Effects of Selecting Growing Male Hair Sheep of Different Flocks for Internal Parasite Resistance on Performance

Katahdin, Dorper, and St. Croix male lambs from farms (2 Katahdin, farms A and B; 1 Dorper and 1 St. Croix) in the south-central US were categorized as resistant (R), moderately resistant (M), and susceptible (S) to Haemonchus contortus based on artificial larvae challenge in a central performance test at Langston University over 3 consecutive years. Animal groups consisted of 17, 15, and 15 Katahdin-A (initial age 3.9 mo and 38.8 kg); 18, 7, and 8 Katahdin-B (3.7 mo, 18.9 kg); 20, 15, and 16 Dorper (5.5 mo, 34.2 kg); and 13, 14, and 19 St. Croix (4.2 mo, 19.2 kg) in yr 1, 2, and 3, respectively. Males were randomly selected in yr 1, whereas progeny of R and M sires were evaluated in yr 2 and 3. The test entailed 2 wk of adjustment and 8 wk of data collection, with free access to a 15% CP and 50% concentrate diet in automated feeders. During adaptation, anthelmintic treatment resulted in low fecal egg count (FEC; <600 eggs/g), after which 10,000 infective larvae were administered orally. Body weight and packed cell volume (PCV) were measured weekly, and FEC was determined 4 to 5 times in wk 6-8. The cubic clustering criterion of SAS^® was used for resistance categorization, which resulted in 49, 35, and 37 R; 38, 33, and 39 M; and 28, 17 and 36 S in yr 1, 2, and 3, respectively. The statistical model included animal group, resistance classification, year, interactions, and covariates; GENMOD of SAS^® was used for mean FEC. There were interactions (P<0.05) in mean FEC between animal group and resistance classification (R: 473, 928, 1089, and 297; M: 1793, 3058, 2199, and 1084; S: 4198, 5073, 3164, and 2176 eggs/g; SEM=144.7) and between animal group and year (yr 1: 1573, 2261, 3196, and 1388; yr 2: 2417, 4793, 1932, and 1006; yr 3: 2475, 2005, 1325, and 1163 eggs/g for Katahdin-A, Katahdin-B, Dorper, and St. Croix, respectively; SEM=146.0). The PCV ranked (P<0.01) R>M>S (29.9, 28.0, and 26.9%; SEM=0.25). Intake of DM, ADG, and ADG:DMI were similar among resistance classifications (P>0.05) and were not correlated with FEC or PCV (P>0.05). In conclusion, hair sheep can be selected for resistance to internal parasites without adversely affecting growth performance, and selection progress appeared greatest for the Dorper flock though FEC were relatively low for the St. Croix farm.