Oestrus in relation to peak oestradiol levels in ovariectomized Galloway cows

The objective of the present study tons to characterize the variation in oe strous behaviour among ovariectomized cows in response to a measured dose o f oestradiol benzoate (OB). In study 1, nine ovariectomized Galloway cows, approximately 10 years old, were challenged with an intramuscular injection of either 0.25, 0.5 or 1.0 mg of OB. Following this, 0.5 mg OB teas chosen as the appropriate nose required to induce oestrous behaviour in ovariecto mized Galloway cows. In study 2, nine cows injected with 0.5 mt: OB were mo nitored for oestrous behaviour using KAMAR(R) heat mount detectors. Blond s amples for plasma oestradiol-17 beta (E-2) assay weve taken every 4 h betwe en 0 to 72 h and once at 96 h, following OB administration. The Plasma E-2 concentrations between 0 to 96 h following OB administration differed signi ficantly (P < 0.001) among cows. The interval to peak E-2 concentrations av eraged 17.42 (s.e. 2.21) h and the mean peak E-2 concentration was 5.86 (s. e. 0.57) ng/l. The mean interval from OB administration until onset of moun ting activity was 24.57 (s.e. 2.38) h and the duration of oestrus averaged 10.5 (s.e. 1.99) h. In all cows, mounting activity occurred following peak E-2 concentrations after an average interval of 7.73 (s.e. 1.84) h. There w as no significant association between peak E-2 concentrations and the inter val to onset of mounting activity or its duration. Since the variation amon g Galloway cows in the manifestation 4 behavioural oestrus was independent of systemic E-2 concentrations, it implies that there are inherent differen ces between individuals in the sensitivity of the hypothalamus to physiolog ical E-2 thresholds. This may help to explain the incidence of silent ovula tion in some animals and the occurrence of overt oestrus associated with fo llicular activity during the luteal phase of the cycle or during the variou s stages of pregnancy, in the presence of high plasma progesterone concentr ations.