The reproductive efficiency of the entire sheep flock could be improved if ewe lambs go through puberty early and produce their first lamb at 1 year of age. The onset of puberty is linked to the attainment of critical body mass, and therefore we tested whether it would be influenced by genetic selection for growth rate or for rate of accumulation of muscle or fat. We studied 136 Merino ewe lambs with phenotypic values for depth of eye muscle (EMD) and fat (FAT) and Australian Sheep Breeding Values at post-weaning age (200 days) for live weight (PWT), eye muscle depth (PEMD) and fat depth (PFAT). First oestrus was detected with testosterone-treated wethers and then entire rams as the ewes progressed from 6 to 10 months of age. Blood concentrations of leptin and IGF-I were measured to test whether they were related to production traits and reproductive performance (puberty, fertility and reproductive rate). In total, 97% of the lambs reached first oestrus at average weight 39.4 6 0.4 kg (mean 6 s.e.m.) and age 219 days (range 163 to 301). Age at first oestrus decreased with increases in values for PWT (P , 0.001), and concentrations of IGF-I (P , 0.05) and leptin (P , 0.01). The proportion of ewe lambs that achieved puberty was positively related with increases in values for EMD (P , 0.01), FAT (P , 0.05) or PWT (P , 0.01), and 75% of the ewe lambs were pregnant at average weight 44.7 6 0.5 kg and age 263 days (range 219 to 307). Ewe lambs that were heavier at the start of mating were more fertile (P , 0.001) and had a higher reproductive rate (P , 0.001). Fertility and reproductive rate were positively correlated with increases in values for EMD (P , 0.01), FAT (P , 0.05), PWT (P , 0.01) and leptin concentration (P , 0.01). Fertility, but not reproductive rate, increased as values for PFAT increased (P , 0.05). Leptin concentration increased with increases in values for EMD (P , 0.001), FAT (P , 0.001), PWT (P , 0.001), PEMD (P , 0.05) and PFAT (P , 0.05). Many of these relationships became non-significant when PWT or live weight was added to the statistical model. We conclude that selection for genetic potential for growth can accelerate the onset of puberty and increase fertility and reproductive rate of Merino ewe lambs. The metabolic hormones, IGF-I and leptin, might act as a physiological link between the growing tissues and the reproductive axis.Keywords: ewe lambs, phenotypic selection, ASBV, reproductive efficiency ImplicationsGenetic selection can improve the rates of growth and muscle development in sheep, and should also permit reproduction at a younger age because the onset of puberty depends on attainment of sufficient body mass. Our data support this hypothesis, suggesting that phenotypic and genetic selection for growth or muscling will improve the reproductive performance of Merino ewes mated to lamb at 1 year of age. These findings will inform bio-economic models and promote genetic selection strategies with a view to improving profitability of sheep production systems by achieving improvements i...

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In ewe lambs, acceleration of growth and accumulation of both muscle and fat leads to earlier sexual maturity and better reproductive performance. The next stage in the development of this theme is to test whether these aspects of growth in young ewes affect milk production in their first lactation and the growth of their first progeny. We studied 75 young Merino ewes that had known phenotypic values for depth of eye muscle (EMD) and fat (FAT), and known Australian Sheep Breeding Values for post-weaning weight (PWT) and depths of eye muscle (PEMD) and fat (PFAT). They lambed for the first time at 1 year of age. Their lambs were weighed weekly from birth to weaning at 10 weeks to determine live weight gain and weaning weight. Progeny birth weight was positively associated with live weight gain and weaning weight (P0.05). The PWT of the sire was positively associated with live weight gain (P0.05). The concentrations of fat, protein, lactose and total solids in the milk were not affected by the phenotype or genotype of the mothers or of the sires of the mothers, or by the sex of the progeny (P>0.05). We conclude that selection of young Merino ewes for better growth, and more rapid accumulation of muscle and fat, will lead to progeny that are heavier at birth, grow faster and are heavier at weaning. Moreover, milk production and composition do not seem to be affected by the genetic merit of the mother for post-weaning live weight or PEMD or PFAT. Therefore, Merino ewes can lamb at 1 year of age without affecting the production objectives of the Merino sheep industry.