SEASONAL PATTERN OF INHIBITION OF OSTERTAGIA-OSTERTAGI IN CALVES IN NORTHEAST MISSISSIPPI

The composition of, and seasonal changes in, populations of gastrointe stinal parasites of calves in northeast Mississippi were determined fo r 10 months post-weaning. After weaning on 15 October, 20 mixed breed beef steers were grazed together on a 4 ha fescue/bermudagrass pasture . From November through August of the following year, two of the calve s were removed each month for necropsy and counting of gastrointestina l nematodes. Eight species of worms were found: Haemonchus placei, Ost ertagia ostertagi, Trichostrongylus axel, Bunostomum phlebotomum, Coop eria spp., Trichostrongylus colubriformis, Oesophagostomum spp., and T richuris ovis. During all months, Ostertagia ostertagi and Cooperia sp p. combined comprised at least 89% of gastrointestinal nematode burden s. Cooperia spp. represented 92.6% of the total worm burden of calves in November but declined to about 56% in January and February. From Ma rch through August, Ostertagia ostertagi comprised at least 79% of the worms from calves. Numbers of inhibited Ostertagia ostertagi increase d markedly from February to March and remained at high levels prior to resumption of development in August. The proportion of Trichostrongyl us axei remained about 4% throughout the year, but the highest numbers were recorded in the summer months. Other species were minor componen ts of the worm population. These data indicate that with respect to Os tertagia ostertagi, northeast Mississippi can be considered a summer i nhibition zone.