D espite years of research, an applied understand- ing of production loss associated with gastroin- testinal nematode infestations in cattle and the factors that accentuate or diminish the impact of those infes- tations remains incomplete. 1,2 A considerable volume of work has been reported, which focuses on anthel- minthic use or efficacy in grazing cattle. 3-6 These investigations have often been conducted under con- ditions that may not reflect common cattle manage- ment or, at best, reflect management of a specific locale. Regional differences in parasite challenge and epidemiology are evident in the veterinary medical literature, but published reports 7-9 of studies investi- gating the impact of gastrointestinal nematode infes- tations in cattle in the US Northern Plains, an impor- tant beef cattle production region, have been sparse. Because of limited precipitation during the short grazing season, pasture parasite loads have been per- ceived as inconsequential and producers in the Northern Plains region have not embraced strategic nematode control programs. There is a need to develop regional estimates of the impact of gastrointestinal nematode infestations in cat- tle through multiple studies of representative popula- tions in actual field situations. 10 The purpose of the study reported here was to assess the effect of subclin- ical, naturally acquired gastrointestinal nematode infestation on weight gain in yearling cattle kept on pasture in the US Northern Plains. Materials and Methods A series of trials involving 11 commercial herds was performed from 1999 through 2001 to estimate the impact of naturally occurring infestation with nematodes on pas- ture gain in yearling beef cattle. A previously described sin- gle-pasture, epidemiologic-based, research approach was used. 11 All trials included yearling beef-breed cattle, pre- dominately Angus or Angus cross with no visible Bos indicus influence, that were grazing native pasture in eastern and central South Dakota. Trials involved home-raised (single source) cattle or cattle purchased from local auction markets (multiple source) at least 30 days prior to spring turnout. Producers participating in the study had never used a spring deworming program in grazing cattle. All producers had extensive experience with grazing yearlings and had used the same grazing pastures for multiple years. Cattle received anthelmintic treatment either at weaning the previous fall or at the time of purchase. No anthelminthics, except the experimental treatment, were administered within 30 days of turnout to pasture. No anthelmintics were used during the grazing period or within the period of study. With the exception of trial 3, during which the producer provided 0.5 to 1 kg/head (1.1 to 2.2 lb/head) of whole shelled corn 3 times/wk to cattle to facilitate observation, no supplemen- tal feed aside from free-choice trace-mineralized salt was offered to grazing cattle. In all trials, cattle grazed as a single pasture group throughout the grazing season. Within 5 days prior to turnout, a subgroup of representative cattle (comprising 20% to 30% of the total group) was sorted from the pasture group. Cattle that were ill or otherwise had signs of abnormalities JAVMA, Vol 226, No. 5, March 1, 2005 Scientific Reports: Original Study 779 RUMINANTS/ POULTRY Assessment of the effect of gastrointestinal nematode infestation on weight gain in grazing beef cattle Keith J. Mertz, MS; Michael B. Hildreth, PhD; William B. Epperson, DVM, DACVPM Objective—To assess the effect of subclinical, natu- rally acquired gastrointestinal nematode infestation on weight gain in yearling cattle kept on pasture. Design—Prospective study. Animals—799 Bos taurus yearlings kept on pasture with 2,805 herd mates in eastern and central South Dakota. Procedure—11 trials were initiated at 9 sites from 1999 through 2001. For each trial, approximately 10% of cattle in each site’s pasture group were identified, weighed, and administered a bolus of ivermectin (sus- tained-release formulation) prior to turnout. A similar subgroup of nontreated control cattle was identified and weighed prior to turnout. For each trial, treated and control groups remained with the larger pasture group throughout the entire grazing season. At the end of the grazing season, weight measurements and fecal samples were obtained from all treated and con- trol cattle; average daily grazing gain was calculated and compared between these 2 groups. Results—Treatment of grazing cattle with ivermectin increased average daily gain by 0.0459 ± 0.01 kg/head/d (mean ± SEM; 0.1 ± 0.02 lb/head/d), com- pared with that achieved in control cattle. Control cat- tle had significantly greater fecal egg counts at graz- ing season end than treated cattle. Conclusions and Clinical Relevance—Compared with anthelmintic-treated cattle, yearling cattle with naturally occurring gastrointestinal nematode infestations kept on pasture in the US Northern Plains had a decreased aver- age daily gain equivalent to 6.6 kg (14.5 lb) less gain in a 143-day grazing season. Strategies for control of nema- tode populations in pastures should be considered to ameliorate this production loss. (J Am Vet Med Assoc 2005;226:779–783) From the Departments of Biology and Microbiology (Mertz, Hildreth) and Veterinary Science (Epperson), College of Agriculture and Biological Sciences, South Dakota State University, Brookings, SD 57007. Dr. Epperson’s present address is the Department of Veterinary Preventive Medicine, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210. Supported by the South Dakota State University Agricultural Experiment Station; Merial Animal Health, Rahway, NJ; and Pfizer Animal Health, New York. Approved for publication as Journal Series No. 3414 by the Director, Agricultural Experiment Station, South Dakota State University. Address correspondence to Dr. Epperson.