ABSTRACT
Context. Breech strike compromises wool production and welfare of Merinos. Long tails contribute to the formation of dags, increasing the risk of sheep attracting gravid blowfly females. Tail-docking is popular globally, as it reduces the incidence of dags. Breeding for a reduced tail length is a more socially accepted measure to replace tail-docking as a management strategy, since stronger legislation for animal welfare is expected. Aim. The study used historic data on tail length (TL), birth weight (BW) and bodyweight at docking (DW) to estimate genetic and environmental parameters for these traits in a well known South African resource flock. Methods. Merino lambs born from 2016 to 2021 (except for 2020 due to Covid-19) of the Elsenburg Merino flock were recorded for TL, BW and DW. The flock was separated by divergent selection for and against a number of lambs weaned per ewe mated in a High (H) and a Low (L) line. Fixed effects included birth year, sex, selection line, dam age and birth type. ASReml was used to analyse the fixed effects so as to obtain an operational model, before adding the random direct genetic (h(2)), maternal genetic (m(2)) and maternal permanent environmental (c(2)) effects. Results. H-line lambs were heavier at birth and tail-docking, with longer tails than those of L-line contemporaries (all P < 0.01). The line difference in TL seemed to be size-dependent, as it was eliminated by adding DW as a covariate to the analysis. The same trend was observed for sex, dam age and birth type. TL was thus affected (P < 0.01) only by birth year in the latter analysis. Regressions of TL on age at tail-docking and DW were highly significant (P < 0.01). Single-trait estimates of h(2) were 0.26 for BW, 0.06 for DW, and 0.30 for TL. TL remained heritable at 0.38 in the analysis including DW as a covariate. Including m(2) improved the random- effects model for TL (0.08) and DW (0.20). TL and DW were genetically correlated (rG = 0.47 and rM = 0.70). Conclusions. TL was moderately heritable, with small m(2) and non-significant c(2) effects. It will be possible to directly select for TL. However, selection for shorter tails will reduce size if breeding values are not adjusted for DW as a covariate. Implications. Further studies on the genetic basis of TL as well as its genetic correlations with other traits of economic importance are warranted.