Effects of altrenogest supplementation in late lactation on reproductive performance of primiparous sows

Genetic selection for hyperprolific sows resulted in higher proportion of low birthweight piglets and greater birthweight variation within the litter, increasing preweaning mortality. There is evidence that altrenogest supplementation improves post-weaning reproductive performance through higher ovulation rates, and increased embryo survival. This study aimed to evaluate the impacts of altrenogest supplementation during the last week of lactation in primiparous sows on subsequent reproductive performance. Ninety-six primiparous females were randomly assigned to two treatments: ALT-sows supplemented orally with 20 mg of altrenogest (n = 46) during the last six days of lactation up to 24 h before weaning; and CON- non-supplemented sows (n = 50). Ten sows from each experimental group were randomly selected to evaluate plasma progesterone concentrations on days 1 and 3 of treatment, at weaning and at 48, 72 and 96 h after estrus onset. Farrowings were monitored and piglets' individual birthweight, total number of piglets born, born alive, stillborn and mummified were recorded. Stillborn piglets were necropsied to assess organ weight. A subgroup of five females per treatment was euthanized on the day of estrus onset for morphological analysis of the reproductive tract. ALT females showed higher progesterone levels 72h after estrus onset (P < 0.05). Moreover, piglets' birthweight, total number of piglets born, liveborn, stillborn, and mummified were similar between experimental groups. Data collected from stillborn necropsies showed that birthweight was the main factor affecting organ weights (P < 0.05). Correlation analysis revealed that piglets weighing 600-800g presented higher brain:liver weight ratio, a determinant factor of intrauterine growth restriction, which was predominant in males (P < 0.05), and ALT females showed fewer piglets within that birthweight range (P < 0.05). Interestingly, ALT sows showed higher number, but smaller endometrial glands (P < 0.05). Taken together, short-term altrenogest supplementation in the last week of lactation increased progesterone levels after estrus onset, improving uterine vascularization and endometrial glands hyperplasia in the pre-implantation period, which benefit embryo development, leading to a decrease in the proportion of low birthweight piglets.