by: Heather Smith Thomas

When treating cattle with antibiotics, dewormers and other drugs, it is important to use proper dosage—which is generally determined by weight of the animal. Thus it is crucial to know the weight, rather than guessing. Underdosing may not give the desired results, and overdosing in some instances can be harmful. In the case of dewormers, underdosing won't kill the parasites and simply leads to drug resistance.     

Veterinarians, producers and economists in the U.S. estimate that annual parasite related losses (which includes poor performance) in the livestock industry at more than $100 million. These losses can be minimized with proper parasite control.

Gary Sides, PhD, a cattle nutritionist with Pfizer Animal Health Veterinary Operations, points to a study in 2004 that looked at the effects of dewormers in feedlots. “At that time there were a substantial number of producers using just a half dose of Dectomax injectable, trying to save money at processing, and believing that this dosage would be adequate. Therefore a group of four veterinarians ran a trial to see if this was effective. They took cattle off grass in California and put them in a feed yard in Nebraska. One third of the cattle were not treated, one third got a half dose of injectable Dectomax dewormer and the other third of the cattle got a full dose of that product,” says Sides.

“When they slaughtered the cattle they weren't able to get feed efficiency results because the cattle were all in the same pen, but they had individual ID on all the cattle and did get carcass weight, daily gain, and percentage of the cattle grading choice/prime. There was a 22 pound difference on carcass weight between the full dose and the no dose cattle, and statistically no difference between the no dose and the half dose cattle,” he says. Thus the half dose had no noticeable benefit. In essence, it was a waste of money.

“Looking at the percent of cattle grading choice/prime, there was also no difference between the half dose and the no dose cattle, but there was a 16 percent advantage in choice/prime in the full dose cattle,” he says. The full dose obviously helped the cattle that were dewormed with the proper doses; they were able to perform better in the feedlot.

Looking at this situation with the cow/calf operation, we need to make sure we are actually giving these animals enough dewormer. “Very few people actually have 1000 pound cows anymore. Most of the cows are closer to 1400 or 1500 pounds. We need to give them an adequate dose for their weight, in whatever product we are giving them,” says Sides.

If we are using vaccines, it's not an issue. “We are giving 2 milliliters or 5 milliliters per head, depending on the product; it's not weight-specific. But with antibiotics like Draxxin, Excede, Advocen, etc. or any of the dewormers, the dosage is always by body weight and cattle need to be dosed accordingly,” he explains.

When pharmaceutical companies do their trials and studies to get FDA approval for their product, it's very important to get the right dosage for the body weight. “This is why the processing chutes that have digital scales are very useful and helpful,” he says. You know immediately and accurately what the animal weighs when it steps into the squeeze chute for treatment. Visual estimates are often misleading, and weight tapes are usually not accurate enough.

Stockmen are trying to cut operating costs in order to survive financially. There are appropriate ways to cut costs and inappropriate ways. Skimping on needed drugs usually ends up costing a producer more in the long run. If dewormers can help cattle be healthier and more feed efficient, using the proper dose pays off—especially when feed costs are high.

“If a feeder is looking at very expensive corn and high priced cattle, if you try to save money by using just a partial dose or underdose of dewormer, those cattle don't perform as well. All we have to is make up one pound of gain or half a pound of feed efficiency to make up that difference,” he says. You haven't saved money; you had to spend more money for feed or had lower weights at the end.

Parasite resistance is another important reason to never underdose. “Most of the parasitologists believe that the continuous exposure to less than adequate dosage is what selects for resistance in the parasites,” he says. The most susceptible parasites might die, but the resistant ones survive—and they become the predominant population.

“They are out there anyway, and we select for more of them if we keep using products incorrectly,” says Sides. This may be more of an issue in a cow/calf operation in a pasture environment, with on-going worm transmission. The feedlot cattle don't stay around as long, and the parasites aren't transmitted/reproducing in a drylot. Thus the worm resistance could become more of a problem in the long run for the cow/calf or stocker operation.

Regarding efficacy, however, deworming at proper dosage is just as crucial in the feedlot. “It was a feedlot study that showed reduced gain and grade with inadequate dosage—in cattle that were consuming the best diet known to science. There wasn't any nutritional stress on these cattle, but they still had a negative response to a half dose,” says Sides.

“Internal parasites inhibit feed intake and reduce digestibility. Thus if I don't get rid of the parasites I can measure the negative response in the feed yard,” he explains.

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