Exactly when you deworm your cows matters; both to the cows and to the worms.

Burt Rutherford, Senior Editor

April 1, 2011

8 Min Read
Exactly When You Deworm Your Cows Matters

Larry Hollis calls it the clean cow-clean pasture concept.

That’s his answer to the question of when you should worm your cows. “The way I look at it,” says the Kansas State University Extension veterinarian, “when you go to clean pasture, put clean cows out there.”

That way, he says, you reduce the level of egg and larvae contamination in the pasture, which is the major source of the worm load that cows and eventually their calves will carry.

He stresses the word “reduce,” however. “No dewormer is 100% effective, so over time, there will gradually be a buildup of eggs that hatch into infective larvae. But you’ll minimize that to the smallest degree you possibly can if you’ll deworm them when you go to a clean pasture.”

That means, he says, deworm your cows when you turn out in the spring and again in the fall when you’re working your cows and calves before turning out on winter pasture.

Warm, wonderful spring

As the poet Tennyson so aptly observed, springtime is when a young man’s fancy lightly turns to love.

Same thing if you’re a brown stomach worm (Ostertagia ostertagi).

In colder northern climates, internal parasites go into a form of hibernation in the animal’s gut, emerging when weather conditions become conducive to larval survival. In hot southern climates, that hypobiosis occurs in summer, with emergence in the fall.

“Optimal larval survival is when it’s warm and wet,” Hollis says. “Any time there’s enough moisture where you get dew on the grass, that’s good conditions for those larvae to survive.”

As the weather heats up and dries out, larval activity on the pasture subsides. But the cows – and likely their calves if they’re grazing – are re-infected. That’s why the strategic philosophy of worm management calls for cows and calves to be dewormed in mid-summer. However, few cattlemen want to gather cows in the heat of summer and work them again, which makes spring and fall deworming, when cattle are being worked anyway, a more palatable management option.

That’s particularly true in the dry desert climates that John Wenzel, New Mexico State University Extension veterinarian, deals with. “The most effective time to deworm cattle is when we have them in the chute in the fall at preg check time,” he says. “Generally speaking, our parasites tend to over-summer because it’s so hot and dry.”

However, even in arid climates, internal parasites can flourish. “In my opinion, I think it’s very important that we deworm those cattle, because we have enough internal parasitism to make a difference. Even though it’s terribly hot and terribly dry, we do see a level of parasitism.”

Typically, the desert Southwest will get summer monsoon rains beginning in late July. That means, in three to four weeks, hibernating worms emerge and begin shedding eggs, making pastures infective by August and September.

He also recommends that cattlemen use a product that controls external parasites, as well. Lice are a problem in late winter, he says, if cattlemen don’t deworm in the fall. “It’s very effective for us to treat those cattle in the fall, kind of clean them up. It seems if we do that, we really don’t have any ectoparasitism.”

Dollars and dimes

Wenzel is a strong advocate of getting the biggest bang for every dollar spent on a deworming program. That’s why he recommends that cattlemen work closely with their local veterinarian and that they use name-brand products. “It really does matter,” he says. “You get what you pay for.”

While generics are cheaper, they use a different, less-effective carrier, he says, and may not have the same activity as a drug that has more research behind it. “To me, if you’re going to spend the money, do the absolute best job. It may cost a little more money in the short term. In the long term, if we get the efficiency out of it, it’s worth every dime we spend.”

That’s because, Hollis says, much of the worm burden that cattle carry is sub-clinical. That’s especially important in calves, which are more susceptible to worm loads than are older cows. By the time cows reach maturity, their immune system has kicked in and is partially controlling the negative effects that worms have.

Not so with calves. “And you can’t tell by looking which are ones are wormy and which ones aren’t,” Hollis says. But untreated calves won’t perform as well as those that have had the worm load knocked back.

“So you may have a calf that has the genetic potential to gain 3 lbs./day and he’s gaining 2.2 lbs. A 2.2 calf looks good, but when you go to weigh them up, you left pounds on the table that the dewormer would have paid for.”

Which is why it’s important to deworm in the fall as well. By then, the calves will have picked up enough of a worm load to make deworming worthwhile. And the cows benefit, too.

“If they’re spring-calving cows, I want to deworm them in the fall to get them as low as I can get them. I want to deworm them again in the spring to keep them as low as I can keep them,” he says.

Tips to thwart resistance

According to Michelle Arnold, University of Kentucky Extension veterinarian, there may be another aspect to strategic deworming in mid-summer that cattlemen may need to consider.

“Traditionally we recommend a strategic deworming around the end of June to the first of July, with the thought that it is hot and dry and there are very few larvae out in the pasture,” she says.

“The thought now is if you chemically deworm with low numbers of larvae in the pasture, the only worms that survive are those resistant to the chemical. So you’re creating populations of chemically resistant worms.”

She says at this point, she hasn’t seen documented chemical resistance problems. But she thinks there’s a potential for resistance to begin creeping into the equation, not only due to the timing of the deworming program, but from the use of generics and the continued use of pour-ons.

“So at this point, my recommendation is to focus on deworming calves and young stock up to two years old, twice a year – spring and fall,” she says. Mature cows need deworming once a year.

She emphasizes deworming young cattle because of the effect that a worm load can have on a young, growing animal, particularly replacement heifers.

“Because they are growing, they have a lot of nutrient demands on their body. When you get up to two years old, they’re either in gestation or they’ve already calved and they’re producing milk. That is a tremendous demand as far as nutrients go. So you don’t want to add parasites to that equation, because parasites are going to consume some of that – they’re going to take away some of that energy and nutrients the animal really needs.”

Arnold also stresses that deworming is just part of the management equation in controlling the worm load in your cattle. Pasture management plays a big role, too.

“If you overgraze and/or overcrowd, your animals are going to pick up many more larvae,” she says. “Larvae don’t migrate up (grass stalks and leaves) more than 4-6 in. So don’t force animals to graze close to the ground or close to manure. Larvae don’t get much farther than a foot beyond the pat of manure. If we force them to graze close to a manure pat and close to the ground, they’re going to pick up more larvae.”

Arnold encourages producers to rotate products, using both macrocyclic lactones and the “white” wormers. The macrocyclic lactones, which includes the avermectin class, have a residual effect; meanwhile, the white wormers kill the adult worms in the gut and hypobiotic larvae, but are only effective for one or two days.

“Change your regimen so you’re not always showing the same chemical to the worms,” she advises.

Thoughts on fecal egg counts

Rotating compounds is important not just in keeping a population of worms around that will stay susceptible to chemical dewormers. It also helps control other species of internal parasites. The macrocyclic lactones are highly effective against brown stomach worms, which are the number-one internal parasite of concern to cattlemen.

“But none of them are very effective against the number-two worm in young cattle, and that’s Cooperia punctata,” Hollis says. “The white or drench dewormers have an advantage there. One of the things we have seen with continuous use of the avermectins is that we have selected for cattle that are loaded with Cooperia.”

Trouble is, you can’t tell. It’s very difficult to tell the difference between Ostertagia and Cooperia eggs when looking at them under a microscope, Hollis says.

But fecal egg counts can be diagnostic, at least in young cattle. “Pull a fecal sample at the time you treat, pull another fecal sample three weeks later, and look at the relative number of eggs.” Hollis says. “If it was high and went virtually to zero, it means you had Ostertagia and you did an effective job. If it was high and stays high, you’re probably dealing with Cooperia and you probably need to go back with one of the drench dewormers.”

But it’s only worthwhile to do fecal egg counts on young cattle. “In cows, it’s misleading because their immune system has kicked in and is trying to fight the worms. One way it fights the worms is to cause them to lower their egg-laying ability,” Hollis says. “On a cow, a low fecal egg count doesn’t mean much.”

That’s why, by and large, veterinarians don’t recommend doing fecal egg counts, because it’s an added cost that doesn’t always tell you all you need to know. “If you have a limited amount of money, put it into deworming the animal rather than testing,” Arnold advises.

But by all means, they say, put your money into deworming. You’ll be dollars and dimes ahead.


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About the Author(s)

Burt Rutherford

Senior Editor, BEEF Magazine

Burt Rutherford is director of content and senior editor of BEEF. He has nearly 40 years’ experience communicating about the beef industry. A Colorado native and graduate of Colorado State University with a degree in agricultural journalism, he now works from his home base in Colorado. He worked as communications director for the North American Limousin Foundation and editor of the Western Livestock Journal before spending 21 years as communications director for the Texas Cattle Feeders Association. He works to keep BEEF readers informed of trends and production practices to bolster the bottom line.

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