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An experiment was conducted to test 3 hypotheses regarding the variation in plasma parameters and their association with lamb growth and composition. The experiment tested whether sire genetics and available nutrition affected specific plasma traits, whether these traits were associated with growth and compositional characteristics, and whether data on plasma traits could improve the prediction of growth and carcass composition. Lambs (n = 103) produced by sires differing in estimated breeding values for growth, muscling, and fat were reared on LOW or HIGH planes of lifetime nutrition. Blood samples were collected immediately prior to weaning and before slaughter at 8 months of age. Concentrations of plasma creatinine (CRE), urea (PUN), insulin like growth factor I (IGF-I) were measured in all samples. Leptin was measured in samples from male lambs only (n = 56). HIGH nutrition increased preweaning and preslaughter IGF-I and leptin concentrations; nutritional effects on PUN and CRE were inconsistent. Lambs of the high muscle sire-type exhibited increased plasma IGF-I, reduced PUN, and an increased PUN : CRE ratio preweaning. Increased genetic potential of the sire for growth reduced PUN and increased PUN : CRE ratio. Plasma IGF-I correlated strongly with lamb growth rate (r2 = 0.49). Interaction of nutrition and sire-type for preslaughter leptin mimicked the interaction occurring for carcass fat, providing an example of selection on genotype inducing a physiological (hormonal) change and resulting in a modified phenotype. Stepwise regressions developed to predict carcass protein and fat mass from information available at weaning or at slaughter indicated that inclusion of plasma parameters in these models caused only small increases in the proportion of variance accounted for. The most significant benefit was an increase in the proportion of variance in carcass weight accounted for (from 65 to 73%) by including IGF-I and CRE as predictors compared with prediction from weaning weight alone. If slaughter data were available, then >90% of the variance in carcass fat and protein masses was explained by carcass weight alone and this was not increased by inclusion of any preslaughter plasma parameter in the model. It was concluded that while nutrition and the genetic capacity of the sire (especially for muscling) affect plasma constituents as well as lamb growth and carcass composition, the inclusion of the measured plasma traits in models to predict final carcass composition from weaning data or slaughter data is unlikely to improve the prediction sufficient to justify the additional measurement cost.

This paper provides an introduction to the special edition of the Australian Journal of Agricultural Research dedicated to an integrated research program to understand the effects of genetic selection for live animal traits (growth, muscle, and fatness) that determine carcass and meat attributes. Combined with this are the effects of nutritional status of the dam and growing lamb. The reasons for, and general approach of, the research is outlined.

Carcasses from 56 lambs representing male progeny of 3 sires selected for muscling (M sire-type), 3 sires selected for postweaning weight (G sire-type), and 3 control sires (C sire-type) were evaluated. Lambs had been raised on low (LOW) or high (HIGH) planes of available nutrition from 10 days of age to approximately 8 months when they were slaughtered at an average cold carcass weight (CCW) of 21.4 kg. When adjusted for CCW, M lambs had more lean tissue in the loin, a greater depth and width of the M. longissimus thoracis et lumborum at the 12th rib, and a greater weight of major hindlimb muscles than did G or C lambs. Although there was no difference in GR tissue depth due to sire-type at an adjusted CCW, there was less total fat in the carcass of M lambs and the amount of fat in the carcass of M lambs on HIGH nutrition was not greater than that on LOW nutrition, as it was for C and G lambs. This reduced propensity of M lambs to deposit fat in the carcass in response to HIGH nutrition was particularly evident in the loin, with fat-trim from the loin decreasing for M lambs in response to HIGH nutrition, whereas fat trim increased for C and G lambs compared at an adjusted CCW.

This is the first paper in a series that reports on a national maternal sire central progeny test program (MCPT) to evaluate the genetic variation for economically important production traits in first- and second-cross progeny of maternal and dual purpose (meat and wool) sires and the scope for genetic improvement. The MCPT program also provides direct linkages between breeds that will contribute information for across-breed genetic evaluation. Crossbred progeny by 91 sires from more than 7 maternal breeds (including Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, and Booroola Leicester) at 3 sites over 3 years with 3 link sires in common at each site and year were evaluated in this study. National estimated breeding values for the sires that were entered by industry breeders indicated that there was some selection for weight, but not other traits and the sires were generally representative of the maternal genetics available to the industry. There were 8377 base Merino and Corriedale ewes artificially inseminated with thawed-frozen semen from the 91 sires and 8193 crossbred lambs born and 6117 lambs weaned. Mixed model analyses of birth and weaning weight, lamb survival, and gestation length as well as fertility, litter size, and lamb weaning rate of the base ewes are reported. Sire breed was significant for birth weight (with a range of 4.0–4.4 kg at an average litter size of 1.8) and weaning weight (19.6–22.5 kg), as well as lamb survival (70–79%) and gestation length (147.1–150.3 days). Birth/rearing type was significant for all traits. Males were significantly heavier than females at birth and weaning, although there was no sex difference for lamb survival or gestation length when birthweight was included in the model. Pregnancy rate from the artificially inseminated base ewes varied from 40 to 84% at the 3 locations in the various years. Sire breed was significant for the number of lambs weaned per ewe lambing, but not for fetal number or litter size born. There were also significant differences in performance due to the source of the base ewes at each site.

The study evaluated post-weaning growth, carcass characteristics, and wool production of crossbred progeny of 91 sires from more than 7 maternal breeds (including Border Leicester, East Friesian, Finnsheep, Coopworth, White Suffolk, Corriedale, and Booroola Leicester). The sires were joined to Merino and Corriedale ewes at 3 sites over 3 years with 3 link sires in common at each site and year. Post-weaning weight at an average age of 200 days of 2841 ewes and 3027 wethers was analysed using mixed model procedures. The wethers were slaughtered at an average age of 214 days and carcass weight, fat, muscle, meat colour, and ultimate pH (24 h post-slaughter) were analysed. For ewes, hogget fleece weight, yield, fibre diameter, and faecal worm egg count (FEC) were analysed. Sire breed was significant (P < 0.01), with a range of 32.8–39.0 kg for post-weaning weight, 19.1–22.8 kg for hot carcass weight, and for carcass fat levels (11.1–17.2 mm at the GR site (FatGR) and 3.4–6.5 mm at the C site, adjusted to 22 kg carcass weight), with the East Friesian cross carcasses being very lean (FatGR 11.1 mm). Sire breed was significant for eye muscle area (P < 0.01) but not for eye muscle depth, meat colour L* (brightness), or ultimate pH. Sire breed was significant (P < 0.01) for greasy and clean fleece weight (CFW), yield, and fibre diameter (FD), with ranges of 0.9 kg (CFW) and 4.6 μm (FD). Sire breed was not significant for FEC. Type of birth and rearing classification was significant for most traits. National estimated breeding values for the sires that were entered by industry breeders indicated that they covered a range of genetic merit for most traits within their respective breeds and were generally representative of the maternal genetics available in the industry. Where there were large differences between the sire mean and breed mean breeding values the effects on crossbred progeny performance would be small and not expected to affect our conclusions about the breed differences reported. The results provide lamb producers with comparative information on sire breeds for growth, carcass and wool traits. The considerable variation among individual sires within the respective breeds will be reported in later papers.

The study was conducted to estimate heritabilities and genetic correlations for liveweight, fat depth at the C site and eye muscle depth, width and area measured by B mode ultrasound on live young sheep. Records on 4338 animals were available for analysis, representing 161 sires and 79 managementlsex groups. The management groups were allocated to young (5-9 months), medium (10-13 months) and old (14-18 months) data sets for analysis, representing 89, 82 and 19 sires respectively. The heritabilities of liveweight, ultrasonic fat depth and ultrasonic eye muscle measurements were estimated after adjusting for managementlsex group, birth and rearing type, age class of dam and age of the animal. The heritability estimates for the three age data sets respectively were: for liveweight 0.19 � 0.06, 0.11 � 0.06 and 0.37 � 0.18; for fat depth 0.01 � 0.03, 0.18 � 0.07 and 0.28 � 0.15; and for eye muscle depth 0.15 � 0.05, 0.11 � 0.06 and 0.l6 � 0.11. After adjusting for sex/management group and liveweight within sex (the other factors listed above being no longer significant), heritability estimates were: for adjusted fat depth 0.11 � 0.05, 0.33 � 0.09 and 0.29 � 0.16 and for adjusted eye muscle depth 0.l5 � 0.06, 0.l5 � 0.06 and 0.19 � 0.12. The heritability estimates for eye muscle area, estimated as the area of an ellipse with dimensions equal to the measured depth and width, were about 10% lower than for eye muscle depth, but the genetic correlations with depth, in each case were 0.94 or higher. The heritability estimates for eye muscle width were less than 0.05 in young and medium age animals and 0.12 to 0.15 in old animals with high genetic correlations with depth which were not different from 1.00. Ultrasound measurements of fat depth at the GR site were available on 2065 lambs under 8 months of age representing 76 sires in 33 groups. The heritability estimates for fat depth adjusted for liveweight, at the GR and C sites in these lambs were both 0.10 � 0.05 with the genetic correlation between sites being 0.76 � 0.19. There is potential for including eye muscle depth in selection indices to improve leanness. Ultrasound measurement of eye muscle width and area will not improve prediction of eye muscle over that attained by measuring eye muscle depth. Predicted responses in liveweight, fat depth and eye muscle depth for five breeding objectives for terminal sires, using the appropriate LAMBPLAN selection indices, are shown and discussed.