“Resistance is a real issue. We’re beginning to see evidence of resistance to the ivermectin type products, the macrocyclic lactones, in cattle as well, in isolated places, so far.”
Veterinarian Mac Devin believes the resistance to de-worming products is beginning to occur because they aren’t being used correctly.
We treat a set of animals, we want 90% of the worms to be killed or more, preferably more. When we get down below that, now we’ve got a population that’s resistant, and they’re the only ones left out there to reproduce, so consequently that population over time becomes more and more and more resistant, if we continue to use the same products time after time, not recognizing that we have a resistance going on.”
Devin says a producer can test for resistance by looking at fecal egg counts.
“We take a sample of feces, generally measured by weight, and we look to see the number of eggs that are in that fecal count before we treat. Then we come back, we treat and wait at least two weeks, if we’re talking about the pour-ons and the injectables, and then we do a second fecal count to see if we got at least a 90% reduction of eggs in that fecal exam. If we have that, we’re not worried about resistance yet.”
It’s very important not to under-dose animals.
“We should be dosing animals in these herds to the heaviest animal in the production group. The heaviest cow, the heaviest bull, the heaviest calf, not trying to hit the average.”
Devin maintains that the proper worming of cattle is a great investment.
“In general, de-worming in the spring will pay us fifteen to twenty dollars a head in additional return on our investment. So if we apply a product that say, costs us a dollar and we get twenty dollars back, I can make that work for me all day long.”
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