But horn flies have developed resistance to the popular pyrethroid ear tags in many areas. That's what happened on Godbey's farm. To counter resistance, entomologists recommend rotating between pyrethroid and organophosphate ear tags. If you used pyrethroid tags for several years, rotating to organophosphate tags should control pyrethroid-resistant flies. Your veterinarian or Extension agent can help you determine which classes of insecticides are in the various brands of ear tags.
Entomologists recommend using pyrethroid tags only one year out of three.
Some manufacturers recommend placing two ear tags in each cow and one in her calf. But Sheppard doesn't recommend tagging calves. When ear tags work, we get perfect control placing tags in both of the cow's ears and none in the calf. To get the most out of fly ear tags, don't put them in cows until the fly population reaches 50 to 100 per side. Tagging cows too early wastes the strongest period of control.