Title page for ETD etd-03052003-105320


Type of Document Master's Thesis
Author Vanimisetti, Hima Bindu
Author's Email Address vbindu@vt.edu
URN etd-03052003-105320
Title Genetics of resistance to Haemonchus contortus infections in sheep
Degree Master of Science
Department Animal and Poultry Sciences
Advisory Committee
Advisor Name Title
Notter, David R. Committee Chair
Greiner, Scott P. Committee Member
Lewis, Ronald M. Committee Member
Zajac, Anne M. Committee Member
Keywords
  • Resistance
  • Haemonchus contortus
  • Sheep
  • Heritability
Date of Defense 2003-02-20
Availability unrestricted
Abstract
Genetic control of resistance to H. contortus was assessed in 198 ewes and 386 lambs of 50% Dorset, 25% Rambouillet and 25% Finnsheep breeding in fall and spring over 2 yr. After deworming, lambs that were approximately 120 d old and ewes that had weaned their lambs at 60 d and dried off were individually dosed with approximately 10,000 infective larvae. After infection, body weight (BW), fecal egg counts (FEC) and packed cell volume (PCV) were measured weekly for 7 wk in lambs and fortnightly for eleven wk in ewes. Summary traits were defined as initial PCV, mean BW (MBW) across all times, and means for FEC, log-transformed FEC (MLFEC), and PCV (MPCV) at wk 3 to 7 post-infection for lambs and wk 3 to 11 post-infection for ewes.

No consistent seasonal variation in FEC was observed. Younger ewes were more susceptible to infection than older ewes. Sex differences in FEC were not observed in lambs.

Heritabilities for summary traits were estimated from a REML analysis that included fixed effects of year and season plus effects of either sex (for lambs) or age category (for ewes). Heritability estimates for MBW, MPCV, and MLFEC were 0.74, 0.57, and 0.27 respectively (all P < 0.01), in lambs, and 0.24 (P < 0.1), 0.25 (P < 0.05) and 0.55 (P < 0.01), respectively, in ewes. Across-year repeatability estimates in ewes for MBW, MPCV, and MLFEC were 0.83, 0.54 and 0.56, respectively (all P < 0.01). Resistance was antagonistically associated with estimated breeding values for growth in ewes but not in lambs. Fertility and prolificacy in ewes were not related to resistance.

Breed differences in resistance to H. contortus were also evaluated in 4 to 6-mo-old crossbred Dorset and Dorper, straightbred Katahdin, and Barbados Blackbelly x St. Croix lambs. Dorpers were not more resistant than Dorsets but appeared to cope better, with higher PCV and similar BW during infection compared to Dorsets. Katahdin and Barbados Blackbelly x St. Croix lambs were more resistant with lower FEC.

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