SELECTION FOR HIGH OR LOW BACKFAT DEPTH IN COOPWORTH SHEEP - JUVENILETRAITS

A selection experiment was established in Coopworth sheep in 1981 to b reed for increased or reduced ultrasonic backfat depth (scan C). Found ation females came from four flocks recorded for scan C and live weigh t, with weight-adjusted scan C data within flock being used for initia l screening and subsequent selection. Three groups of animals per sour ce pock, comprising proportionally the fattest 0.12, a random sample, and the leanest 0.12, were used to establish the F, control and L line s, respectively. Ewe flock numbers from 1981 to 1992 averaged 51 per l ine. Foundation rams were selected in the same manner from four differ ent farms (two sources each in 1981 and 1982) to provide F line (propo rtionally the fattest 0.04), control line and L line (leanest 0.04) ra ms for use in 1982 and 1982, with four mated per line per year. Therea fter homebred rams were selected, with 41 or 42 homebred rams being us ed per line until 1992. Average generation intervals were 2.13 years a nd annual inbreeding rates per line 0.004. Bivariate heritability esti mates for log scan C and log live weight, and a univariate estimate fo r log scan C using restricted maximum likelihood with an animal model, were 0.28, 0.22 and 0.38, respectively (all with s.e. 0.03). There wa s a suggestion of lower heritabilities in the L line for log scan C af ter adjustment for live weight. Realized heritabilities in the F and L lines were 0.34 (s.e. 0.07) and 0.26 (s.e. 0.03), respectively. Devia tions of back-transformed weight-adjusted scan C in the last 2 years o f F and L data analysed (1991 and 1992 birth years) from the control f locks were 2.08 and -0.85 mm, which corresponded to responses of +2.50 and -1.03 phenotypic standard deviations, respectively In addition th ere were relatively large responses in live weight taken at scanning, with F and L lines averaging 34.0 and 40.2 kg, compared with 38.0 kg f or controls in the 1991 and 1992 birth years. The genetic and phenotyp ic correlations between log scan C and log live weight at scanning wer e 0.16 (s.e. 0.07) and 0.46 (s.e. 0.01) respectively.