Relative resistance to gastrointestinal nematode parasites in Dorper, Katahdin, and St. Croix lambs under conditions encountered in the southeastern region of the United States

A study was conducted to evaluate the relative resistance to a natural or induced gastrointestinal (GI) nematode infection in hair and wool breeds of lambs. In Experiment 1, natural GI nematode infection was evaluated among 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 (FECs) and blood packed cell volume (PCV) were determined between weaning and 56 days 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 days of age; April) to 115 +/- 1.3 days of age (June). In both studies, lambs grazed bermudagrass overseeded with ryegrass, previously grazed with sheep, and were supplemented with 225-500 g of corn/soybean meal supplement. In Experiment 3, the relative resistance was examined among DO (n = 8), KA (n = 8), and SC (n = 9) ewe lambs to induced GI nematode infection (20,000 Haemonchus contortus L3). Lambs were weaned at 62 +/- 2.3 days of age, dewormed 22 days later and inoculated 6 days later (Day 0). In Experiment 1, FEC was greater (P < 0.04) 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 (breed x time, P < 0.003). In Experiment 3, FEC was greater in DO and KA lambs compared with SC lambs (P < 0.02). By Day 42, four DO, six KA, and three SC lambs had been dewormed. With a low to moderate challenge, resistance to infection was similar among the three hair breeds examined and greater than the 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. Published by Elsevier B.V.