Effect of age on peripheral levels of progesterone and oestradiol-17β, and duration of oestrus in Romney Marsh ewes

Abstract

Abstract Nine hoggets (10 months) and nine ewes (4 years) were run with teaser rams. Jugular blood samples were taken by venipuncture at 8-hourly intervals from days 4 to 14 of the cycle, and then at 4-hourly intervals until 2 days after oestrus. Concentrations of progesterone and ocstradiol-17β were determined by radioimmunoassay on non-chromatographed plasma extracts. Duration of oestrus was significantly shorter in the hoggets than in the mature ewes (18.7 ± 2.9 h(SE) v. 27.6 ± 1.9 h, P < 0.05), but there was no difference in cycle length (hoggets 16.4 ± 0.4 days v. ewes 16.3 ± 0.3 days). There were no differences between age groups in the levels or in the patterns of oestradiol-17β and progesterone throughout the cycle. From these data it appears that factors other than concentrations of jugular venous progesterone and oestradiol-17,β determine the duration of behavioural oestrus in sheep.