Previous work at the Edinburgh School of Agriculture has suggested that sheep, like pigs, can select amongst foods which differ in protein:energy content according to their protein needs. The proportion of high protein feed which was selected by young sheep gradually diminished as the animals grew towards maturity over a period of measurement of 10 weeks (Cropper, 1988). The experiment reported here was designed to test this idea further by allowing sheep, initially differing in age and weight, to select between a high and low protein food (both available ad libitum) throughout their growth to a stable mature weight and body composition.

Two pelletted feeds were formulated and prepared which differed in protein concentration but with similar calculated energy (metabolisable energy, ME) (Table 1) concentration and abundant in minerals and vitamins. Seven sheep (Suffolk x Greyface wethers) aged 3 months (n = 3), 18 months (n = 2), and 30 months (n = 2) at the start of the experiment were individually penned and given experience of each of the two feeds individually before having ad libitum access to both. Tap water was freely available from a bucket. Voluntary consumption of each feed was recorded daily by weighing and drying refusals. The sheep were weighed weekly and scanned using a Vetscan ultrasonic scanner to estimate fat and muscle depths at the 13th rib on days 126, 225, 336, 436 and 539.