Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2002
Publication Date: 11/1/2002
Citation: BURKE, J.M., MILLER, J.E. 2002. RELATIVE RESISTANCE OF DORPER CROSSBRED EWES IN GASTRO-INTESTINAL NEMATODE INFECTION COMPARED WITH ST. CROIX AND KATAHDIN EWES IN THE SOUTHEASTERN UNITED STATES. VETERINARY PARASITOLOGY. 109:269-275. Interpretive Summary: Current anthelmintics available to sheep producers, as well as some approved for other species, have become ineffective for controlling internal parasites. Some breeds of hair sheep (St. Croix, Katahdin) possess a greater resistance to these parasites as compared with wool breeds. Resistance of the Dorper breed is unknown. Therefore, parasite resistance among hair and wool breed ewes was examined. The Dorper ewes appeared to be nearly as resistant to infection as the Katahdin and St. Croix breeds and more resistant than the Hampshire (wool) ewes when the challenge was low to moderate. Dorper crossbred ewes maintained good or excellent body condition and demonstrated relative resistance to natural or experimental infection. This information is important to producers and extension agents with an interest in hair sheep.
Technical Abstract: A study was conducted to evaluate relative resistance of Dorper crossbred (DO), Katahdin (KA), St. Croix (SC), and Hampshire (HA) ewes to natural and experimental gastro-intestinal (GI) nematode infection over a 20 month period. The objective of Experiment 1 was to evaluate breeds for resistance to infection acquired naturally from mixed grass pastures. In Year 1 (May to December 2000) deworming of ewes occurred during wet, hot conditions in July and during late pregnancy in December. In Year 2 (January to December 2001), ewes were dewormed after fecal egg count (FEC) for a breed group rose above 1000 eggs per gram (epg) or after blood packed cell volume (PCV) of an individual ewe fell below 20. FEC was determined every 28 d and PCV every 14 to 28 d. In both years, ewes were pastured together, except during the 28 d breeding periods, on tall fescue, bermudagrass, or ryegrass, and rotated among pastures dependent on forage availability. Ewes were in good or excellent condition (body condition score of 3 to 4 out of 5) throughout the study. The objective of Experiment 2 was to evaluate the breeds for relative resistance to an experimental infection with Haemonchus contortus infective larvae. Both PCV and FEC were determined every 7 d from 14 to 42 d after inoculation with 30,000 infective larvae per ewe. In Experiment 1, Year 1, FEC was slightly greater and PCV was lower from July through September in DO ewes (breed x time, P<0.001). In Year 2, deworming occurred 14 d later in DO ewes compared with other breed types. Otherwise PCV and FEC were similar among the hair breeds and higher and lower, respectively, compared with HA ewes (breed x time, P<0.001). In Experiment 2, FEC and PCV were similar among hair breeds; FEC was lower and PCV higher in hair breeds compared with that of HA ewes (P<0.01). Relative resistance of mature Dorper crossbred ewes was comparable to that of Katahdin and St. Croix ewes and superior to that of Hampshire ewes.