When it comes to combating internal parasites in cattle, it’s all about the timing , says Thomas Craig, a professor in Texas A&M University’s Department of Veterinary Pathobiology. And the “best” time to deworm depends on region and climate, and whether it’s a cow-calf operation or stockers. “Suckling calves are not as adversely affected as weaned calves and yearlings,” he says.
Craig says that, in Texas, worms thrive on winter pastures because it’s cool and moist. But the brown stomach worm, Ostertagia, can’t survive the South’s 100° summer days, so it goes dormant in the cow in summer.
Meanwhile, in the North, worms can’t survive in winter outside the cow; so they hibernate in the cow’s abomasum wall when environmental conditions are poor for larval survival in pastures. That’s important, he says, because you can reduce transmission of Ostertagia if you treat when they’re dormant utilizing a drug that’s effective against that arrested stage. This kills any that would “wake up” later to start laying eggs when weather improves.
“Ostertagia  is the important worm in adult cattle and can be devastating to young animals. Most other internal parasites aren’t an issue in adults. By the time cows are two years old, they’ve developed some immunity to most worms,” Craig says.
A numbers game
Parasitism is a numbers game. Cattle can tolerate a few worms, but heavy infections are detrimental, Craig says. Worms are more devastating in young animals because they haven’t yet developed much immunity.
Research indicates it’s probably optimal to deworm calves  after two months of age, as internal parasites reduce growth rate in young animals. However, calves with their mothers on rangeland won’t pick up many worms, because conditions are drier and stocking rates tend to be lower. Heavy worm burdens are more likely to occur with intensive management where many cattle are grazed in small areas.
Meanwhile, intensive grazing on wet pastures is ideal for worm transmission. “These cattle need more deworming,” Craig says. “Short-duration, high-density grazing systems are modeled on how long it takes high-quality forage to regrow so cattle can come back to it. This is often the same time it takes worm eggs to hatch and become infective again. High cattle density puts more worm eggs in a smaller area, and the animals are forced to eat everything, even grass next to manure piles.
If a pasture is used through summer for rotational grazing, the worms gain the advantage. By contrast, in a mob-grazing program where cattle don’t come back to the same plot for a year or more, there won’t be many viable worm larvae left.”
Young cattle concerns
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.
Notes on drug resistance
Louis Gasbarre, a retired research leader in USDA’s Bovine Functional Genomics Laboratory who is now a consultant, says drug resistance  in some worm populations has shown that we can’t completely control parasites.
“Prior to the 1980s, producers controlled parasites with good management – rotating classes of livestock on pastures, monitoring animals, and not overstocking pastures. When the new wonder drugs became available, we thought we didn’t need to do anything except deworm the animals,” Gasbarre says. “But, if we keep using dewormers indiscriminately, we may lose the efficacy of these tools.”
Yazwinski adds that to avoid the development of drug resistance by parasites, the treatments must be targeted for when they are most effective, and done as infrequently as possible.
“This utilizes the refugia (population of pathogens on your farm that hasn’t been exposed to drugs); they are the most vulnerable and easiest to control,” he says. “Refugia parasites cross with resistant parasites and help keep the total population vulnerable to chemical control.”
The larger this refugia population, the more they will intermingle with any worms that have developed drug resistance, thus diluting them.
“The refugia helps make sure current and future worms can be removed. If you just treat the cow one time, and the rest of the year let her get some parasite exposure (to worms that aren’t resistant), and shed some eggs, she contributes to the population that hasn’t been selected for drug resistance,” Yazwinski says. This also allows calves to be exposed and start to develop immunity to worms.
In stocker cattle, which are prone to several types of worms, it’s more complicated. In a stocker operation, it’s important to know which worms you’re dealing with and what they are doing, when choosing drugs. “You must think about drug resistance, which you generally don’t need to worry about on a cow-calf operation,” Yaswinski says.
“Resistance in some worms is enough of a concern at the stocker and feeder levels that some people are combining drugs to maintain efficacy,” Yazwinski says. An example is using a “white” dewormer (oxfendazole, albendazole or fenbendazole) in conjunction with a macrocyclic lactone (ivermectin, doramectin, eprinomectin or moxidectin).
“Since you’re combining drugs that have been available awhile, you’re selecting for dual resistance. But, to get the most economic effect, many people are going this route today.” When combining drugs of different classes, be sure to use full doses of each, he cautions.
Sidebar: Drug classes
We have three families of dewormers,” says Thomas Craig, Texas A&M University veterinary pathobiologist. “The benzimidozoles (white paste oral dewormers) have a broad spectrum of activity, but no residual effect.” They are very effective, however, against worms in the digestive tract.
Levamisole also is only effective against adult worms, has no residual effect, and can’t reach arrested larval stages. “It’s given as an injectable or oral. It’s a good drug, but not as good as newer products. There are some situations where it’s useful in stocker calves – where there’s drug resistance to other dewormers,” Craig explains.
“Another class is the macrolides (ivermectin, moxydectin) that retain high blood level for a period of time, so any incoming worms will be killed,” he says.
The best drug depends on your region/climate, and class of cattle, but be careful. “Products like Ivomec are now off patent, so there are many generics available. They all have ivermectin in them, but some don’t do the job,” Craig says.
Heather Smith Thomas is a rancher and freelance writer based in Salmon, ID.