Benzimidazoles (white dewormers):

2. Macrocyclic lactones (MLs, avermectins/milbemycins):

3. Imidazothiazoles/tetrahydropyrimidines (levamisole/morantel):

No New Classes on the Horizon

Unlike antibiotics, where new drugs are occasionally developed, the anthelmintic pipeline for cattle is essentially empty. The last new class (macrocyclic lactones) was introduced in the 1980s. There are no new drug classes in late-stage development for cattle. This means the three classes we have now are the three classes we will have for the foreseeable future. Preserving their efficacy through responsible use is not optional; it is the only path to long-term parasite control.

The Resistance Crisis

How Bad Is It?

Research from the University of Georgia found resistance to at least one drug class on 92 percent of cattle operations tested. Cooperia resistance to ivermectin-class drugs is nearly universal in the southeastern United States. Ostertagia resistance, once rare, is now documented in multiple states.

Resistance develops when susceptible parasites are killed by treatment while resistant parasites survive and reproduce. Over time, the resistant population becomes dominant. The more frequently a drug is used, and the more completely it eliminates susceptible parasites, the faster resistance develops.

What Drove the Problem

Several common practices accelerated resistance development:

Modern Parasite Management

The Refugia Concept

The single most important concept in modern parasite management is refugia: the population of parasites that are not exposed to a deworming treatment and therefore not selected for resistance. Refugia includes parasites on pasture (as eggs and larvae), parasites in untreated animals, and parasites in treated animals that escape drug exposure.

Maintaining refugia dilutes resistant genes in the parasite population. If you treat 100 percent of your herd and kill 95 percent of susceptible worms, the 5 percent that survive are disproportionately resistant. But if you leave 10 to 15 percent of the herd untreated, the susceptible parasites in and from those untreated animals interbreed with the resistant survivors, slowing the spread of resistance genes.

This is counterintuitive. The best way to preserve dewormer efficacy long-term is to intentionally leave some animals untreated. The animals you leave untreated should be those with the lowest parasite burdens (typically mature cows with strong immunity), not young stock or animals showing clinical signs.

Targeted Selective Treatment

Targeted selective treatment (TST) treats only the animals that need it rather than the entire herd. This maximizes refugia while still protecting the animals most vulnerable to parasitic disease.

Criteria for identifying animals that need treatment:

Combination Treatments

Current veterinary parasitology research supports using combination treatments (two or more drug classes simultaneously) rather than rotating between classes. The rationale: if a parasite is resistant to Drug A but susceptible to Drug B, a combination of A and B kills it. Single-drug rotation allows parasites resistant to one class to survive during that class’s rotation cycle.

A practical combination protocol: administer a benzimidazole (oral fenbendazole) and a macrocyclic lactone (injectable ivermectin or moxidectin) simultaneously. This is legal and supported by veterinary parasitologists, though the label for each product only references single-drug use. Work with your veterinarian to develop a combination protocol appropriate for your operation.

Fecal Egg Count Reduction Test (FECRT)

The FECRT is the standard tool for detecting resistance on your operation. The process:

  1. Collect fecal samples from 15 to 20 animals before treatment
  2. Treat those animals with the dewormer you want to test
  3. Collect fecal samples from the same animals 14 days after treatment
  4. Compare pre-treatment and post-treatment egg counts

A reduction of 95 percent or greater indicates the drug is fully effective. A reduction of 90 to 95 percent suggests developing resistance. Below 90 percent indicates established resistance, and the drug should no longer be used as a standalone treatment on your operation.

Merck Animal Health provides free FECRT kits through its Fecal Solutions program. Contact your veterinarian or Merck representative for details.

A Practical Deworming Schedule

Spring (Turnout to Pasture)

For cow-calf operations with spring calving, the strategic deworming window is at or shortly after turnout to spring pasture. Periparturient cows have suppressed immunity and elevated egg output, contaminating pastures that calves will graze. Treating cows within 4 to 6 weeks of calving reduces pasture contamination during the critical period when calves begin grazing.

Use a combination treatment (benzimidazole plus macrocyclic lactone) for cows identified as needing treatment based on body condition or fecal egg counts. Leave 10 to 15 percent of the healthiest, best-conditioned cows untreated to maintain refugia.

Mid-Season (Calves at 3 to 4 Months of Age)

Calves begin ingesting significant quantities of forage around 3 to 4 months of age, coinciding with their first significant parasite exposure. A mid-season treatment for calves at this age targets the early parasite burden before it affects growth and performance.

Fall (Weaning)

Weaning is a common deworming point because calves are already being gathered and processed. Treat weaned calves that will remain on the operation. For calves being sold, treatment adds value for the buyer (preconditioned calves with documented deworming sell at a premium, as discussed in our auction buying guide).

For mature cows, fall treatment is not always necessary if spring treatment was effective and pasture management has been adequate. Base the decision on fecal egg counts rather than the calendar.

Winter

In northern climates, parasite transmission on pasture effectively stops during winter. Treating cattle in mid-winter provides little benefit for current infection control but can target inhibited Ostertagia larvae that are dormant in the stomach wall and will emerge in spring. If inhibited Ostertagia is a concern on your operation (history of Type II ostertagiasis in late winter), a strategic winter treatment with fenbendazole (which is effective against inhibited larvae) may be warranted.

Pasture Management for Parasite Control

Non-Chemical Tools

Parasite management does not begin and end with dewormers. Pasture management practices reduce parasite exposure and complement chemical control:

Frequently Asked Questions

How often should I deworm my cattle?

There is no single answer. The old recommendation of “twice a year” is outdated and contributes to resistance. The modern approach is to deworm based on need, using fecal egg counts to identify animals with significant burdens, and to leave some animals untreated to maintain refugia. For most cow-calf operations, one to two strategic treatments per year, timed to periods of highest risk (spring turnout and weaning), provides adequate control without accelerating resistance.

Which dewormer should I use?

Start by determining which drug classes are still effective on your operation using a fecal egg count reduction test. Use combination treatments (two drug classes simultaneously) rather than single-drug rotation. Work with your veterinarian to develop a protocol specific to your region, your parasite species, and your resistance status.

Is pour-on ivermectin still effective?

Pour-on ivermectin has the highest rates of treatment failure among all dewormer formulations, for two reasons: Cooperia resistance to ivermectin is widespread, and pour-on products are frequently underdosed because producers estimate rather than weigh cattle. If you are using pour-on ivermectin as your sole deworming strategy, conduct a FECRT to verify it is still working. Many operations find that switching to injectable formulations (which achieve more consistent blood levels) or combination treatments with a benzimidazole restores efficacy.

Can I deworm organic cattle?

USDA organic standards allow the use of approved dewormers but with restrictions. Ivermectin is prohibited in organic production. Fenbendazole is allowed but may have extended withdrawal periods under organic certification rules. Organic cattle operations rely more heavily on pasture management, nutrition optimization, and genetic selection for parasite resistance. Consult your organic certifier for current approved practices.

How do I know if my cattle have parasites?

All grazing cattle harbor some internal parasites. The question is whether the parasite burden is causing economic losses. Signs of significant parasitic infection include poor weight gain despite adequate nutrition, rough hair coat, diarrhea, bottle jaw (swelling under the jaw from Haemonchus), and anemia (pale mucous membranes). Subclinical infections cause production losses without obvious signs, which is why fecal egg counts are valuable for identifying animals with high burdens before clinical signs appear.

Next Steps

  1. Log deworming treatments in your animals’ health records on Creatures, including the product used, dose, date, and any fecal egg count results, to maintain a complete parasite management history.
  2. Review your vaccination program alongside your parasite management plan to ensure both components of your herd health program are current and coordinated.
  3. Connect with producers in your area through the Creatures directory to learn what parasite challenges and resistance patterns are common in your region.

Read More

Winter Cattle Care: Feeding, Shelter, and Cold Weather Management

Winter Changes Everything Cold weather transforms cattle management. Nutrient requirements increase, water sources freeze, body condition becomes harder to assess under thick winter coats, and the decisions you make between November and March determine whether your herd emerges in the spring ready to breed back on schedule or thin, weak, and behind. The core challenge […]

Cattle Mineral and Nutrition Supplements: What Your Herd Actually Needs

The Mineral Program Most Herds Are Missing Mineral supplementation is one of the most impactful and least understood management practices in the beef cattle business. Evaluations of thousands of liver biopsy samples show that over 60 percent of beef cattle are deficient in copper alone. Zinc, selenium, and manganese deficiencies are similarly widespread, depending on […]

Beginner’s Guide to Raising Livestock on a Small Farm

Before You Buy a Single Animal The dream of raising livestock on a small farm is compelling: green pastures, healthy animals, and the satisfaction of producing something real with your hands. The reality is also compelling, but it comes with a learning curve that catches many beginners off guard. The most successful small-scale livestock owners […]

Cattle Vaccines: What Every Owner Needs to Know

Why Vaccination Matters for Every Cattle Owner Vaccination is one of the most cost-effective management practices in cattle production. A well-designed vaccination program prevents diseases that can cause death, reduce weight gain, impair fertility, and devastate your herd’s productivity. For a few dollars per head, you protect animals worth thousands. Whether you run a commercial […]

Ethical Breeding: Principles Every Breeder Should Follow

Why Ethical Breeding Matters Breeding animals carries real responsibility. Every breeding decision affects the health, temperament, and longevity of the offspring, the welfare of the parents, and the long-term viability of the breed itself. Ethical breeders understand this weight and make decisions accordingly. The difference between ethical breeding and irresponsible breeding isn’t subtle. It shows […]