I think the manual spraying doesn't get as good and regular coverage as the rubs and may lead to more resistance build up in flies. I mix up a gallon or two of diesel and pour it on the rub out of a plastic fuel can. With the liquid poured on the rub there is less exposure to the permethrin than a spray. I'm not an expert in cattle by any means, but the rub seems to be the safer & lesser evil compared to flies all over. Just my limited experience.

Jim at Dawn