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.