Sex-related susceptibility of bulls to gastrointestinal parasites.

The effects of sex and anabolic implants on fecal egg counts and pasture contamination were examined in 77 naturally infected yearling bulls, steers, and heifers of mixed beef breeds grazing the same contaminated pasture in southern Ohio in the spring of 1990. Fecal egg counts were compared in seven groups of 10-14 animals each, twice before anabolic implants and twice after implants. Comparisons were made between untreated bulls, steers, and heifers, between steers with and without implants, and between heifers with and without implants. Bulls had significantly (P < 0.01) higher fecal egg counts than steers, and counts from steers were not significantly different to those from heifers. There were no significant differences between counts from implanted and control steers or heifers. The results have important practical implications both for parasite control and for bovine parasite research. There is a need to pay special attention to young bulls when designing parasite control programs, and to distribute the sexes equally between groups when doing research trials with cattle of mixed sexes. Some previous studies on worm population dynamics and anthelmintic evaluation may need to be re-examined in the light of sex differences.