Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Biology and Microbiology


In many Northern regions of the United States, gastrointestinal nematode infections are largely sub-clinical, with relatively few cases of clinical disease. At subclinical levels, the effects of nematodes are less apparent but lead to decreased rates of production, and decreased immunity to various infectious diseases. Sub-clinical burdens may make it difficult for producers to recognize the loss, and the need to control these parasites. A convenient method to measure sub-clinical losses under actual herd production conditions is needed to enhance a producer's ability to evaluate the costbenefit of nematode control programs. The primary objective of this thesis was to utilize a single pasture limited treatment approach to quantify losses associated with gastrointestinal nematodes in several herds within South Dakota to aid producers and practioners in making herd health decisions. Prior to spring turn out, sentinel animals, consisting of approximately 20% of a pasture group, were selected. Sentinel animals were weighed and individually identified with an ear tag. Half of the 20% (10% of the total pasture group) received one Ivomec SR Bolus® (Merial Animal Health) (experimental group) and the other half was designated as untreated controls for the study. Animals were assigned to the experimental and control groups through systematic allocation during initial weight data collection. Both experimental treatment and control groups were pastured with the rest of the pasture group, and then re-weighed at the end of the grazing season. Average daily gain was determined by subtracting the end weight from the start weight and dividing by the number of days the animals were on pasture. Difference in weight gain was attributed to gastrointestinal nematode burdens, as all other environmental factors were identical except for the anthelmintic application. Fecal samples were obtained at study end, and egg counts were performed to estimate nematode burden. Fourteen pasture groups consisting of steers, spayed heifers or replacement heifers were utilized during the 1999-2001 grazing seasons. Pasture groups were located at separate sites throughout South Dakota and stocking densities as well as overall pasture condition varied with site. After controlling for trial site, statistical analysis indicated that treated animals had a 0.05 kg/day ADG advantage over control animals (p=0.0347). The second objective of this thesis was to determine the availability of over-wintering infective gastrointestinal nematode juveniles on pasture, in relation to spring strategic deworming practices. The premise of strategic deworming programs is to synchronize the application of persistent anthelmintics with the parasite's life cycle to diminish future generations of nematodes. To maximize the use of anthelmintics and reduce the number of internal parasites on pasture, strategic deworming programs must be synchronized to coincide not only with the emergence, but also the decline in first generation infective juveniles on pasture. A pasture, divided into 12 paddocks, was "seeded" with gastrointestinal nematode eggs by daily rotation of eight yearling replacement heifers, with known levels of subclinical nematodiasis, through the paddocks. Infective juveniles were allowed to develop from the subsequent eggs and over-winter on paddocks until the following spring. The following spring, four groups of 15 fall born calves were placed on designated paddocks at two-week intervals starting on May 31, and ending on July 27. The calves were removed from designated paddocks after two weeks and placed in a controlled non-grazing environment for 21 days. Measurement of juvenile levels on pasture was performed through analysis of individual fecal samples obtained from the calves 21 days after removal from paddocks. The arithmetic average eggs/gram declined from 22.86 to 8.52 from period one to period two. The average eggs/gram for periods three and four were 3.13 and 0.40 eggs/gram respectively. Geometric transformation, to control for aggregation of parasite loads in host populations, revealed a linear decline over the four time periods (p=0.0001, R2=0.8163). The decline in eggs/gram from grazing periods 1 to 2 and throughout the rest of the grazing season is indicative of infective juvenile decline on summer pastures.

Library of Congress Subject Headings

Cattle -- Parasites -- Control Cattle -- Losses -- South Dakota Cattle -- Wintering -- South Dakota Nematoda



Number of Pages



South Dakota State University