Two groups of six heifers of mean age 284 days and mean body weight 233 kg were offered respectively, a barley-beef (BB) diet and a diet based on dried grass and sugar beet pulp (DG) at a rate calculated to give a live-weight gain of 0·5 kg/day. Three heifers from each group were fed twice daily, and the remaining three were fed through an out-of-parlour concentrate dispenser, and, during the period of blood sampling, hourly. Following a period of adaptation, blood samples were taken hourly during 24 h and the plasma analysed for insulin, prolactin and growth hormone (GH). The experiment was repeated 28 days later using the same heifers offered the same diets except that the twice-daily fed heifers were fed hourly and vice versa.
Comparing the heifers fed the BB diet and those fed the DG diet, insulin in plasma was higher (7·11 v. 3·54 mU/1, P < 0·001) and GH lower (2·48 v. 3·41 uxg/l, P < 0·05) in heifers fed the BB diet. There were no overall significant differences between the two diets for prolactin or between the two frequencies of feeding for all three hormones.
The residual variation among and within heifers was used to estimate the standard error of a treatment mean for different numbers of hourly samples on different numbers of heifers. Little was to be gained by taking in a day more than eight hourly-samples for insulin and more than 12 hourlysamples for GH and prolactin. Precision was most improved by increasing the number of heifers sampled. Feeding the heifers more than twice daily did not improve precision significantly.