Relation of hormonal variations to nutritional studies and metabolism of ruminants

Variations in concentrations of growth hormone, insulin, glucagon, prolactin, adrenal corticoids, and thyroid hormones in blood plasma of ruminants as related to circadian rhythms, environmental factors and nutrition are reviewed. Rhythms in prolactin concentrations are related to light and temperature. Concentrations of prolactin in plasma are higher in warm environments and during longer photoperiods. There is episodic secretion of growth hormone in cattle with frequent peaks in plasma concentrations during 24 h. Marked increases in environmental temperature result in greater secretion of growth hormone for short times followed by a decline in plasma concentration with prolonged exposure. Extreme cold temperature also increases growth hormone concentrations in plasma. Secretion of thyroid hormones increases in cold and decreases in warm environments. Circadian rhythms are definite for plasma concentrations of adrenal corticoids in ruminants adjusted to their surroundings. Peaks occur late in dark periods, and lows occur during the end of light periods. Circadian patterns have not been consistent for secretion of insulin. Prolonged subjection to stresses such as noise, handling, and restraint can modify concentrations of prolactin, growth hormone, and adrenal corticoids. Changes in concentrations of insulin may occur if blood glucose is increased. After feeding, concentrations of growth hormone in plasma decrease and concentrations of insulin and glucagon increase. Relationships of these changes with metabolism are discussed.