Growth, body composition and body wrinkle are favourably correlated with reproductive performance in 2-8-year-old Merino sheep
Authors: Chapman J. A. , Hebart M. L. , Brien F. D. 
Published: August 2021

Context Despite the increased economic importance of reproductive rate in Australian Merinos, there have been low genetic improvements in reproductive performance over the past several decades. Genetically improving reproductive traits through direct selection is restricted by low heritability, low selection intensity directly on reproductive traits, high recording costs and lack of accurate maternal pedigrees in the majority of industry breeding programs. However, indirect selection via correlated traits may be useful as a supplement to direct selection or even on its own where reproduction records are not collected. Aims The aims of the present study were to determine whether various production and quality traits have the potential to be used as indirect selection criteria for genetically improving lifetime reproductive rate, but also whether their selection could be detrimental to reproduction via unfavourable correlations. Methods Reproductive traits studied included pregnancy rate (ewes scanned pregnant per ewe joined), fetal number (number of fetuses scanned per ewe joined), number of lambs born (per ewe joined), number of lambs weaned (per ewe joined) and ewe rearing ability (number of lambs weaned per fetuses scanned). Using data from the SA Selection Demonstration Flocks project (1997–2005), reproductive traits were modelled against various visual and production traits to estimate correlations. Key results Favourable genetic correlations with reproductive traits were estimated with adult ewe bodyweight (0.37–0.50), hogget eye muscle depth (HEMD, 0.40–0.57), fat depth (HFAT, 0.27–0.48) and hogget body (HBWS) and neck wrinkle scores (–0.13 to –0.50). However, the estimates for genetic correlations with hogget eye muscle depth and fat depth were lower when bodyweight was fitted as a covariate, being 0.11–0.35 and 0.17–0.32 respectively. Genetic correlations with ewe rearing ability were generally unfavourable (except those with hogget body and neck wrinkle scores, which were –0.24 and –0.15 respectively), but were either negligible (adult ewe bodyweight, hogget eye muscle depth, fibre diameter and fleece weight) or low in magnitude(fat depth). Unfavourable genetic correlations were observed between fibre diameter and reproductive traits (0.13–0.33). Fleece weight had negligible genetic correlations with all reproductive traits studied (–0.08 to 0.10). Predictions of response to index selection using indirect criteria of one or more of yearling weight, HBWS, HEMD and HFAT measurements projected substantial genetic gains in the number of lambs weaned per ewe joined (NLW). Without reproduction records on the dams of candidates for selection, indirect selection using all four indirect criteria (yearling weight, HBWS, HEMD and HFAT) was predicted to achieve 112–168% of the genetic gains of direct selection for NLW. When all indirect and direct criteria for NLW are combined as part of index selection, even larger gains for NLW are predicted (from 164–215% of direct NLW genetic gains). Conclusions Findings from the present study suggest that bodyweight, HEMD and HFAT and HBWS could be potentially beneficial as indirect selection criteria for lifetime reproductive rate. Selection for reduced fibre diameter is potentially detrimental to reproductive performance; however, selection for improved fleece weight is unlikely to have any effect on genetic gain for reproductive traits. Implications Use of indirect selection for lifetime reproductive could allow for genetic gain when either used with or without the direct selection criteria of reproduction records.