CSIRO PUBLISHING www.publish.csiro.au/journals/ajar Australian Journal of Agricultural Research, 2006, 57, 627–639 Lamb myofibre characteristics are influenced by sire estimated breeding values and pastoral nutritional system P. L. Greenwood A,C , G. E. Gardner B , and R. S. Hegarty A Australian Sheep Industry Cooperative Research Centre, Armidale, NSW 2350, Australia. A NSW Department of Primary Industries Beef Industry Centre of Excellence, University of New England, Armidale, NSW 2351, Australia. B School of Rural Science and Natural Resources, University of New England, Armidale, NSW 2351, Australia; Present address: School of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, WA 6150, Australia. C Corresponding author. Email: paul.greenwood@dpi.nsw.gov.au Abstract. The objective of this study was to establish the extent to which lamb myofibre characteristics at ∼8 months of age were influenced by sire (n = 9) estimated breeding values (EBVs) and pasture-based nutritional systems (low and high quality and availability, LOW and HIGH) from birth to slaughter (n = 56). Immunostaining of myosin heavy chain isoforms in longissimus muscle fibres revealed that as the post-weaning eye muscle depth EBV (PEMD) increased, the percentage of type 2B/X myofibres increased and the percentage of type 2A myofibres decreased. The percentage of type 2B/X myofibres also increased with increasing PEMD in the semimembranosus muscle, but not in the semitendinosus muscle. Post-weaning fat depth (PFAT) EBV was negatively related with the percentage of type 2B/X myofibres in longissimus and semitendinosus muscles and with the relative area of more glycolytic to more oxidative fibre types, although there was an interaction with nutritional system in the semitendinosus muscle, with this ratio increasing with the PFAT EBV among the LOW nutrition lambs. Overall, post-weaning weight (PWWT) EBV had less effect on myofibre characteristics than PEMD and PFAT EBVs. Average cross- sectional area of myofibres was more affected by the LOW than HIGH nutritional system in longissimus muscle (-25%) compared with semitendinosus (-21%) and semimembranosus (-15%) muscles. LOW nutrition resulted in a significantly smaller cross-sectional area of all myofibre types in longissimus muscle and of types 2A, 2AB/X, and 2B/X in semimembranosus and semitendinosus muscles. Overall, the relative area of glycolytic myofibres increased and/or more oxidative myofibre types decreased in LOW compared with HIGH nutritional system lambs in longissimus and semimembranosus, but not in semitendinosus muscle. The results demonstrate significant influences of sire EBVs on myofibre characteristics of lambs, the extent of which depends upon muscle type and nutritional regimen. More specifically, they suggest continued selection for increased muscling and reduced fatness in lambs are associated with shifts in the proportions of myofibre types, with potential for adverse effects on eating quality. They also show that the direction and magnitude of the effects of chronic, moderate postnatal nutritional restriction at pasture on myofibre characteristics vary between muscles with different contractile, metabolic, and/or functional characteristics. Additional keywords: sheep, meat, muscle, genotype, growth. Introduction Myofibre characteristics of lambs may differ with age (White et al. 1978; Suzuki and Cassens 1983), growth rate, live or carcass weight, gender, and breed or selection lines (Moody et al. 1970, 1980; Solomon et al. 1981; Kadim et al. 1993; Hawkins et al. 1985; Arnold and Meyer 1988). Specific genes associated with muscle hypertrophy such as Callipyge (Koohmaraie et al. 1995; Carpenter et al. 1996; Lorenzen et al. 2000) and Carwell (Greenwood et al. 2006), and acute, severe nutritional restriction (Suzuki 1973) or nutritional regimens associated with different production systems (Moody et al. 1980; Arnold and Meyer 1988; Greenwood et al. 2006) have also been shown to influence the percentage and size of muscle fibre types. Despite this research, there is no information on the extent to which genotypic effects on muscle fibre characteristics of lambs relate to differences in estimated breeding values (EBVs) for traits such as muscling, growth, and fatness. Furthermore, the extent to which EBV-related effects on myofibre characteristics may be modified by environmental influences such as nutrition, and whether such influences © CSIRO 2006 10.1071/AR04318 0004-9409/06/060627