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

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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.