Single trait selection for yearling fleece weight or liveweight in Romney sheep—correlated responses in liveweight, fleece traits, and ewe reproduction

Abstract

An experiment was carried out to study direct and indirect responses to selection in Romney sheep. There were two selection lines, one selected for greasy fleece weight (GFW) and the other for liveweight (LW), maintained alongside a control line (CO). Data from lambs born in 1967-90 were analysed to quantify the correlated responses to selection. By 1990, there had been an average of nine generations of selection with an average generation interval of 2.7 years. The correlated responses analysed included yearling and ewe greasy and clean fleece weights, washing yield, yearling and ewe fleece quality traits (staple length, mean fibre diameter, fibre diameter variation, loose wool bulk, Commission Internationale de l'Eclairange (CIE) Y value (brightness) and CIE Y-Z value (yellowness)), birth weight, weaning weight, yearling liveweight, ewe pre-mating weight, and ewe reproductive rate (fertility, litter size, weaning percentage, and lamb survival). Correlated responses were estimated as the deviation of selection lines from the CO line, and genetic correlations were obtained by restricted maximum likelihood techniques. Direct responses measured in the GFW and LW yearlings were 1.7 and 1.2% per year, respectively. Correlated annual responses were 0.7% in yearling liveweight in the GFW line and 0.2% in yearling fleece weight in the LW line. Corresponding annual changes in ewe performance were 1.5% in greasy fleece weight and 0.5% in pre-mating weight in the GFW line, and 0.0 and 1.2% respectively in the LW line. Mean fibre diameter increased in yearlings and ewes of the GFW line, but did not change in the LW line. There were small but significant increases in staple length in both lines. Loose wool bulk among GFW yearlings was reduced by 0.12% per year but did not change significantly in ewes. Yearlings and ewes of the LW line both showed an increase in bulk (0.15 and 0.43%, respectively). Relative to the CO flock, CIE Y-Z among yearlings increased significantly by 1.35% per annum and 0.25% per annum in the GFW and LW flocks respectively. Among ewes it increased in the GFW line whereas the opposite occurred in the LW line. There was no significant change in percentage ewes lambing as a result of GFW or LW selection. Litter size increased and lamb survival to weaning decreased significantly, but there was a net increase per year in lambs weaned per ewe lambing (0.27% per year for the GFW line and 0.48% per year for the LW line). A95024

Genetic and environmental factors affecting lamb survival at birth and through to weaning

“…In another experiment at Whatawhata and Tokanui, treating the survival data as a trait of the ewe instead of the lamb, we found that the h 2 for lamb survival was also very low (0.02), and the repeatability was 0.07 (Morris et al 1996). In conclusion, genetic parameters for perinatal and preweaning survival were small under all conditions evaluated, and within-breed opportunities for genetic improvement of survival appear quite limited.…”

“…However, corresponding calf:dam weight data in cattle (Morris et al 1986) were associated with differences in perinatal survival. Selection for yearling liveweight in sheep leads to correlated changes in dam weight (e.g., pre-mating weight) and in BW (Morris et al 1996). Further analyses would be required to test whether different BW optima in Romneys might be associated with genetically different ewe weights.…”

“…These values are required in formulating genetic improvement plans, and for evaluating the relative merits of animals in such improvement programmes. Some of these parameters have been put together by Morris et al ( 1996), but their study did not include lamb survival (as a lamb trait). Lopez-Villalobos & Garrick (1999) reviewed earlier genetic studies, and then used up-to-date statistical methods to analyse direct and maternal genetic parameters for the survival to weaning of Romney lambs in a single flock.…”

Estimated variance components and breeding values for pre-weaning growth criteria in Romney sheep

“…After mating, ewes were separated from rams and kept as one group until the lambing season which usually starts in August to October. (Morris et al 1996;Morris et al 2000;Ibrahim 2015, respectively).…”

Divergent selection for total fleece weight in Angora rabbits: Correlated responses in wool characteristics

“…It is unclear, however, whether a higher fleece weight is associated with an increase in other fleece characteristics (length, diameter, compression and secondary to primary follicle ratio) of Angora rabbits. In sheep, Morris et al (1996) found an unfavourable Available online at www.sciencedirect.com Livestock Science 113 (2008) 8 -13 www.elsevier.com/locate/livsci correlated response in mean fibre diameter when selecting for high fleece weight. In a companion paper, Rafat et al (2007) presented results of a divergent selection experiment on total fleece weight in French Angora rabbits.…”