Performance of autumn- and spring-lambing ewes was compared in a year-long trial. Ewes were shorn in November and mated to lamb in May or August. Midside wool samples and plasma were collected at seven-week intervals to determine wool-growth rate, fleece characteristics, and prolactin concentrations. Autumn lambs were lighter (P<0.001) at birth and weaning (3.92±0.1; 15.6±0.7 kg) than spring lambs (5.27±0.1; 25.3±0.7 kg). Plasma prolactin concentrations were high (150 ng/ml) at mating (Dec) of autumn-lambing ewes, but low (50 ng/ml) at mating (March) of spring-lambing ewes. Both groups exhibited periparturient prolactin surges, but in autumn-lambing ewes this created high plasma prolactin concentrations during early pregnancy when plasma prolactin is usually very low. In autumn-lambing ewes, clean fleece weights (3.75±0.11 kg), midside wool growth over 345 d (374±12 mg/cm2) and fibre diameter (39.8±0.5 mm) were significantly higher (P<0.05) than those of spring-lambing ewes (3.26±0.12 kg; 331±14 mg/cm2; 38.2±0.5 mm respectively) whilst bulk and colour did not differ. Bulk was higher (P<0.05) in single-bearing ewes (21.1±0.4 cm3/g) than in twin-bearing ewes (19.2±0.7 cm3/g). It is postulated that elevated plasma prolactin concentrations (at key times) are related to seasonal differences in birth and fleece weights. Autumn lambing is beneficial for wool growth but detrimental for lamb birth weight and growth. Keywords: out-of-season lambing; sheep; prolactin; wool growth; birth weight
New Zealand Journal of Animal Science and Production, Volume 81, Online, 129-135, 2021
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