Spring turnout is a good time to think about parasite control. This time of year, both internal and external parasites are important as flies are getting well established and intestinal worms are blooming on new spring grass. Parasites, particularly internal parasites like worms and coccidia, impact young animals the most because of their naïve immune system. Based on their exquisite sensitivity to parasites and the economic impact of parasites on growth, it is important to deworm calves appropriately.

There are three areas of calf deworming worth considering:

Age at Deworming

Baby calves nursing their mothers have little exposure to parasites. As those calves begin to develop their mature gastrointestinal tract, they begin to actively graze and start to pick up parasites. For this reason, there is no need to deworm baby calves. Wait until at or near weaning to deworm them. At that time, they will have a population of worms in them that will need to be treated, so we are targeting the use of our available drugs to a time when they really need it. Using dewormers is similar to using antibiotics – we want to use them when we need them and not use them when they are not really necessary. Using dewormers when they are not necessary helps select for resistance and results in a loss in the drug’s effectiveness over time.

Selection of Drug

Calves are impacted by different worm species than their dams. Therefore, as everyone goes through the cattle chute, we should deworm calves differently than their moms. One of the, if not the biggest, threats to growing calves is a worm called Cooperia. Cooperia has been widely and repeatedly shown to be resistant to the avermectin dewormers, the class that includes ivermectin. For this reason, ivermectin is not a great choice for deworming calves, especially if it’s the only dewormer used. Deworming calves with ivermectin or its cousins, eprinomectin, doramectin, and moxidectin, leaves a major parasite untouched, which will drag on the calf’s growth and productivity. Cooperia does generally respond to what we call the “white wormers.” These are the chalky white suspensions available, which include fenbendazole, albendazole, and oxfendazole. For this reason, I recommend that calves at or near weaning receive a “white wormer,” which is given orally with a mouth hook. It is less convenient, but it is targeted and effective for what we know is the most important parasite in the calf.

Use of Pour-Ons

The pour-on dewormers belong to the avermectin class. Based on what I said above regarding Cooperia, it should be clear that pour-on dewormers in calves just won’t get the job done. Because of the quality of the carrier, there is an additional concern of unpredictable absorption of some brands. A drug carrier is a compound that delivers the drug to the animal. Some generic pour-on dewormers have poor quality carriers, making them ineffective as dewormers. This results in a low dose of dewormer being delivered and, like taking only part of your dose of antibiotics, results in resistance. So, I do not recommend using pour-on dewormers in calves at all and, if you are using them in cows, make sure to talk with your veterinarian about the quality of the brand you are using.

Many folks like pour-ons because of their ability to control flies, which is a legitimate consideration. Because of the issue of absorption and resistance of Cooperia to pour-on dewormers, I suggest using a pure insecticide to control flies rather than using a deworming product for fly control. This will also save you money.

When it comes to deworming, calves aren’t “little cows.” They have their own susceptibility to certain worms, which must be considered. Calves need a customized parasite control plan to get the most bang for your deworming buck. Be sure to ask your veterinarian what he or she recommends and select brands of products that have been shown to be highly effective.


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