The most common gastrointestinal nematodes found in cattle in Midwest are Ostertagia ostertagi and Cooperia oncophora. Other parasites include Haemonchus placei, Bunostomum phlebotomum and Nematodirus helvetianus. Parasite numbers are generally low in this region, as indicated by fecal egg counts and tracer calf worm recoveries, and the numbers of parasites decrease as one travels farther north. This decrease in parasite numbers also corresponds to a similar reduction in the prevalence of clinical disease. The dynamics of fecal egg shedding are similar throughout the Midwest. Cow fecal egg counts begin to increase just prior to calving in the spring, continue to rise, reaching a peak in July and August, and return to low numbers (2 or less eggs per gram of feces (epg)) in October. Calves born in the spring (March-May) show increasing numbers of eggs in their feces throughout the grazing season, with peak recoveries in October and November, after which the numbers decline as the calves develop an immunological response. Tracer calf worm recoveries demonstrate pasture contamination for the period grazed. These recoveries show adult O. ostertagi increasing throughout the grazing season then falling to low levels over the winter. Larval inhibition of O. ostertagi, as shown by recovery of inhibited fourth stage larvae (L4) occurs throughout the year, with increasing numbers of larvae recovered in the fall. The production robbing effects of these parasites has been demonstrated by the use of a controlled strategic deworming program. Weaning weight advantages of 14 to more than 20 kg have been reported in this region of the country when compared with an untreated group. There were also significant (more than 12%) reproductive advantages in treated cows. Recent studies have suggested that N. helvetianus, in moderate numbers, may have a significant effect on productivity.