The influence of radiant heat load on reproduction in the Merino ewe. II. The relative effects of heating before and after insemination

Abstract

The effect of radiant heat load imposed both before and after artificial insemination (AI) on the reproductive performance of Merino ewes was tested in two experiments. In the first experiment ewes were allocated to four groups: controls (group 1); hot-room conditions for 5.5 days prior to AI (day 0) (groups 2 and 4); hot-room conditions for days 1–9 after AI (groups 3 and 4). Ewes in the second experiment were allocated to three groups: controls (group 1); hot-room conditions for 5.5 days prior to AI then for days 1–4 after AI and days 5–8 after AI (groups 2 and 3 respectively). Hot-room conditions for both experiments ranged over 41–43°C and 2.07–3.08 kPa vapour pressure, and control room conditions over 22–27°C and 1.19–2.10 kPa. When subjected to hot-room conditions the rectal temperatures and respiration rates for all heated ewes were significantly higher than those of controls by c. 1.0°C and 120 respirations/min.

In both experiments the number of ewes exhibiting oestrus was significantly reduced by about one-third following heating. Despite this most ewes ovulated. Heated ewes which showed oestrus in experiment 2 had significantly longer oestrous cycles than control ewes (17.9 days v. 16.9 days, P < 0.01). In this experiment lack of cyclical oestrous activity was the main cause of reduced fertility in heated ewes.

The number of ewes lambing was depressed in heated ewes in both experiments, but was affected most severely in experiment 1. In this experiment the poorest lambing performance of 21.5% compared with 70.6% in controls resulted from heating both before and after insemination. Interference with cyclical oestrous activity by exposure to a high radiant heat load at the time of insemination (either natural or artificial) may contribute significantly to reproductive wastage associated with ewes which mate, apparently conceive, but fail to lamb.