The Effect of Age of Dam and Birth Rank on the Reproductive Performance of Ewes as One- and Two-Year-Olds

SIMPLE SUMMARY: Lambs that are born to ewe lambs are not commonly selected as replacement ewes for sheep flocks in New Zealand, as they are born smaller and later than lambs born to mature ewes, hindering their chances of being heavy enough to breed as ewe lambs themselves. By selecting lambs born to ewe lambs, farmers can utilize animals that are available, and potentially increase genetic gain and ewe production efficiency. This experiment found that lambs born to ewe lambs are lighter than those born to mature ewes until two years of age, but have similar body condition scores, and rates of lamb production, if they are heavy enough to be bred as ewe lambs themselves. Therefore, farmers could select lambs born to ewe lambs that are heavy enough to achieve breeding targets. ABSTRACT: Currently, 30–43% of New Zealand sheep farmers breed their ewe lambs, but few retain the offspring as replacements for their flock. No difference in lamb production as a yearling among singletons and twins born to ewe lambs and twins born to mature ewes has been reported, provided the ewe lambs had reached the 60–65% of their likely mature weight prior to breeding at seven to eight months of age. The aim of this experiment was to determine the lamb production from singletons and twins born to ewe lambs and twins born to mature ewes during their first two years of lambing. The experiment included 8-month-old ewes born as twins to mature ewes (M2, n = 135), singletons born to ewe lambs (L1, n = 135), and twins born to ewe lambs (L2, n = 88), bred during the same period to the same rams, over two years. The efficiency of lamb production (total litter weight at weaning divided by the pre-breeding weight of the ewe, for all ewes presented for breeding) after two years of production was not significantly different (p > 0.05) among the groups (0.40 ± 0.02, 0.39 ± 0.02, and 0.39 ± 0.03, for M2, L1, and L2, respectively).