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N. McHugh, T. Pabiou, E. Wall, K. McDermott, D. P. Berry, Impact of alternative definitions of contemporary groups on genetic evaluations of traits recorded at lambing, Journal of Animal Science, Volume 95, Issue 5, May 2017, Pages 1926–1938, https://doi.org/10.2527/jas.2016.1344
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ABSTRACT
The objective of this study was to quantify the impact of alternative contemporary group definitions for lambing traits on genetic evaluations in the Irish multibreed sheep population. Three lambing traits were considered for analysis: lambing difficulty, birth weight, and survival. Eight alternative contemporary group definitions were investigated for each lambing trait; all contemporary groups were formed within flock of lambing and included (flock by) week of lambing, week of lambing by litter size (i.e., singles vs. multiples), 2-wk interval (i.e., fortnight) of lambing, fortnight of lambing by litter size, month of lambing, and month of lambing by litter size or were based on an optimized algorithm that creates contemporary groups based on animals from the same flock that are born in close proximity of date. Three alternative scenarios were modeled for each of the lambing traits using the contemporary group definitions: the first scenario (termed Current Scenario) represented the editing criteria currently employed in the Irish national genetic evaluations; the second scenario (No Restriction Scenario) removed any restriction on number of records per contemporary group, and the final scenario (Variation Scenario) included only data from contemporary groups with some variability in the dependent variable. Variance components and EBV for each of the 3 lambing traits were estimated using linear mixed models. The direct heritability estimates ranged from 0.09 ± 0.02 to 0.29 ± 0.02 for lambing difficulty, 0.11 ± 0.01 to 0.24 ± 0.01 for birth weight, and 0.05 ± 0.02 to 0.10 ± 0.02 for lamb survival. Irrespective of lambing trait, greater estimated accuracy of the sire EBV was achieved with the No Restriction Scenario. Results for the ability to predict future lambing characteristics, based on only the direct and maternal EBV, revealed that the area under the receiver operator characteristic curve for the dichotomized lambing assistance phenotype varied from 0.56 to 0.66; a lambing event predicted to be in the worst 10% risk category of a difficult lambing on the basis of genetic merit alone was 5.48 times (95% CI: 3.94 to 7.61; P < 0.001) more likely to require assistance at lambing compared to a lambing event in the best 10%. Results show that the use of contemporary groups formed over short time periods, coupled with moderate editing of the data, yielded superior predictions for all lambing traits.
