EPIDEMIOLOGIC-STUDY OF NEMATODE INFECTIONS IN A GRAZING BEEF COW-CALFHERD IN MISSISSIPPI

The epidemiology of gastrointestinal nematodes was studied in a spring calving herd in northeast Mississippi, Pregnant, mixed breed beef cow s (n=15) were placed on a 10 ha fescue/bermudagrass pasture from Janua ry 1990-February 1992. In both years, calves were born from February-A pril and were weaned and removed from the pasture in mid-October. Feca l egg counts (EPG) and generic composition of nematodes in fecal cultu res were determined monthly for cows and calves. Estimation of numbers of third-stage larvae on herbage also was determined monthly from Mar ch 1990-February 1992, Worm-free tracer calves (2-3 per month) were al lowed to graze for 1 month periods and slaughtered for counting and id entification of gastrointestinal nematodes, The mean monthly EPG of co ws was consistently low (0.23-3.41); EPG of calves increased from spri ng through fall of both years. Five nematode genera were identified fr om fecal cultures of cows and calves, Ostertagia and Trichostrongylus spp, were the predominant nematodes in cows, while Ostertagia and Coop eria spp. were predominant in calves. Numbers of third-stage larvae on herbage declined from spring through summer and remained at low level s until late fall/winter, when numbers increased markedly. Eleven nema tode species were identified from tracers, but O. ostertagi and Cooper ia spp. predominated in most months, Seasonal changes in tracer worm c ounts coincided with similar changes in counts of third-stage larvae o n herbage. Inhibition of O. ostertagi occurred in tracer calves during spring, but did not give rise to a marked increase in egg production in cows during fall.