Foetal stunting in sheep: 1. The influence of maternal nutrition and high ambient temperature on the growth and proportions of Merino foetuses

Abstract

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.

Effect of chronic heat load during pregnancy on birth weight, behaviour and body composition of Australian feral goat kids

“…Chronic heat stress in sheep and cattle causes diversion of the blood supply to the superficial tissues with a reduction of blood flow to the uterus (Brown and Harrison, 1981;Reynolds et aL, 1985) and is associated with an increase in metabolic rate due to increased tissue temperature (Cartwright and Thwaites, 1976). Over 56 days C and H twin-bearing does consumed 25.3 and 26.2 kg DOM equivalent to 466 and 482 mJ DE.…”

“…development in sheep (Cartwright and Thwaites, 1976), partly by reduced uterine blood flow in sheep and cattle (Brown and Harrison, 1981;Reynolds, Ferrell, Nienaber and Ford, 1985). This is caused partly by reducing placental.…”

Altered arterial concentrations of placental hormones during maximal placental growth in a model of placental insufficiency

“…This regime allows developmental or growth impairment to be attributed to the effects of chronic heat exposure. When TN ewes are fed the same feed intake of gestational age matched heat stressed ewes, heat stressed lamb birthweights are significantly reduced, while TN weights are unaffected (Cartwright & Thwaites 1976, Brown et al 1977. Furthermore, both groups of ewes consumed approximately 10·7 MJ per day, an amount greater than the estimated 10·5 MJ required metabolizable energy intake of pregnant ewes during the first 105 days of gestation (Subcommittee on Sheep Nutrition 1998).…”

Thermal Environment and Human Birth Weight

“…Uterine and umbilical blood #ow are also reduced, decreasing the availability of oxygen and glucose to the foetus (Bell et al, 1987;Dreiling et al, 1991). Weight reduction in the foetus varies by organ (Cartwright & Thwaites, 1976), with brain weight being preserved at the expense of liver weight (Bell et al, 1989). Thus, heatstressed foetuses are not merely small at birth, but show stunted growth similar to that induced by undernutrition (Cartwright & Thwaites, 1976).…”

“…Weight reduction in the foetus varies by organ (Cartwright & Thwaites, 1976), with brain weight being preserved at the expense of liver weight (Bell et al, 1989). Thus, heatstressed foetuses are not merely small at birth, but show stunted growth similar to that induced by undernutrition (Cartwright & Thwaites, 1976).…”