Selenium concentration in the blood of ruminants grazing in northern New South Wales. 3. Relationship between blood concentration and the response in liveweight of grazing cattle given a selenium supplement

Approximately 700 yearling (Experiment 1) or unweaned (Experiment 2) cattle grazed commercial farms on the New England Tablelands of New South Wales, and were supplemented or not supplemented with selenium (Se). Whole blood and plasma Se concentrations, and liveweight gains were measured over periods of 3-7 months. In a third experiment, 96 castrated male weaned calves on a farm with a long history of unthriftiness were allocated to four groups, and were supplemented with either Se or copper in a factorial design.In Experiments 1 and 2, mean whole blood and plasma Se concentrations of unsupplemented animals on individual farms ranged from 0.004 to 0.139, and from 0.002 to 0.058 8g Se/ml respectively, but growth rates of only two of the twenty herds responded significantly to Se supplementation, and both showed diarrhoea and ill thrift at the start of the experiment. The herd in Experiment 3 also showed ill thrift, and growth rate responded to supplementation with Se, but copper depressed growth. Se concentrations in blood and plasma were generally poor indicators of responsiveness. Possible reasons are suggested.