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.”