The effect of level of nutrition on thyroid activity in beef cattle

Abstract

Four experiments were conducted to determine the effect of different nutritional levels on thyroid activity in beef cattle. In experiment 1, plasma protein-bound iodine (P.B.I.) levels in eight steers on poor pasture increased by over 50% 24 hr after supplying a supplement of 20 lb good quality lucerne hay per animal per day. In experiment 2, 17 grazing spayed heifers given a supplement of 2 lb peanut meal per day over a 4-month period also had higher P.B.I.'s than 24 controls. In experiment 3,10 growing steers given a constant amount of lucerne hay throughout the year showed decreases in P.B.I. and thyroid secretion rate consistent with a decline in feed intake per unit of metabolic body weight. A Latin square design was used in experiment 4, to study the effects of ad libitum feeding of three different rations: lucerne hay, non-legume hay, and ground sorghum plus non-legume hay to 18 steers. All rations caused sharp first-day increases in P.B.I., the magnitude of which depended on the quality of the ration. As feeding continued the P.B.I.'s decreased somewhat. At the end of 5 weeks, P.B.I.'s and thyroid secretion rates were similar in steers fed on lucerne or sorghum plus non-legume hay, but significantly lower (P < 0.05) in those fed on non-legume hay alone. The failure of intramuscular injections of vitamin A to increase P.B.I. in animals on low-carotene pasture and hay ruled out the carotene content as a source of the nutritional effect. These studies indicate that various types of improved nutrition can have pronounced effects on the thyroid, and that seasonal and animal differences in thyroid activity could result from differences in quality or quantity of pasture intake.