Growth pattern to the end of the mating period influences the reproductive performance of merino ewe lambs mated at 7 to 8 months of age

•A 5 kg greater live weight at the start of the mating period increased the reproductive rate of Merino ewe lambs mated at 7 to 8 months of age by 20%.•Gaining an extra 100 g/day of live weight during the mating period further increased their reproductive rate by 20%.•Ewe lambs from sires with higher breeding values for fat measured post-weaning achieved a higher reproductive rate.•The effects of managing growth pattern until the end of the mating period and sire genetics on fertility and reproductive rate were additive.•Survival of twin born progeny from ewe lambs was lower than the survival of single born progeny (68 vs 93%; P < 0.001).