Submitted to: Small Ruminant Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2003
Publication Date: 8/1/2004
Citation: Burke, J.M., Miller, J.E. 2004. Resistance to gastrointestinal parasites in Dorper, Katahdin, and St. Croix lambs in the southeastern United States. Small Ruminant Research. 54:43-51. Interpretive Summary: Current anthelmintics available to sheep producers, as well as some approved for other species, have become ineffective for controlling internal parasites. Lambs are very susceptible to internal parasites. Some hair sheep (St. Croix) possess a greater resistance to these parasites compared with wool breeds. Resistance of Dorper and Katahdin lambs is unknown. Therefore, parasite resistance among hair and wool breed lambs was examined and found to be similar among hair breeds when the parasite challenge was relatively low. Dorper lambs tolerated the infection more so than Suffolk (wool) lambs when fecal egg counts were low. However, as the challenge escalated, parasite resistance was greatest in St. Croix lambs, followed by Katahdin and Dorper lambs. This information is important to producers and extension agents with an interest in hair sheep.
Technical Abstract: A study was conducted to evaluate resistance to a natural or induced gastrointestinal (GI) nematode infection in hair and wool breeds of lambs. Experiment 1 evaluated parasite resistance of fall born Dorper crossbred (DO; n = 17) or St. Croix (SC; n = 17) lambs. Ewe and wether lambs were weaned at 84 ± 1.1 days of age and dewormed. Fecal egg counts (FEC) and blood packed cell volume (PCV) were determined between weaning and 56 d later. In Experiment 2, natural GI nematode infection was evaluated among spring born DO (7/8 or 3/4 Dorper; n = 24), Katahdin (KA; n = 26), SC (n = 8), and Suffolk (SU; n = 10; wethers only) ewe and wether lambs from weaning (60 ± 1.3 d of age; April) to 172 ± 1.3 d of age (August). Lambs were dewormed if the breed average FEC was 1000 eggs/g. Treatment for all breeds occurred in early and late June. In both studies, lambs grazed bermudagrass overseeded with ryegrass, previously grazed with sheep, and were supplemented with 225 to 500 g of corn/soybean meal supplement. Experiment 3 examined resistance of DO (n = 8), KA (n = 8), and SC (n = 9) ewe lambs to induced GI infection (20,000 L3 stage Haemonchus contortus). Lambs were weaned at 62 ± 2.3 d of age, dewormed 22 d later and inoculated 6 d later (Day 0). In Experiment 1, FEC was greater (breed x time, P<0.09) at weaning in DO lambs and PCV was similar between DO and SC lambs. In Experiment 2, FEC was greatest and PCV least in DO lambs when compared with other breeds. After deworming, PCV was lowest in SU lambs (breed x time, P<0.001). In Experiment 3, on Day 28, FEC was greater (P<0.05) in DO and KA lambs compared with SC lambs. By Day 42, 4 DO, 6 KA, and 3 SC lambs had been dewormed. With a moderate biological challenge, parasite resistance was similar among the three hair breeds examined and greater in hair than wool breed lambs. When the challenge escalated, St. Croix lambs were most resistant and Dorper crossbred lambs were similar or less resistant than Katahdin lambs.