The influence of radiant heat load on reproduction in the Merino ewe. I. The effect of timing and duration of heating

Abstract

Merino ewes were subjected to a source of radiant heat during early pregnancy in two experiments in which the incidence of embryonic death was examined as reflected by the proportion of ewes that failed to return to service but did not lamb. Ewes were artificially inseminated (day 0) and exposed to hot-room conditions during days 1–3, 4–6 or 7–9 or not at all (control) (experiment I), and during days 1–3, 1–6 or 1–9 or not at all (control) (experiment 2). Conditions in the hot room and control room were respectively 40–43°C and 1.36–3.06 kPa vapour pressure, and 22–26°C and 1.12–1.96 kPa. Ewes exposed to hot-room conditions had significantly higher rectal temperatures and respiration rates during treatment than did control ewes (40.2° v. 39.3° and 186 v. 61 respirations/min, P < 0.001). The number of ewes which conceived and eventually lambed was reduced by exposure to high environmental temperatures shortly after insemination. The most critical period was during the first 3 days after insemination. If ewes were heated for periods longer than this, fertility was not reduced significantly further. This suggested that the timing of the heating was more critical than the duration of heating in early pregnancy. The reduction in the number of ewes lambing (experiment 1) or in the number of ewes pregnant at 30 days (experiment 2) was due almost entirely to either fertilization failure or early embryo death. Most infertile ewes showed oestrus within a normal cycle (16–19 days). Only in experiment 1, where heating lasted 3 days, did ewes return to oestrus after extended intervals from insemination. In experiment 2 the proportion of ewes which showed oestrus after an extended period from insemination was very small. It was concluded that death of developed embryos due to the effects of high environmental temperatures during early pregnancy was insufficient to explain the large reproductive wastage associated with ewes which apparently conceive but fail to lamb.