Well, you got me on both the feedlot and the vet's.  My vet loves cattle, knows what he is doing and I would assume his cattle are well fed but don't know that for a fact.  In a feedlot sure the low rate of pinkeye may be somewhat due to high energy diet, but the concrete and high animal density may have something to do with it too. They are also not sticking their heads down into tall stiff summer seed heads from grass that's gone to seed and causing drippy eyes which attract disease bearing flies. However I concede on both counts.

The main reason I drag pastures as soon as the cattle leave is not to control flies, although that seems to be a side benefit, sort of like those lick barrels that try to kill flies in the manure.  The main reason I drag is to distribute the fertility and reduce the avoidance zones next time they rotate through that paddock.

After I drag a paddock the scattered fresh manure usually washes into the soil with the next decent rain, when we get one. Along with higher density rotational grazing distributing the manure, dragging really helps pasture fertility. Overall I believe there is a significant positive return on the very minimal cost of running my Wingfield drag over a pasture at 6-8 mph.

Jim