DIET SELECTION IN SHEEP - THE ABILITY OF GROWING LAMBS TO SELECT A DIET THAT MEETS THEIR CRUDE PROTEIN (NITROGEN X 6.25) REQUIREMENTS

To test the proposition that sheep are able to select a diet that meet s their crude protein (N x 6.25; CP) requirements, feeds L, A, B, C an d H with the same energy content (11 MJ metabolizable energy/kg feed) but different CP contents (78, 109, 141, 172 and 235 g CP/kg fresh fee d respectively) were formulated. In addition, feed U, which was feed L plus 21.4 g urea/kg (CP content 132 g/kg), was also made. The feeds w ere offered ad lib. either singly (n 4 per treatment) or as a choice b etween feed H and another feed (pairs LH, AH, BH, CH and UH; n 9 per f eed pair) to individually penned Suffolk x Scottish mule wether lambs, over the live-weight range 25-45 kg. On the single feeds the rates of live-weight gain were 273, 326, 412, 418, 396 and 407 g/day (SE of di fference (SED) 34; P < 0.01) and protein (excluding wool) gain were 27 , 32, 44, 45, 41 and 39 g/d (SED 4; P < 0.001) for feeds L, A, B, (C, H and U respectively. When sheep were given a choice between a feed be low (L or A) and a feed above their CP requirements (H; as judged by t he single-feeding treatments) the CP concentration selected was not di fferent between the two pairs: 131 (SE 4) v. 133 (SE 4) g CP/kg feed f or pairs LH and AH respectively. On the choices BH and CH (a choice be tween two feeds above requirements) the feed lower in CP was constantl y preferred (874 (SE 33) and 910 (SE 33) g feed B and C respectively p er kg total feed intake; CP selected was 157 and 178 g CP/kg respectiv ely). However, this was not the case with the UH choice on which sheep consumed only 599 (SE 61) g feed U/kg total feed intake, resulting in a selection of a higher CP in their diet (173 g CP/kg). The live-weig ht gains of the animals given a choice between two feeds were 416, 387 , 415, 410 and 383 g/d (SED 37) and protein gains were 45, 40, 46, 50 and 43 (SE 7) for pairs LH, AH, BH, CH and UH respectively, which were comparable with the best performance achieved on a single feed. The r esults suggest that sheep were able to select a diet that meets their CP requirements and avoid, at least to a certain extent, excess of pro tein intake. It is also possible that sheep discriminate against a pro perty of feed U, such as an excess of urea, when this feed is paired w ith a feed high in CP.