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.”