Mating Season Influences Placental Characteristics, Plasma IGF Concentration, and mRNA Expression without Affecting Lamb Birth Weight in Akkaraman Ewes

Seasonal variations significantly impact lambs' birth weight and post-natal growth of lambs, yet the underlying physiological mechanisms remain insufficiently explored. Therefore, this study examined the effects of mating season on lamb birth weight, placental characteristics, maternal concentrations of insulin-like growth factors (IGFs) in plasma, and placental mRNA expression levels of IGFs in Akkaraman ewes reared under extensive conditions. Ewes were mated in the breeding season (September; n = 35) and out-of-breeding season (April; n = 27). Blood samples were taken from the jugular vein of all ewes using a vacutainer in sterile heparin tubes every month from mating to parturition. Post-lambing, both dam and lamb weights were recorded, and placental characteristics were documented within 12 hours of parturition in both seasons. The number and weight of cotyledons were higher (P<0.05) in ewes mated during the breeding season than those mated during the out-of-breeding season. Also, ewes mated during the breeding season demonstrated thicker large cotyledons (P<0.05). Although placental and cotyledonal efficiencies were similar in both mating seasons, volumetric cotyledon efficiency was higher (P<0.05) in ewes mated during the out-of-breeding season than in those bred during the breeding season. Additionally, ewes mated in the breeding season exhibited higher plasma concentration of IGF-I at three months of gestation than ewes mated in the out-of-breeding season (123.8 +/- 9.8 vs. 99.3 +/- 11.4 ng/ml, respectively; P<0.05). Although the mating season did not affect the IGF-I gene's placental mRNA expression levels, the IGF-II gene's expression level was notably higher (P<0.05; fold change: 2.84) in ewes mated during the breeding season. The results suggest that mating season influences placental characteristics, maternal plasma IGF-I concentrations, and IGF-II gene expression levels of cotyledons due to alterations in cotyledon numbers and size on the surface of the placenta.