We used it back in the 1970s. Think we still had a few clients who were still pouring it on cows at herd-working time, even into the mid 1980s. IIRC, you had to be careful with it on cattle with Brahman influence.
I have not seen a cow with grubs since about the time Ivomec came on the scene, around 1981.
As to killing birds... that was mainly when folks intentionally laced corn/grain with it to kill nuisance birds, or injected carcasses with it to take out coyotes, dogs, etc. When owls, eagles, hawks, buzzards start dying (and are reported) around carcasses, or from eating smaller birds poisoned by it, the Feds get all up in arms.
Carbofuran (Furadan), an organophosphate insecticide widely used in crop production, has largely taken the place of Warbex as the 'drug of choice' for dastardly folks poisoning dogs. We used to see cases of Furadan poisoning far too frequently at the diagnostic lab - usually targeting neighbor's dogs, but occasionally, when birds of prey were affected by someone purposely contaminating a deer or calf carcass... the Federal Wildlife folks came swooping in, and people have gone to jail &/or received big fine$.
fasterhorses... cattle don't develop 'immunity' to anthelminthics - but underdosing (whether accidentally or purposefully) and 'rotating' dewormers, as practiced by many folks, certainly selects for populations of parasites that are resistant to multiple classes of anthelminthics.
