Postnatal but not prenatal exposure to artificial long days enhances growth and IGF-1 secretion in goat kids

Under long-day conditions, melatonin secretion is suppressed, leading to enhanced growth, partly through the upregulation of insulin-like growth factor 1 (IGF-1). During gestation, photoperiodic information is transmitted in utero to goat fetuses through the maternal melatonin rhythm. However, whether maternal photoperiodic cues influence fetal or neonatal growth or milk composition in goats remains unknown. We tested the hypothesis that exposure to artificial long days, either prenatally and/or postnatally, would increase birth weight and postnatal growth in goat kids compared with a natural photoperiod. During the fall, pregnant goats (n = 18) were allocated by body weight (54.6±1.0 kg) and body condition score (2.5 ± 0.1) and were exposed to artificial long days (16 h light: 8 h dark) for 30 days before parturition, while controls (n = 40) remained under natural daylength. After birth, kids from long-day-treated dams continued under long days until 30 days of age. Kids from control dams were assigned to one of two treatments: (i) continued exposure to the natural photoperiod, or (ii) transfer to artificial long days starting at 4 days of age. Body weight and milk samples were recorded weekly, and blood samples were collected for IGF-1 quantification. Each animal was considered an experimental unit, and the data were analyzed using mixed models and repeated measures of SAS. Gestational long-day exposure did not influence birth weight, but pre-natal and/or postnatal photoperiod treatment significantly increased offspring's daily weight gain (P < 0.001) and weaning weight (P < 0.05). Artificial long-day exposure also elevated circulating IGF-1 concentrations (P < 0.01), which were positively correlated with both daily weight gain (r = 0.56, P < 0.05) and weaning weight (r = 0.57, P < 0.05). Milk components (fat, protein, and lactose) did not differ between treatments (P > 0.05). These findings demonstrate that continuous long-day exposure (pre-natal and postnatal) or postnatal to artificial long days accelerates growth in fall-born goat kids, representing a practical management strategy for producers. Nevertheless, continuous long-day exposure does not confer additional benefits beyond postnatal treatment.