Genetic and environmental effects on lambing and neonatal behaviour of Dormer and SA Mutton Merino lambs

Abstract

Data were accumulated for birth weight, lambing and neonatal behaviour of Dormer and SA Mutton Merino lambs from 1989 to 1998. Numbers of records amounted to 1167–1298 for SA Mutton Merinos and 1036–1118 for Dormers, depending on the trait analysed. Behaviour traits were expressed as time intervals in minutes. Length of parturition was defined as the period between the first definite sign of impending parturition in the ewe to the birth of a specific lamb. In multiples, the birth of the preceding litter mate was regarded as the beginning of parturition for subsequent litter mates. Other observations on lambs included the intervals from birth to standing for >10 s and from standing to apparently suckling. Single lambs were heavier than twins in both breeds, which were heavier than triplets ( P<0.05). Subsequent multiple lambs had shorter parturitions than singles and first-born multiples ( P<0.05). Triplets were slower ( P<0.05) to progress from standing to suckling than singles. Ram lambs were heavier, with longer parturitions than ewes. Lamb birth weight was curvilinearly related to dam age, while length of parturition tended to decrease with an increasing dam age in SA Mutton Merino lambs. Lambs borne by 2-year-old maidens and 7-year-old ewes generally tended to take longer to progress from standing to suckling. Direct heritability estimates ( h 2±S.E.) in SA Mutton Merinos were 0.11±0.05, 0.03±0.04, 0.10±0.05 and 0.08±0.05, respectively, for birth weight, length of parturition, and intervals from birth to standing and standing to suckling, respectively. Corresponding estimates for Dormers were 0.22±0.06, 0.04±0.05, 0.12±0.05 and 0.12±0.06, respectively. The inclusion of the maternal genetic variance ratio ( m 2) resulted in an improvement in the log likelihood ratio for length of parturition in SA Mutton Merino and Dormer lambs, yielding estimates of, respectively, 0.15±0.04 and 0.14±0.04. Maternal permanent environment variance ratios ( c 2) were 0.18±0.03 for birth weight and 0.17±0.04 for the interval from standing to suckling in SA Mutton Merino lambs. A corresponding c 2 estimate of 0.15±0.06 was computed for birth weight in Dormer lambs. On a phenotypic level, length of parturition was positively related to birth weight in both breeds. Corresponding maternal permanent environmental correlations between the traits amounted to 0.44±0.14 in SA Mutton Merinos and 0.47±0.16 in Dormers. Phenotypic correlations of birth weight with neonatal lamb progress were negative in both breeds, but the only significant maternal permanent environmental correlation was an estimate of −0.75±0.23 between birth weight and the interval from birth to standing. Direct genetic correlations between traits rarely exceeded their standard errors. Lambs that succumbed during parturition and prior to weaning generally had higher ( P<0.05) direct and maternal breeding values for length of parturition. Corresponding results were found when derived breeding values for neonatal progress were related to post-parturient deaths, particularly in Dormer lambs. These results are discussed with reference to sheep production.