Effect of birthweight and birth rank on the survival of single and twin lambs born to ewe lambs

Benefits of ewe lamb mating include improved lifetime production, increased rates of genetic gain and higher net profits. These benefits are only achieved if the ewe lamb successfully rears its offspring to weaning. A meta-analytic approach was used to assess the effects of birthweight and the interacting effect of birth rank on the survival to weaning of lambs born to ewe lambs. The data were from eight studies where birthweight and weight at weaning had been recorded for lambs born from ewe lambs. Data were for single- (n = 1392) or twin-born lambs (n = 456). The twin lambs were further classified as the ‘heavier’ or ‘lighter’ of the pair to give three birth rank categories. To compare the probability of dying before weaning between birth ranks, but with lambs compared at a similar birthweight, lambs were categorised by their birthweight as heavy (>3.8 kg) or light (<3.8 kg) and also categorised within their birth rank on their placement around the mean birthweight (birthweight deviation). A mid-range birthweight deviation fell between one standard deviation above or below the mean for each birth rank, whereas a low- and high-range birthweight fell below and above the mid-range, respectively. Logistic regression was used to assess the effect of birth rank (single, heavier twin and lighter twin) and birthweight on lamb mortality up to weaning. To compare the probability of surviving up to weaning between the different birth ranks, relative risk ratios were calculated. Birth ranks were compared for lambs within the same birthweight group (heavy or light) and same birthweight deviation (low-, mid-, high-range). Decreasing birthweight increased mortality before weaning in twins and the change in mortality was greater for the lighter twin. Birthweight did not alter mortality before weaning in singles. Comparison of the birth ranks showed that there were differences (P < 0.05) in the probability of dying for lambs of different birth ranks but similar birthweight. The results suggest that management options for ewe lambs that increase the birthweight of the lighter twin could increase the weaning percentage.

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