Genetic evaluation of ewe productivity and its component traits in Katahdin and Polypay sheep

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Virginia Tech

The objectives of this dissertation were to evaluate genetic influences on ewe productivity, its growth and reproductive components, and measures indicative of accelerated lambing performance. Genetic parameters were estimated for total weight of litter weaned per ewe lambing (TW) and its components, number of lambs born (NB), number of lambs weaned (NW) and average weight of lambs weaned (AW), measured as traits of the ewe, and lamb survival (LS) and weaning weight (WW), measured as traits of the lamb, in Katahdin sheep. Heritabilities of TW, NB, NW, and AW, were 0.12, 0.12, 0.09, and 0.13, respectively. Heritability of WW was 0.15 to 0.20. Genetic effects on LS were negligible. Genetic correlation of TW with NB, NW, and AW averaged 0.30, 0.90, and 0.74, respectively, those of NB with NW and AW averaged 0.72 and 0.01, respectively, and that between NW and AW averaged 0.50. Direct genetic effects on WW were independent of NB and NW, but correlation between maternal genetic effects on WW and animal genetic effects on NW averaged 0.35.

Ewe fertility, NB, LS, and WW were modeled using stochastic simulation and used to derive NW, AW, and TW to test alternative predictors of genetic merit for TW. A random 8% of WW observations were set to missing values and AW and TW were recalculated to evaluate the effects of data reporting inconsistencies on efficacy of different prediction strategies. Four alternative predictors of estimated breeding values (EBV) for TW involved direct univariate prediction (TW1), an index of EBV for NW and AW (TW2), indirect prediction using data for NW and AW and genetic correlations among NW, AW and TW (TW3), and indirect prediction augmenting TW3 with data and genetic correlations involving NB (TW4). To validate efficacy of predictors, daughter data sets were generated from the original ewes and their realized TW were regressed on alternative predictors. Regression coefficients from TW1, TW3, and TW4 were close to the expected value of 0.50 whereas those from TW2 were less than 0.50. Model Rsquare statistics were similar among predictors when there were no missing WW data but regressions involving TW1 had lowest model R-square when some WW data was missing.

Ewe lamb fertility (ELF), ages at first, second, and third lambings (AGE1 to AGE3), first and second lambing intervals (INT1 and INT2), and number of lambings by 38 mo of age (LAMB3) were evaluated for an accelerated lambing Polypay flock. Relationships among these traits and NB and WW were estimated. Heritability of ELF, AGE1, AGE2, AGE3, INT1, INT2, and LAMB3 were 0.14, 0.39, 0.28, 0.36, 0.00, 0.09, and 0.27, respectively. Heritability of AGE2 and AGE3 were negligible after accounting for variation in AGE1. Genetic correlations of ELF with AGE1 and AGE2 were -0.89, -0.91, respectively, and that with LAMB3 was 0.89. Genetic correlations of LAMB3 with AGE1 and AGE2 were -0.49 and -1.00, respectively. Genetic correlations of ELF and LAMB3 with direct genetic effects on WW were close to -0.70, but correlations with maternal genetic effects on WW were 0.88 and 0.58, respectively. Prolificacy was independent of ELF and LAMB3.

Ewe Productivity, Fertility, Accelerated lambing, Sheep, Heritability, Reproduction