Factors Affecting Birth Weight, Dystocia and Preweaning Survival in Sheep

Abstract

Data from 3,535 purebred (PB) and 3,178 crossbred (XB) lambs were analyzed to determine the attention that should be given dystocia and lamb survival in selection programs for terminal sire breeds. Both dystocia and lamb mortality were quadratically related (P<.01) to birth weight. Dystocia was minimal (9 to 15%) at birth weights of about 3.5 kg, whereas mortality was minimal (26 to 30%) at about 5.5 kilograms. Dystocia increased lamb mortality by 8.6 (PB) and 4.8% (XB). Single-born lambs were heavier at birth (4.6 ± .03 vs 3.6 ± .04 kg, PB; 4.7 ± .03 vs 3.6 ± .03 kg, XB) and had fewer deaths (28.6 ± 1.8 vs 47.7 ± 2.3%, PB; 27.7 ± 1.8 vs 46.6 ± 1.7%, XB) than multiple-born lambs. Single-born XB lambs also had more dystocia than multiple-born lambs (13 ± 1.0 vs 8 ± .9%); however, an unexplained breed by type of birth interaction (P<.01) negated the effect of type birth on dystocia of PB lambs. The .13 ± .03 heritability estimate for dystocia was not changed by accounting for differences in birth weight. Large differences in percent lamb mortality, which were not attributable to birth weight or dystocia, were found among Suffolk- (41.9 ± 1.9), Hampshire- (36.8 ± 1.9) and Oxford-sired (32.8 ± 1.8) lambs; however, the .06 ± .03 heritability estimate for lamb survival discourages direct intrapopulation selection for viability. Also, the low heritability estimate for vigor (.10 ± .03) and its low genetic correlation with lamb survival (.16 ± .28) would discourage its use as a selection criterion for improving survival. Selection for increased size and growth rate in terminal sire breeds with little concern about dystocia or preweaning survival seems warranted, especially if they are to be mated with highly prolific ewes. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.