Short term heat stress and embryo mortality in the ewe

Abstract

Groups of mature Merino ewes were exposed to high air temperatures in a hotroom for three-day periods beginning on days 1, 2, 3, or 4 of the cycle (oestrus=day O). A fifth group was similarly treated for seven days beginning on day 1. Controls were kept in similar housing, but at prevailing air temperatures (11-21¦C). Returns to service were significantly increased only in the group which received heat treatment between days 1 and 7 (75 versus 19 per cent in controls). Comparable figures for ewes treated on days 1-4, 2-5, 3-6, and 4-7 were 50, 25, 37.5, and 37.5 per cent respectively. Ovum wastage was significantly increased above the control level of 13.6 per cent in only the day 1-7 and 1-4 hotroom groups (83.3 and 54.4 per cent respectively; P<0.001). The results confirm that the sheep embryo is most susceptible to maternal heat stress during the period immediately after mating. In most sheep breeding areas, however, air temperatures sufficiently high as to induce embryonic death normally occur only during short term 'heat waves'. The present results indicate that such conditions are capable of causing embryo death, but that due to differences in mating dates, only a small proportion of ewes in a flock are likely to be affected at any one time.