You can look at it the same way you look at weeds in your pastures, says Mike Hildreth. The weeds are there and, if they get too thick, need to be controlled. But no matter how hard you try, you'll never completely knock them out.

Same with internal parasites. “Some cattle guys will say their cattle don't have any worms,” says Hildreth, a South Dakota State University parasitologist. “That's as ridiculous a statement as saying: ‘my pasture has no weeds in it.’ ”

While it may not be necessary to spray your weeds every growing season, controlling cattle worms is going to pay every year, he says. That's because, just as you can count on green grass after spring rains (assuming, of course, that it rains), you can also count on overwintering larvae of the brown stomach worm (Ostertagia) and intestinal Cooper's worm (Cooperia) emerging from their long winter naps and looking for a cow to infest.

While intestinal worms are present, it's the brown stomach worm that causes the greatest concern. And even though producers in the Northern Plains and other colder climates won't see clinical signs of a worm infestation, they're there and stealing efficiency and profit with every bite your cows take.

Strategic deworming

Enter the concept of strategic deworming. It's called that because what you're trying to do, Hildreth says, is hit the life cycle of the parasites when they're most vulnerable. In the Northern Plains, that's in the spring. “Essentially when the cows are going out on pasture, the overwintering larvae are coming up out of the grass. If you can set it up so the cows are protected with some kind of long-term dewormer, then the worms have two choices — starve to death (if they don't get ingested by a cow) or be killed by the drug if they are ingested. So you're attacking when they're most vulnerable. And you get the biggest bang for your buck.”

How much bang is that? Enough to make it worth your while, research shows. According to Hildreth, in cooler climates where internal parasites are subclinical, the main damage is done by appetite suppression.

“Because they (internal parasites) decrease appetite, they depress the grazing activity of the cattle. The cattle just don't graze the pastures as hard. Up here, where the grazing season is pretty short, we want to get those animals out there and take advantage of that forage.”

For stockers, the advantage is weight gain. In research trials Hildreth and others conducted, they found treated stockers gained about 15 lbs. more than untreated cattle in a 143-day grazing season. A recent Louisiana study found treated steers gained 0.4 lb./day more than untreated steers.

For cows, that increased efficiency shows itself several ways. According to a research review by Bert Stromberg, a University of Minnesota parasitologist, researchers have reported weaning weight advantages in calves from treated cows ranging from 31-44 lbs. over untreated cows.

That's likely due to increased milk production. According to Stromberg, a substantial increase in milk production in beef cows was observed after spring deworming, where anthelmintic-treated cows had an average of 7.7 lbs. of milk/suckling vs. 6.1 lbs. of milk/suckling for untreated cows carrying a parasite load.

What's more, the extra nutrition going to the cows instead of going to the worms helps reproduction. Stromberg says research shows treated cows had a significant advantage in reproductive performance, with an average of 94.2% of the treated cows being pregnant compared with 82.1% of the untreated cows at the end of the grazing season.

“Put a pencil to that and you can figure out what the housing costs of those worms are,” Hildreth says. “Because you're paying to house those worms just like you're paying to house the cattle.”

What about the calves?

Hildreth says he's often asked if calves should be dewormed along with cows at turnout. It depends on how old the calves are, he says. If you start calving your heifers in February and don't turn out until mid-May, you may want to deworm the older calves. But for the most part, he says, the recommended practice is to deworm the cows in the spring when you turn out, and the calves at weaning in the fall.

And, he adds, deworming is even more important in tough times. When pastures are dry, having all systems on “go” becomes even more critical, he believes, because a depressed appetite when forage is short can really hurt performance.

“It's just like you,” he says. “If your appetite isn't good, you're a lot fussier about what you're eating. So during times when there's not a lot of good forage, I think the worms create even more of a problem.”

Worms in the South

In warmer, more humid southern climes, the same general rules apply, says Carla Huston, Mississippi State University Extension outreach veterinarian. But because the climate is more conducive to worm survival, it may be necessary to worm more frequently.

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