Genetic and environmental parameters for birth weight and neonatal behaviour of Merino lambs in relation to cold stress

Abstract

Lamb mortality remains a significant welfare and production issue, constraining ethical and sustainable sheep production. Lamb survival benefits from early suckling and colostrum intake, while it is impaired by inclement weather. The effect of cold exposure on neonatal behaviour and progress to suckling in lambs needs to be understood to curb lamb mortality. This study used historic data to establish the relationship of neonatal lamb behaviour with cold stress and derive genetic parameters for early lamb behaviour. Data of Merino lambs from 1993 to 2002 of the Elsenburg Merino flock were assessed for birth weight (BW) and behaviour latencies (intervals in minutes), namely: from birth to first standing for > 10 sec (LTBS) as well as from standing to first suckling for > 10 sec (LTSS) in relation to a cold index (CI). The CI was derived from a combination of wind, rain, and temperature. The flock consisted of a line selected for number of lambs weaned per mating (NLW; the High or H-Line) and a line selected against NLW (Low or L-Line). Fixed linear and random cubic spline components of LTBS and LTSS on the CI and genetic parameters were derived. Overall, H-line lambs were somewhat heavier at birth and progressed faster from standing to first suckling than L-line lambs (P < 0.05). Random spline components as well as interactions of the regression variables with selection line were not significant (P > 0.05) and the analyses reduced to modelling the fixed linear component. Increased levels of cold stress resulted in faster progress in LTBS (P < 0.05). Expressed relative to mild conditions at a CI of 800 kJm⁻²h⁻¹, LTBS was reduced by 17.6 % on the observed scale at a higher CI of 1200 kJm⁻²h⁻¹ (P < 0.05). In contrast, LTSS was compromised at higher CI values (P < 0.05), increasing markedly by 76.0 % from 800 kJm⁻²h⁻¹ to 1200 kJm⁻²h⁻¹. Direct single-trait heritability estimates were 0.16 ± 0.05 for BW, 0^.22 ± 0.07 for LTBS, and 0.06 ± 0.05 for LTSS. The inclusion of the maternal genetic variance ratio resulted in an improvement in the log-likelihood ratio for BW and LTBS, yielding single-trait estimates of 0.37 ± 0.03 and 0.06 ± 0.03, respectively. The single-trait dam permanent environment variance ratio was 0.09 ± 0.03 for LTSS. Further research is required to better understand the responses to cold stress between LTBS and LTSS.