Lower birth weights of autumn-born than of spring-born lambs may be due to a seasonal impairment of placental development, established by d84 of gestation, and perhaps mediated by high summer prolactin concentrations. This study attempted to determine whether reducing circulating summer prolactin concentrations in ewes during early gestation, using bromocriptine and melatonin, would improve placental and fetal growth and birth weights of autumn-born lambs. Pregnant mixed-age Romney ewes were randomly allocated to three treatment groups, balanced for age, live weight (LW) and source flocks, on d15 after mating. Ewes in the Bromocriptine group (n=20, LW 58.3±6.7 kg) were injected with 50 mg Parlodel®LA, the Melatonin group (n=20, LW 58.6±6.6 kg) each received one 18-mg Regulin® implant, while the Control group (n=21, LW 58.5±5.8 kg) was injected with saline solution (0.9% NaCl). At d139 of gestation, ewes were randomly allocated to a ‘Slaughter group’ sacrificed at d140-2 (n=30 LW 61.2±5.6 kg) and a ‘Live-birth group’ (n=31, LW 60.0±7.1 kg). Maternal plasma prolactin concentrations were significantly reduced by both bromocriptine (Control vs. Bromocriptine): 48.3±9.2 vs. 0.7±9.9 ng/ml (P<0.001) at d20 of gestation, and 82.6±8.5 vs. 4.5±9.1 ng/ml (P<0.001) at d40 of gestation, and melatonin treatments (Control vs. Melatonin): 82.6±8.5 vs. 18.4±9.5 ng/ml (P<0.001) at d40 of gestation, and 16.8±3.7 vs. 2.9±4.2 ng/ml (P<0.05) at d60 of gestation. Fetal plasma prolactin concentrations were not affected by treatments (Control vs. Bromocriptine vs. Melatonin): 32.4±18.3 vs. 32.4±19.1 vs. 59.8±16.4 ng/ml. At d140 of gestation, there were no treatment effects (adjusted for sex and litter size) on fetal weight, crown-rump length, placental parameters, and maternal and fetal organ weights, except thymus reduced 20% (P<0.05) by melatonin treatment. Birth weights and subsequent growth rates were similar among groups. Despite treatments reducing plasma prolactin concentrations there was no affect on placental or fetal growth, indicating that high prolactin concentrations early in pregnancy are not responsible for low birth weight in ewes lambing in May.
Proceedings of the New Zealand Society of Animal Production, Volume 79, Palmerston North, 74-79, 2019
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