The trichostrongylid egg output during spring in feedlot cattle.

Larval cultures prepared from faeces collected at two-week intervals from feeder cattle originating from Michigan, Oklahoma and Kentucky, and kept in a feedlot in Michigan, were examined during the winter and spring of 1979. The animals received anthelmintic treatment shortly after entering the feedlot. The larval output, consisting mainly of Ostertagia larvae, was less than 20 larvae per g, but did show a seasonal increase during spring in the cattle originating from Kentucky and Michigan. This "spring rise" was not observed in the animals originating from Oklahoma which is thought to be related to a difference in transmission pattern of Ostertagia in the southwest as compared to the eastern and midwestern parts of the U.S.A.