The influence of radiant heat load on reproduction in the Merino ewe. III.* Duration of oestrus, cyclical oestrous activity, plasma progesterone, LH levels and fertility of ewes exposed to high temperatures before mating

Abstract

The effects of high ambient temperatures on the length of the oestrous cycle, duration of oestrus, ovulation rate, fertility and the levels of luteinizing hormone (LH) and progesterone in blood were examined in mature Merino ewes.

In experiments 1 and 2, after synchronization of oestrus half the ewes in a group were exposed to hot-room conditions for 6 days prior to the onset of the next expected oestrus. Conditions in the control pen and hot room were, respectively, 22–24°C with vapour pressure 2.10 kPa and 40.0–41.5°C, 2.80–3.50 kPa. In experiment 1, blood samples were taken 4-hourly for 2 days before and after expected oestrus, and analysed for LH and progesterone.

In experiment 3, two of three groups of synchronized ewes were exposed to hot-room conditions for days 8–11 inclusive and days 12.5–15.5 inclusive of the oestrous cycle after synchronizing. In all experiments the two heat-treated groups had significantly higher rectal temperatures and respiration rates during treatment than the third (control) group.

In experiments 1 and 2 approximately one-third of the heat-treated ewes did not display oestrus, although all ewes in experiment 1 had LH peaks and ovulated. Interference with cyclical oestrous activity was confined to the oestrus immediately after heating.

In experiment 2, heat treatment before mating reduced by 30–50% the period that most ewes were receptive to the ram, and one-third did not show oestrus. Ewes heat-treated for 2–3 days immediately before oestrus showed oestrus for a significantly shorter time than those heat-treated between days 8 and 11 of the cycle or controls.

Oestrus was completely blocked in 35% of ewes only when heating lasted 6 days and included the critical period immediately before oestrus. If heating was restricted to the critical period, only the duration of oestrus was affected.

The oestrous cycle during which treatment was imposed was up to 4 days longer (mean 1.7 days) in most ewes heated prior to oestrus in experiment 1, and by an average of 0.6 day in experiment 2. Progesterone in venous plasma of these heat-treated ewes remained high for longer than in controls, and consequently the LH surge before ovulation was delayed. The fertility of ewes which showed oestrus after heat treatment was not affected, and heated ewes and controls had similar ovulation rates.

High temperatures may cause disruption to cyclical oestrous activity by (a) blocking behavioural oestrus, (b) reducing the duration of oestrus, or(c) increasing the length of the oestrous cycle. The severity of such effects depends upon both the duration of heating and on the stage of the oestrous cycle when ewes are exposed. It is suggested that some of the observed disruptions may have a hormonal basis, and possible interference with hormonal or neural mechanisms which determine behavioural oestrus is discussed.

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*Part II, Aust. J. Agric. Res., 30: 1143 (1979).