Abstract
Summary - Second and third parity twinning rate of Israeli Holsteins was analyzed by linear (LM) and threshold models (TM). Data were 124 553 calving records of daughters of 179 sires. Twinning rates were 4.8 and 6.9% for second and third parity, respectively. Heritabilities, as estimated by REML for the LM analysis, and the counterpart of REML for the TM analysis, were 2.2 and 10.1%. The correlation betwen LM and TM evaluations was 0.98. Both distributions of sire evaluations were positively skewed, but only the LM distribution differed significantly from normality. Heritability as estimated from the maternal grandsire effect on twinning rate, correlations between sire and maternal grandsire evaluations, intraclass correlations among half-brothers, and son-sire regressions were all consistent with the hypothesis of polygenic additive inheritance. Sire evaluations for twinning rate were economically favorably correlated with evaluations for dystocia. Breeding for twinning rate may be feasible by selection of sires with high repeatability evaluations, and will not result in significant undesirable correlated responses. cattle / twinning / threshold model / selection / additive inheritance
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