Lightweight stocker calves are susceptible to several parasites, including Cooperia (intestinal nematodes). But, as they age, cattle will develop immunity to these worms.

Bert Stromberg, a professor in the Department of Veterinary and Biomedical Sciences at the University of Minnesota, says Cooperia can significantly impact young cattle. He points to a yearling study that found Cooperia impacted weight gain by 7.4%, with parasitized animals consuming 1.5 lbs./day less feed.

“Deworming 250- to 400-lb. calves and yearlings can be beneficial,” Craig adds. “You must choose the proper dewormer, depending on which worm you’re targeting.”

Another devastating worm, particularly in the South, is Haemonchus, which can cause death loss in lightweight stocker cattle by inducing anemia, he adds.

“We see this sometimes on irrigated pastures, or outbreaks when we get rain in summer. These worms thrive in calves and sometimes older animals that had no previous exposure (no immunity),” he says. If range-raised calves are brought to wet pastures, they could develop a high level of worms.

It’s a big concern for Craig in Texas. “The last couple of years, we’ve had very little rainfall, so some animals haven’t been exposed (to internal parasites). If they’re mixed with cattle that have enough worms to contaminate pasture, we could see severe problems if we get rain,” he says.

It’s best to deworm a group after they’ve been on the pasture long enough to pick up worms, but before the worms are mature enough to shed eggs and recontaminate the pasture, he says. “Leave the cattle on it for a month to allow them to pick up worms (and start to develop immunity), then kill those worms before they lay eggs,” he says.

He says several trials have examined various deworming strategies to determine what’s best in cow-calf operations. The work compared different protocols – deworming only the cow, deworming the cow and calf, or deworming calves only.

“Though there was variation in results due to different environments on different ranches, and some years were more conducive to greater transmission of worms, the final results showed no advantage to deworming in young calves, but a definite benefit to older calves,” Craig says. In some instances, it’s best to wait until calves are close to weaning age, unless there’s evidence of heavy worm burden, he says.

Which animals to deworm?

Craig advises deworming the cattle “most likely to have problems.” His first priority would be bulls of any age. “Their hormones can depress immunity against parasites; and they have higher worm egg counts than the herd average,” he explains.

Another vulnerable group is first-calf heifers, because they’re still growing, feeding a calf, and more stressed than adult cows. Weaning-age calves should also be dewormed as part of their backgrounding as stockers or replacement heifers, Craig advises.

Stromberg says studies show benefits of deworming weaning-age calves at least two weeks before vaccinating, thus allowing them to mount a better vaccine response. In cow-calf operations, he usually recommends deworming the cows (to minimize pasture contamination), but not calves.

Tom Yazwinski, University of Arkansas parasitologist, recommends deworming cows about the time of calving – when they are immunologically compromised. Hormones and parturition stress make a cow more vulnerable; this is when worms in her body will be most active. Worms are also easier to kill because most drugs work best when worms are active rather than dormant.

“We can hit the worms hard at a time when the animal needs to get rid of them, and she will cycle quicker to rebreed, and milk better – and you get a healthier, stronger, faster-growing calf,” he says.