1922
Effects of hair sheep breed on performance response of ram lambs to artificial infection with Haemonchus contortus
Twelve Dorper (D; 4.5 ± 0.44 mo, 31.9 ± 1.75 kg), 18 Katahdin (K; 3.8 mo, 24.3 ± 0.76 kg), and 12 St. Croix (C; 4.5 ± 0.17 mo, 19.7 ± 0.99 kg) ram lambs were used to investigate among and within breed differences in the first year of a centralized test for growth performance and response to artificial infection with Haemonchus contortus. Rams were randomly selected from 3 commercial farms in Missouri and Oklahoma. The test at Langston University entailed an adjustment period of 2 wk followed by 8 wk of data collection. Breeds were housed separately in adjacent pens with automated feeders allowing free-choice access to a 15% CP (DM) and 50% concentrate pelletized diet. During adaptation, anthelmintic treatment resulted in low fecal egg count (FEC; < 550/g), after which 10,000 infective larvae were administrated orally. Packed cell volume (PCV) was measured weekly and FEC was determined 4 times in wk 6-8. For analysis, initial BW and FEC were covariates, and the logarithmic transformation ln(x + 2,000) was used for mean FEC (MFEC). Variability in MFEC and mean PCV (MPCV) was homogenous among breeds. Breed affected (P ≤ 0.01) DMI (2.5, 2.2, and 1.9 kg; SEM = 0.10), MFEC (3,431, 1,273, and 1,241 eggs/g, original scale; SEM = 90.7), and MPCV (29.1, 29.7, and 32.9% for D, K, and C, respectively; SEM = 0.68). Residual feed intake (RFI), ADG, and ADG:DMI were similar (P > 0.05) among breeds. Rams were categorized into 3 groups within breeds based primarily on MFEC and MPCV (High, Medium, and Low resistance) using the cubic clustering criterion of SAS, which resulted in unbalanced numbers in the groups (5, 5, and 2 for D, 12, 5, and 1 for K, and 8, 1, and 3 for C, respectively). Group means were similar (P > 0.05) in ADG, DMI, and RFI but varied (P < 0.05) in MFEC (627, 2,137, and 3,302 egg/g; SEM = 109.7) and MPCV (32.3, 30.2, and 29.2% for High, Medium, and Low, respectively; SEM = 0.72). In conclusion, D appeared less resistant than C or K based on MFEC after an artificial challenge with H. contortus larvae in a standardized environment. However, variability in MFEC and MPCV within breed was sufficient to allow assignment to different classes for use in a breeding program to enhance flock resistance.
Keywords: internal parasitism, resistance, sheep