I'm certainly no vet, and not sure what he is basing that on. I used it last year, the product worked great for me, cows looked good slick and shinny( could be a great grass year) I have always wormed twice a year but this last fall I did a sample, out of 5 groups only 1 group had any eggs, so no worming in the fall except for that 1 group, I think last year was done mid April, samples taken around 1st of Oct. This year gonna do it later April so the fly thing will last a little longer.