dr. buddy faires of the texas ag extension, college of veterinary medicine says (quote) don't ever think about stomach worms being in the stomach of cattle year around. they are not! they only occur at certain times of the year. for worms to be in the cows stomach, they have to have come in there in the last two months, cause that is all they will live. he says the right time to deworm is after 3-6 weeks of continuous rain & when soil temp is 55-85 degrees, if it is below 55 worms are hibernating & if it is above 85 they come out for cows to swallow. work at the university of mo proved that most cows have an immunity to worms after they are two, they found they usually had half of a dozen cows that would look wormy & they would end up worming the whole herd, they decided it was easier to sell the ones that could not handle the worms. they also proved if you only wormed the calves & not the cows, it would make you the biggest return on your dollar. most worm problems come from malnutrition
