Genetic and phenotypic correlations among calving difficulty score (CDS) and measures of size of the pelvic inlet were estimated using 547 records of 2-yr- old heifers from three synthetic breed groups. Heritabilities were also estimated, but in this case CDS was treated first as a trait of the dam and then as a trait of the calf. The calving difficulty scores ranged from 0 to 3 (0 * normal birth, 3 - most difficult delivery requiring a hard pull, veterinary assistance or surgical intervention). Analysis of CDS was on three scales: the raw scores, scores transformed by Snell's procedure, and a binary (0, 1) scale. Heritabilities(h2) of CDS on the raw and transformed scores were similar and moderate in magnitude but low on the binary scale, suggesting that it might be easier to reduce the degree rather than the incidence of calving difficulty by selection in these populations. The h2 estimates for CDS as a trait of the dam and as a trait of the calf were similar. Estimates of h2 for the horizontal and vertical pelvic diameters and the area of the pelvic opening were high, implying that pelvic size might be readily modified by selection. The correlations between CDS as a dam trait and pelvic dimensions were all negative and moderate genetically but negative and low phenotypically. As a calf trait, the correlations on all three scales of CDS and calf birth weight were positive and high genetically but positive and low phenotypically.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XV. Beef cattle, sheep and pig genetics and breeding, fibre, fur and meat quality., , 287–290, 1990
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