Seasonal differences in lamb birthweight do not arise from inherent differences in the oocyte and/or early embryo

The aim of this study was to determine whether previously 'observed seasonal differences in conceptus development in ewes are attributable to inherent differences in the oocyte and/or early embryo. Day 6 embryos were recovered from 50 ewes subjected to a standard oestrus synchronization, superovulation and laparoscopic artificial insemination protocol during October (peak breeding season) and April (transition to anoestrus). During the following October, 40 grade 1 and 2 embryos from each month, which had been cryopreserved at the late morula or unexpanded blastocyst stage, were thawed and transferred in singleton to synchronous recipients. Resulting pregnancies were monitored to term. For ewes receiving October- and April-produced embryos, overall mean ± SEM liveweight at the time of embryo transfer was 72 ± 0.7 kg, body condition score was 3.1 ± 0.04 units, and the number of corpora lutea on the ovaries was 2.7 ± 0.11 per ewe. Thirty-one and 27 ewes, respectively, became pregnant and their gestation lengths were 147 ± 0.2 and 147 ± 0.3 days. There was no effect of month of embryo production on peripheral ovine pregnancy-associated glycoprotein concentrations during pregnancy or on fetal and placental characteristics at term, but, for each month, male lambs were heavier than females and were associated with larger placentae. Lamb birthweight was positively correlated with placental weight (r² = 0.474, P<0.001) and the total weight of cotyledonary tissue (r² = 0.429, P<0.001), but not to the number of cotyledons. Results demonstrate close relationships between fetal and placental weights at term, and that seasonal effects on conceptus development in ewes do not arise from inherent differences in the oocyte and/or early embryo.