Deworming Cattle on Pasture

Published on Thu, 04/28/2022 - 2:46pm

Deworming Cattle on Pasture.

 By Heather Smith Thomas.

 Internal parasites (stomach and intestinal worms) rob cattle of nutrients, reducing growth rate and weight gains in young animals and hindering optimum production in all classes of cattle.  Heavy infestations create health problems. Worms are an added stress and can make the host animal more vulnerable to disease.  Deworming cattle at proper times of year, at the most appropriate stage in the life cycle of a certain parasite to eliminate egg-laying adults in the GI tract—to minimize re-contamination of pasture with worm eggs--can keep reinfection to a low level.

Ray M. Kaplan, DVM, PhD, DACVM, DEVPC, Professor of Parasitology, University of Georgia says there is no such thing as a one-size-fits-all parasite control program; parasite control must be tailored to the farm.  “However, there are some general recommendations that apply in most situations.  Beef cows in poor body condition due to sub-optimal winter nutrition should be given a treatment in late winter.  This treatment is usually best given just before calving, but optimal timing will vary depending on the time of calving,” he said.  Once grass growth begins, improved nutrition will enable the cow’s immune system to respond, and the cow will limit her own worm burden.

Thomas Craig, DVM, PhD, Professor emeritus, Department of Veterinary Pathobiology at Texas A&M, says the best time to deworm may vary from region to region, depending on climate.  “We also need to differentiate between cow/calf operations and stocker cattle.  Suckling calves may not be as adversely affected as weaned calves on pasture, and yearlings,” he explains.
“Here in Texas, winter is when the worms are thriving on pastures because it’s cool and moist but not too cold, whereas in northern parts of the U.S. the worms can’t survive outside the cow so they go dormant in the GI tract during winter and aren’t laying very many eggs.  Most winter days here, our temperature gets up to 50 degrees, and the worms do very well out on the pasture.  What they can’t stand is 90 to 100 degrees; they go dormant in the cow during summer heat.  Summer is the best time here to deworm—when it’s dry and hot.  In the north, by contrast, winter is a good time to deworm.”

You can greatly slow down transmission of worms if you hit them when they are dormant, killing any that would otherwise “wake up” to start laying eggs when weather conditions improve.  You can get a head start on keeping pastures more free of worms.

“I think Ostertagia (brown stomach worm) is the most important parasite in cattle, in most of North America.  It’s the only one very important in adult cattle (because it continues to do well in the host even as the animal gets older) and can be devastating to young animals, as well.  Most of the other internal parasites in cattle are not an issue in adults. By the time cows are 2 years old, they have developed some resistance to most worms and have low numbers.  When a cow is 4 or 5 she’s either developed enough immunity to keep parasites at a reasonable level or she’s been removed from the herd because she’s not doing well,” he explains.

It takes a while however for this immunity to develop, and ranchers usually don’t see clinical signs of worm infection.  Signs are very subtle.  “One of the main things this worm does to the host is reduce appetite.  If cattle don’t eat as much, they don’t do as well.  An animal might look normal yet is not eating to its potential—and not milking quite as well or gaining quite as well (if it’s a calf).”  This is why deworming at the proper time of year might improve production.

Ostertagia goes into arrested development (dormant stage, embedded in the stomach wall, in the abomasum) when environmental conditions are not conducive to larval survival in pastures.  The dormant worm emerges again later—to mature and lay eggs—when conditions improve.

 “In the south, just prior to the time of year when it becomes hot and dry, most of these worms, instead of becoming adults, go dormant.  When Autumn rains begin, they resume development,” said Craig.  Thus they are ready to lay eggs that pass out with manure at a perfect time for the hatching larvae to thrive and migrate onto forage plants near the manure pat.

It’s amazing how the worms know when to go dormant and when to resume their life cycle.  “There is a difference between north and south, and some places in between, like Missouri, where the worms seem to have an identity crisis.  There are populations of worms that evolved in northern regions to over-winter in a dormant stage, and others that go dormant over summer (like they would in hot weather of the south).”  It’s the same species of worm, and they all act the same way, but they have adapted to these two different environments.  

“When these worms adapted to the various environments, we weren’t shipping cattle around the country so much.  It took a period of time for those populations to adapt and become tolerant to whatever environment they were in,” he said.

“The main thing to understand, if you are in an area with cold winter and arrested development, is that the only way the worms can survive in any numbers is inside the host.  If we treat the cow during winter with a drug that’s effective against that arrested state, we’ve done the job.  The same results occur with summer deworming here in Texas.  We may not get 100% of the worms, but we will knock them down below a level that would lead to disease (signs of worm infection),” explains Craig.  

There are still enough worms on the pasture that cattle are exposed to them and begin to develop resistance, but not enough to cause problems in the cattle.  Numbers would be below the threshold that would hinder growth, production and good health.  “Parasitism is a numbers game.  Cattle can tolerate a few worms, but heavy infections are detrimental,” he said.

Worms are generally more devastating in young animals because they have not yet developed much immunity.  If calves are grazing pastures heavily contaminated with worms, they may develop large worm burdens.  Craig says the young calf may not pick up worms as readily as an older calf.  “The cow picks up parasites from a pasture, and the calf doesn’t pick up as many.  But if you have a lot of older calves running together on a pasture, shedding worm eggs, this can be a recipe for problems,” he said.

There are several types of worms that can be a problem in calves and stockers, and it may take different strategies to combat them.  You need to know the life cycle of the worms, how a particular worm affects the animal, and when the best time might be (and which drug) to deworm them.  Calves pick up many kinds of worms, eventually develop immunity to some of them, and then good pasture management can help keep the worm transmission to a fairly low level in adult cattle.

Research has shown that it probably pays to deworm calves after they are past 2 months of age.  Even though they may be doing well because they are nursing their dams and on good feed, they generally have a few worms.  “Most studies indicate that if we treat calves above 200 pounds or 2 months of age, they gain more by weaning time than untreated calves, even though the untreated animals may not show clinical signs of worms.  It made a difference that would more than pay for the drug,” said Craig.

If a rancher is working cattle (branding, vaccinating, etc.) when calves are 2 to 3 months old, it’s generally a good practice to deworm them, especially if they summer on irrigated pastures rather than dry rangeland.  Cattle on rangeland won’t pick up many worms, partly because conditions are drier (inhibiting worm transmission) and because stocking rate is low; the animals have less opportunity to be exposed to worms.  By contrast, cattle in lush green pastures face higher numbers of worms since the larvae thrive in wet conditions and cattle density is usually much higher.

“When calves come into a pasture that’s had other young cattle on it, you might have problems.  Intensive grazing on irrigated pastures creates ideal situations for worm transmission.  These cattle need more diligent deworming.  Many short duration, high-density grazing systems are modeled on how long it takes to get high quality forage regrown again so the animals can come back to it.  The time interval for that is often the same time it takes worms to develop—for the eggs to hatch and become infective again,” he explains.  And the cattle density makes transmission conditions ideal; there are more worm eggs in a smaller area.

“The quality of the forage in a rotation system may be such that most animals are able to tolerate a few more worms, but at some point the added worm numbers may catch up with them,” said Craig.  It may make a difference whether or not the pasture is used as hay first, before being grazed—since this gives a longer time interval, and cutting the mature grass leaves a shorter stubble, opening the remaining plant to heat and drying.  The worms won’t survive as long on the pasture in hot weather, unless they are protected by a fecal pat.  If a pasture is used season-long for grazing in a rotation system, however, this tips everything in favor of the worms.

Stockers and replacement heifers are at greatest risk for production loss from parasites, according to Kaplan.  Use of anthelmintics will be important to their health and production.  “However, optimal strategies for applying anthelmintic treatments will vary greatly, depending on the management and grazing system.  Thus, it is impossible to provide a general recommendation.  In general, strategic treatments, resistance-mitigating approaches, and sound pasture management must be used together,” he said.

“Finally, it should be understood that any recommendation given will not be uniformly accepted by all parasitologists and veterinarians who work with cattle.  There is no single best worm control program, and there is plenty of room for disagreement among experts; the best and most cost-effective strategy will differ from region to region and farm to farm depending on many factors,” said Kaplan.  

“What is optimal today will not be optimal tomorrow.  Constant vigilance, paying attention to changes in the host-parasite-environment dynamic (including emergence of drug resistance) is required.  The problem of anthelmintic resistance must be considered.  Drug resistance is extremely common and worsening all the time.”

Drug Resistance Problems
Beginning with phenothiazine in the 1950s, followed by benzimidazoles in the 1960s, the imidazothiazole/tetrahydropyrimidines in the 1970s and the avermectin/milbemycins in the 1980s, a new class of anthelmintics was introduced into the marketplace each decade.  “This arsenal of highly effective and relatively inexpensive drugs led to recommendations for parasite control that were based almost solely on the frequent and/or strategic use of anthelmintics, the goals of which were to maximize livestock health, productivity, and profitability,” said Kaplan.  

Though this approach was very successful for a number of decades, we are now experiencing ever-increasing levels of anthelmintic resistance in all drug classes, and involving all of the most economically important parasites of all livestock species.  “Resistance in parasites of cattle was slower to develop than in small ruminants and equines, but over the past decade we have seen rapid escalation in the levels and distribution of anthelmintic resistance in parasites of cattle worldwide,” he said.

Craig says there have been a few problems in stocker calves, infected with Haemonchus that were resistant to both major drug families used on cattle in the U.S.—the macrolydes (ivermectin and moxidectins) and benzimidazoles.  This doesn’t leave us much to work with.

“One of the older drugs that was used 50 years ago, levamisole, seems to be effective when these worms are resistant to newer drugs,” Craig said.  But levamisole is only affective against an active worm.  “If the worm is not feeding in the digestive tract, it doesn’t take in the drug.  If the worms are active it might kill them, but if they aren’t, it won’t.  This is one of the things, particularly with Ostertagia, if we can whack them before they start laying eggs, then we can do a really good job,” said Craig.

In looking at drug resistance issues, we must be careful to not overuse dewormers or use them incorrectly.  For instance, if we don’t give an adequate dose, only a portion of the worms are killed, and the ones that survive are the more resistant ones, producing future generations of resistant worms.

“We shouldn’t get carried away with deworming programs in cow/calf operations.  I don’t think we have enough problems in that population to justify regular deworming—except in a few situations on certain ranches.  Sometimes we use a drug several times and don’t really know whether it’s working or not.”  It pays to check some of the cattle at the time of deworming (with fecal egg counts) and check them again a few weeks later to see if we actually reduced the worms significantly.

“This is very important when deworming lightweight animals like stockers.  It pays to check from time to time.  Otherwise you may be using a drug that’s not working anymore and you don’t know it until you have a wreck,” said Craig.