Single trait selection for yearling fleece weight or liveweight in Romney sheep—direct responses

Abstract

An experiment was carried out to study the responses to selection for yearling fleece weight and liveweight in Romney sheep. There were two selection lines, one selected for fleece weight (FW) and the other for liveweight (LW), and a control (CO) line was maintained. Data from lambs born 1967-90 were analysed to quantify the direct responses to selection. By 1990 there had been an average of 9 generations of selection with an average generation interval of 2.7 years. Realised heritabilities and selection responses were estimated from the deviation of the selected lines from a control line and by restricted maximum likelihood (REML) techniques. Realised heritabilities were 0.36 ± 0.04 for both fleece weight and liveweight respectively when estimated using the regression of response on selection differential. The corresponding REML estimates of heritabilities were 0.39 ± 0.03 for fleece weight using data from FW + CO and 0.42 ± 0.03 for liveweight using LW + CO. The direct responses to selection, derived from deviation from CO line, were 1.20 kg (39%) and 11.9 kg (28%) for fleece weight and liveweight respectively. The corresponding annual rates of response were 0.047 kg/year (1.5%/year) and 0.53 1 Deceased A93069

Selection response to fleece weight, wool characteristics, and heritability estimates in yearling Romney sheep

“…The HF flock had significantly higher wool growth in winter by 32 and 37% in adult ewes and hogget respectively compared with CR (Wuliji et al, 1995). Heritability estimates for live weights, fleece weight and wool characteristics were in agreement with the reports of Baker et al (1979), Clarke and Rae (1977), Johnson et al (1995), Clarke et al (1995), and the earlier estimates of the flocks (Wuliji et al, 1991).…”

“…He predicted the genetic improvement in a crossbred wool flock of 200 breeding ewes, subject to single trait selection, to be 0.192 kg fleece weight per generation, which was 0.064 kg/year. The average yearly response in fleece weight since establishment in 3 other comparable fleece weight selection Romney flocks in NZ were 0.034 kg in the Massey University line (established in 1956;Blair et al, 1985), 0.047 kg in the Tokanui line (established in 1967;Johnson et al, 1995), and 0.065 kg in the Woodlands line (established in 1973;. Those flocks were selected for a similar environment, flock sizes and selection intensity.…”

“…Though selection objectives involving various traits in sheep have been proposed (Clarke and Rae, 1977), few recommendations were made to select on wool characteristics. Selection response for fleece weight in long wool sheep from random base flocks were estimated (Blair et al, 1985;Johnson et al, 1995). However, intensive screening for a trait in commercial flocks prior to selection is predicted to be advantageous over those flocks established from a random base.…”

Long-term selection experiment with Afrikaner cattle 3. Selection applied and response in calf growth traits

“…Thompson & Juga (1989) showed that the method of Newman et al (1973) is biased downwards as a result of a reduction in variance due to linkage disequilibrium (Bulmer, 1971) and proposed a recursive method to estimate realized heritabilities. Reporting on a selection experiment with sheep, Johnson et al (1995) found lowest realized heritabilities estimated by the method of Newman et al (1973) followed by those estimated using the method of Thompson & Juga (1989) which were similar to, but still lower than, the REML estimates. The low selection response for weaning weight was due to the low REML heritability estimate which was influenced by the presence of sire x year interaction (Beffa et al, 2009b).…”

“…The low selection response for weaning weight was due to the low REML heritability estimate which was influenced by the presence of sire x year interaction (Beffa et al, 2009b). In theory, REML techniques using an animal model and complete relationship matrix yield unbiased estimated of base population parameters (Johnson et al, 1995). However, Meyer & Hill (1991) found that REML analyses did not account for a decrease in genetic variance in the analyses of 23 generations of selection in mice.…”