Effects of selection for divergent ultrasonic fat depth in rams on progeny fatness

Abstract

Fat depths of Southdown and Suffolk rams were ultrasonically measured several times prior to 18 months of age. Ultrasonic measurements were regressed on live weight and deviations from this regression were standardized and summed to select rams with high and low fat depth to produce crossbred progeny for slaughter. An average of 3-24 standard deviations separated high and low sires. Progeny were slaughtered at two ages to determine differences in carcass fatness and its development. A significant difference of -0-5 mm fat depth over the loin eye muscle was found between progeny of low and high sires. Progeny from Suffolk high sires had greater tissue depth over the rib and higher kidney fat weight than those from Suffolk low sires but differences were small or reversed in Southdown progeny. Low progeny had significantly less estimated carcass chemical fat than high progeny. Fat depth C increased less in low progeny between early and late slaughter ages. However, no difference in partition of carcass weight gain to fat weight gain was observed between high and low progeny. Differences in fat weight between high and low progeny, as well as between Southdown and Suffolk progeny, appeared to have occurred prior to the earlier slaughter age.

Carcass characteristics of progeny from ram lambs selected for high or low ultrasonic backfat thickness

“…The weight-adjusted ultrasonic backfat thickness of the two groups of selected sires differed by around 4 standard deviations and the results of the progeny test indicate that significant progress in breeding leaner lambs can be achieved by selecting sires on this basis. The realised heritability estimates of backfat thickness of 0.24--0.25 are similar to that from an earlier experiment at Invermay (Fennessy et al 1982) and to that calculated from the data of Bennett et al (1988) who used selected Southdown and Suffolk sires. In each of these instances, a different type of ultrasonic device was used, although in the latter situation potential sires were measured on several occasions in contrast to both the present and earlier Invermay experiments where the sires were measured only once.…”

“…Although repeated measurements can be expected to increase the heritability estimate of a trait, especially when repeatability of the measurement is low, the repeatability of the ultrasonic fat measurement using the AIDD-type scanner is regarded as moderate (Purchas et al 1981), and thus only small increases in heritability could be expected with repeated measurement. Other published estimates of heritability of backfat thickness based on slaughter data range from around 0.15 to 0.50, with most being in the 0.2-0.3 range (Botkin et al 1969;Bowman & Hendy 1972;Bradford & Spurlock 1972;Cotterill & Roberts 1976;Wolf et al 1981;Thorsteinsson & Bjornsson 1982;Parratt et al 1987;Bennett et al 1988), so that the present estimate is within the usual range.…”

Selection for lean growth in terminal sire sheep to produce leaner crossbred progeny

“…With slaughter at an age rather than a weight, Bennett et al (1988) and Cameron (1992) found that the carcass composition of a lamb depended on whether its sire originated in a lean or fat line. In both studies, rams were selected on live weight and in vivo measure of fat depth combined to achieve the goal of reduced fat with little change in live or carcass weight.…”

“…In both studies, rams were selected on live weight and in vivo measure of fat depth combined to achieve the goal of reduced fat with little change in live or carcass weight. Bennett et al (1988) recorded the selection criteria repeatedly from weaning until selection at 18 months of age whilst the candidate rams grazed a mixed sward of ryegrass and clover. With lambs slaughtered at an early age (15 or 19 weeks) Bennett et al (1988) found that progeny of lean rams had less fat weight; these lambs, however, did not differ in lean weight.…”

“…Bennett et al (1988) recorded the selection criteria repeatedly from weaning until selection at 18 months of age whilst the candidate rams grazed a mixed sward of ryegrass and clover. With lambs slaughtered at an early age (15 or 19 weeks) Bennett et al (1988) found that progeny of lean rams had less fat weight; these lambs, however, did not differ in lean weight. At a later age (26 or 28 weeks), the initial difference in fat remained.…”