The effect of 2-bromo-alpha-ergocryptine (bromocriptine) on the induction, initiation and maintenance of milk secretion was studied in post-parturient ewes and in ovariectomized ewes artificially induced to lactate by treatment with oestrogen plus progesterone and then dexamethasone. Treatment with bromocriptine (about 0.4 mg/kg every 3 days) lowered and maintained the plasma concentration of prolactin at less than 12 ng/ml. Ewes receiving bromocriptine concurrently with oestrogen plus progesterone during the priming phase had a significantly lower (P less than 0.05) mean cumulative milk yield than control ewes, although the milk of the treated ewes contained normal amounts of fat, protein and lactose. Administration of bromocriptine during dexamethasone-induced lactogenesis had no significant effect on the mean cumulative milk yield but significantly (P less than 0.05) increased the milk fat and protein content. In established lactation, bromocriptine markedly reduced the milk yield in both intact and ovariectomized ewes. The concentration of protein was not significantly affected although the milk fat content was higher in the bromocriptine-treated than in the control ewes. The effects of bromocriptine on milk yield and composition during galactopoiesis could be reversed by concurrent infusion of prolactin and the results suggest that prolatin is an important hormone during mammogenesis and galactopoiesis in the sheep.

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The impact of heat stress on the feed intake, milk production, water intake and urine output of undernourished lactating ewes and on the growth rate and water intake of their lambs was measured both in a climate chamber and during exposure to natural high ambient temperature conditions. Similar liveweight increases occurred in both stressed and unstressed lambs. Feed intake was depressed in heat-stressed ewes in the climate chamber but not under natural heat-stress conditions. During the first and second weeks of lactation calculated milk yield (200-500 mljday) and composition were unaffected by heat stress per se. However, undernutrition due to the poor quality roughage offered apparently depressed milk production of all ewes. Increased water intake (27 %) and plasma prolactin concentrations (220 %) were recorded in heat-stressed ewes, but daily urine output (27-36 mljkg body wt) was unaffected. No relationship between prolactin concentrations, milk production and antidiuretic activity was obvious. At the age of 5-6 weeks water intake accounted for 67 % (500 mljday) and 80 % (1000 mljday) of total fluid intake (water plus milk) of unstressed and heat-stressed lambs respectively. These data indicate the importance of making water freely available to lambs subjected to extensive grazing systems of tropical regions.

Abstract. Six Holstein bulls were housed in a climate-chamber under constant light regimen and after two weeks of preconditioning at 15°C, 60% relative humidity RH (day) and 12°C, 60% RH (night) were subjected to two weeks of heat exposure. This involved one week at 30°C and 60% RH (day) and 25°C and 60% RH (night) and a further week at 35°C, 60% RH (day) and 30°C, 60% RH (night). Three bulls were untreated (group I) and 3 bulls were treated (group II) just before and during heat exposure with a prolactin inhibitor to study the possible physiological role of prolactin on the regulation of water, potassium and sodium. Serum prolactin levels increased significantly (P < 0.01) in group I from the control value of 6 ng/ml to 33 and 44 ng/ml when the ambient temperature was increased (weeks 3 and 4) and then decreased to 21 and 12 ng/ml after reduction in temperature during weeks 5 and 6, respectively. For group II prolactin values decreased under the treatment with the prolactin inhibitor to 0.5 ng/ml and remained at this level throughout the experiment. GH levels were unaffected by heat treatment or by treatment with prolactin inhibitor. There were no differences between groups I and II in respiratory rate, pulse rate and rectal temperature. Water intake increased in both groups under heat exposure but decreased significantly afterwards only in group II. Differences in urinary excretion volume and blood serum osmolality were not significant. Urinary potassium and sodium excretion were unchanged in group II but increased with heat exposure in group I. During heat exposure 2 bulls of group II lost weight despite maintaining food intake.