1. Oestrous activity was studied in three groups of eight Southdown ewes exposed respectively to natural lighting (30½° S.), accentuated reversed seasonal lighting, and equatorial lighting for a period of 2 years.2. Control ewes exhibited a restricted breeding season confined to the autumn and winter months of each year. The breeding season averaged 102–5 days in length and during it the ewes experienced an average of 7·14 ± 0·47 oestrous periods.3. The natural breeding season was almost completely reversed by artificial reversal of the normal seasonal variation in daylength. This constitutes presumptive evidence that daylength is the major factor controlling seasonality of oestrus in Southdown ewes.

1970). Photoperiodism in the ewe: 2. The effects of various patterns of decreasing daylength on the onset of oestrus in clun forest ewes. SUMMARYIn six light-proofed buildings experiments were conducted with Clun Forest ewes to determine whether the date of first oestrus was affected by the absolute size of reduction in the case of an abrupt decrease in daylength and by the rate of reduction in the case of a given total decrease in daylength. The mean number of days from the onset of the light treatments to first oestrus (the reaction interval) for ewes receiving gradual decreases in daylength of 3-5, 7-2 or 10-9 min/day from 1 July were 66-4, 53-7 or 33-6 days, respectively. The reaction interval for ewes receiving abrupt decreases in daylength of 3-75, 7-75 or 11-75 hr on 1 July were 59-5, 44-8 or 33-6 days, respectively. A control group of ewes at pasture on natural daylength had a mean reaction interval of 66-2 days. It is concluded that the various light treatments applied significantly affected the date of onset of oestrus in Clun Forest ewes.

Two experiments designed to compare lambs born to Merino ewes subjected to either nutritional deprivation or high ambient temperatures (diurnal: 8 h at 42-2 °C, 16 h at 32-2 °C) during the last two thirds of gestation are described.In Expt 1, lambs from ewes group-fed to maintain maternal body weight at high ambient temperature were lighter (P < 0-01) and had shorter metacarpal bones (P < 0-01) than those from ewes fed to either lose, maintain or gain weight at prevailing temperatures (-2-0 °C to + 16-4 C C). The regression of metacarpal length on the cube-root of birth weight ('linear equivalence') was linear. Simple maternal undernutrition was thus not indicated as a cause of foetal stunting in heat-stressed ewes, though the proportions of affected lambs closely resembled those of the nutritional dwarfs.In Expt 2, in which all ewes were individually fed, heat-stressed ewes fed ad libitum consumed 40 % less feed and produced lambs which were markedly lighter than those from controls at prevailing temperatures. Other control ewes pair-fed at the level of intake of heat-stressed ewes gave birth to lambs of similar weight to those fed ad libitum. Thus, although feed intake was substantially reduced at high temperatures, the amounts actually eaten by heat-stressed ewes were sufficient to enable control ewes to produce lambs of normal weight. The relationships between birth weight and the weight of the adrenals, cerebellum, cerebrum, heart, kidneys, liver, spleen and thyroid were linear, irrespective of treatment, and as in Expt 1 metacarpal length was linearly related to the linear equivalence of birth weight. However, not all changes in body components were directly proportional to changes in the body as a whole, and heat-affected lambs were thus neither true miniatures nor achondroplastic dwarfs. Until these relationships are clarified it is suggested that 'stunting' is the most appropriate description of the influence of high ambient temperature on the sheep foetus.The findings are consistent with the suggestion that the adverse effects of high temperature arise from an extreme form of foetal undernutrition.

Continuous hot-room exposure during the first 15 days of pregnancy raised the rectal temperatures and respiratory rates of mature Merino ewes to about 104 to 105\s=deg\F and 170 respirations/min respectively and led to the death of 83% of embryos.Similar ewes subjected to a diurnally variable heat stress, which raised their rectal temperatures and respiratory rates to about 104 to 105\s=deg\F and 220 respirations/min during an 8-hr 'day' and allowed them to fall to about 102\s=deg\F and 140 respirations/min respectively during a subsequent 16-hr 'night', experienced an embryo mortality rate of only 35%.Control ewes, housed similarly, but at natural temperatures (33 to 65\s=deg\ F), had rectal temperatures and respiratory rates of about 101 to 102\s=deg\F and 20 respirations/min, respectively, and suffered the loss of 19% of their ova.The results illustrate the mitigating effects of a daily period of respite from high temperatures and indicate that this, rather than differences in acclimatization to heat and lack of radiant heating in the hot-room, is the major factor to be considered when comparing the effects on embryo mortality of heat stress in the field with those of periods of continuous heat stress in the hot-room.

In two separate experiments during the autumn of 1965 mature ewes of the Merino and Southdown breeds were continuously exposed to elevated temperatures in a hotroom for the first 20 days of pregnancy. Hotroom conditions in the two experiments were adjusted so as to stress similarly the ewes of both breeds. In both experiments hotroom treatment elevated rectal temperatures by approximately 2\ m=. \ 5\ s=deg\ F and respiratory rates by approximately 150 respirations/min. Such conditions led to the death of 100% of embryos in treated ewes. In both experiments approximately 75% of this embryonic death occurred at an early stage which did not interfere with subsequent returns to service after an oestrous cycle of normal length. The remaining 25% of the embryonic deaths occurred at later developmental stages and resulted in the presence of degenerating embryos in utero at autopsy on Day 23. It would appear that breed differences in heat-induced embryo mortality under similar environmental conditions are largely a consequence of breed differences in heat tolerance.