Parasite Control for Cow Calf Operations


Source: South Dakota State University

Written collaboratively by Written collaboratively by Joe Darrington, former SDSU Extension Livestock Environment Associate and Taylor Grussing, former SDSU Extension Cow/Calf Field Specialist.


Use of dewormer compounds can significantly improve the average level of production; however, care must be taken to avoid a buildup of resistant populations. A short list of deworming drugs can be seen in Table 1. Important differentiations between drugs include the overall effect on the larval state of internal parasites, the effect on external parasites, and the route of administration. General recommendations regarding the use of these drugs should be discussed with your herd veterinarian and in compliance with the label claims of your product.


Drug Class Active Drug Effective Against Adults Effective Against Larvae Common Routes of Administration
Benzimidazole Albendazole Yes Yes Oral Drench
Benzimidazole Fenbendazole Yes Moderate Oral Drench
Benzimidazole Oxfendazole Yes Yes Oral Drench
Imidathiazole Levamisole Yes Moderate Pour-on, Oral Drench, Injectable
Imidathiazole Morantel Yes No Bolus, Crumbles
Avermectin Ivermectin Yes Yes Pour-on, Oral Drench, Injectable
Avermectin Doramectin Yes Yes Pour-on, Injectable
Avermectin Moxidectin Yes Yes Pour-on, Injectable


Internal & External Parasites


Brown Stomach Worm (Ostertagia ostertagi)

  • Symptoms: Diarrhea, anemia, slow growth.
  • Treatment: Any of the above anti-parasitics.*


  • Symptoms: Diarrhea, in-appetent, slow growth.
  • Treatment: Any of the above anti-parasitics.*


  • Symptoms: Anemia, slow growth.
  • Treatment: Any of the above anti-parasitics.*


  • Symptoms: Diarrhea, slow growth.
  • Treatment: Any of the above anti-parasitics.*


  • Symptoms: Diarrhea, slow growth.
  • Treatment: Any of the above anti-parasitics.*

*Cattle develop resistance to worm infestations slowly. Cow’s older than 3-4 years should not require maintenance dosing. Weaned calves and heifers are the most affected. Watch for resistance.



  • Symptoms: Hair loss, scrapes, discomfort.
  • Treatment: 2 to 3 topical treatments separated by 2-3 weeks with ivermectin (be careful if your herd has a history of grubs) or topical insecticide.


  • Symptoms: Hair loss, ulcers, discomfort, secondary infections.
  • Treatment: Topical insecticides. Work with your veterinarian as mange organisms are reportable infestations in the US.


  • Symptoms: Warbles (larval heel flies) appear in the spring along the backs of animals. Painful.
  • Treatment: In the Fall, systemic ivermectin or an organophosphate to kill migrating larva. Do not apply systemic insecticides in the winter or early spring, can cause paralysis and/or bloat.

Treatment Considerations


There are many ways that products can be administered to cattle with the most common methods including: topical pour-on (with or without systemic absorption), injectable, and oral drenches. For treating internal parasites, injectable products and oral drenches ensure delivery of the desired dose of drug. Pour-on products can be effectively absorbed systemically and provide a good dose; however, there is more variability, especially if the weather does not cooperate. The most important aspect to ensure is that each animal receives an adequate dose based on its body weight, regardless of the route of administration. Basing the dosage off of body weight helps attain the best efficacy, as under dosing will not eliminate all the parasites while promoting resistance, and overdosing can be harmful to the animals and 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 will develop. Monitoring effectiveness of treatment can help you determine if and when switching products is necessary.


If you are concerned about how well your current deworming protocol works, you can work with your veterinarian to perform a fecal egg count reduction test (FECRT). This test consists of evaluating the baseline level of parasite egg shedding in your herd, applying your standard deworming strategy, and then rechecking the level of egg shedding in your herd. If a deworming compound reduces the fecal egg count by more than 95% the dewormer works well. If the reduction is less than 95% there is resistance in your herd. As a reminder to veterinarians, best practices indicate conducting the FECRT on paired samples from individuals in the herd.


Cattle build up resistance to parasite infestations slowly and younger animals are more at risk of clinical disease. The most important populations to manage for parasites are weaned calves, heifers, and 2nd calf cows. Older cows have had the opportunity to develop resistance and should not require annual or semiannual treatment in the absence of clinical signs, work with your herd veterinarian to figure out a parasite control strategy that fits your situation.

Other Management Tips

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 very effective in promoting overall herd performance, reducing the development of resistant parasites, all while reducing the level of contamination of pasture. 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. This population of susceptible parasites in the animals and environment are called refugia, and can lengthen the time that anthelmintic drugs will be effective. However, if mass treating in the spring, the use of a persistent product (check labels for duration of residual effect) for parasite control should be used to decrease new fecal shedding and pasture contamination. 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. As a last thought, maintaining appropriate stocking densities to prevent overgrazing can also help limit parasite exposure.

The best way to manage parasites in your herd will be unique to your situation. Hopefully this article has given you a better understanding of current issues and your management options.


  • Smith, B.P., 2009, Large Animal Internal Medicine 4th Edition, Mosby Elsevier Publishing
  • FDA CVM, 2015, List of Approved Animal Drugs.


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