arasitism continues to be economically important. The American Association of Veterinary Parasitologists recommended that seasonal transmission and epidemiologic information on gastrointestinal parasites be developed to combat economic losses due to internal parasites.;The transmission and epidemiologic aspects of gastrointestinal nematodes are not fully known in Utah. The objective of this study was to ascertain the presence of gastrointestinal nematodes and the economic need for nematode treatment of livestock in southern Utah utilizing seasonal pastures.;The effect of gastrointestinal parasite control on free-grazing cattle and sheep and their offspring was measured in two consecutive years. Cows and ewes were wintered on cold-desert rangeland. After calving and lambing, all animals were randomly assigned to replicate treatment (T) and control (C) groups before a 120-day summer grazing period on high-elevation (2900 m) range. Animals in T and C groups were switched in alternate years. T and C groups were further stratified into duplicate single (S) or joint (J) species and continuous (CON) or deferred rotation (ROT) pastures. Animals in T groups were administered recommended doses of ivermectin ;Before turn-out on summer pastures, all animals were weighed and fecal grab samples were randomly taken from approximately 10% of the animals to determine pretreatment internal parasite burdens. Fecal flotation revealed a low, subclinical level of nematodes laying eggs.;Calf BW was not affected by treatment. BW was significantly
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