External parasites have also de-
veloped resistance to 2 other classes of chemicals used in tags and pour- ons: pyrethroids and organophos- phates. “The trick there is combination
Develop a total
parasite control program.
usage of the products and rotational use,” Newcomb says. “If we’re us- ing an ivermectin ear tag, you might want to use a pyrethroid-type pour-on...or, if you’re using an iver- mectin-type pour-on on the cattle but deworm in the spring, then you might want to use a pyrethroid ear tag or an organophosphate ear tag, or a combination. They’re less likely to build resistance to 2 compounds than they are to 1.” In addition, producers need to apply fl y control
products at the right time of year and should institute a whole fl y program that may involve an insect growth regulator (IGR) supplemental to decrease the number of fl y larvae. Newcomb says stocker operators are more attuned
to using combinations of anthelmintic classes. “The cow-calf guys have been a little bit slower to adapt to that principle,” he says, “but more and more of them
are, especially as we have seen those who have used the ivermectin family, macrocylic lactones, for the past 10, 15 or 20 years and have never rotated out of that class. They will be the ones that show up with some level of resistance most of the time.” They may also be less likely to recognize
it because they may not measure the performance of cattle as closely as do feedlot or stocker operators. Merck has been working with a polymerase chain
reaction (PCR) lab operated by Dr. Lora Ballweber at Colorado State University. Newcomb says, “We can actu- ally send samples to her and get a view of what genera of parasites are actually present in the fecal samples.” This information is important because if the test is done with a fecal egg count reduction test (FERCT) it can help determine what parasites may be left behind with a particular class of anthelmintic. The take-home message is that producers need to
deworm at the proper time, check if their dewormer is working by doing a FECRT, and work with their vet- erinarian or parasitologist to develop a total parasite control program for their operation.
tscra.org
June 2014 The Cattleman 51
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