Response of lambs fed wheat straw-based diets to supplementation with soybean hulls

Abstract

A lamb growth and digestion trial and an incubation study were conducted to evaluate the responses to supplementation of a wheat straw (WS)–soybean meal (SB) basal diet with 0, 10, 20, 30, and 40% of soybean hulls (SH) or 29% ground maize and 1% urea (M/U) on a dry matter (DM) basis. DM intake (DMI) increased with SH inclusion up to 30% of the diet, the increase being higher than with the M/U diet. Straw DMI decreased with supplementation, the depression being more severe with M/U than with 30% of SH (405 v. 522 g/day). Digestibilities of diet organic matter (OM) and neutral detergent fibre (NDF) increased, whereas OM digestibility and DM degradation rate (DMc) of WS decreased with SH level. M/U reduced diet NDF digestibility (from 45.9 to 42.3%). OM digestibility and DMc of WS were more severely depressed with M/U than with SH at the similar level of inclusion (from 41.5 to 33.0 v. 39.0% and from 2.63 to 2.16 v. 2.49%/h, respectively). Daily liveweight gain (LWG) and LWG per MJ ME intake (MEI) increased quadratically with SH. Predictions of metabolisable protein (MP) either from the data of the incubation study or from the microbial protein synthesis as assessed by microbial purine excretion, showed that MP may have been in excess of the requirements (11–40%) in diets with 30–40% SH inclusion. Although MEIs of these diets were similar, they gave rise to significantly different LWGs (61.7 v. 84.4 g/day; P < 0.05). By contrast, MEI of the M/U diet was 22% lower than of the diet having 30% SH, but both diets promoted similar LWG. On the whole, the results of this study are interpreted as further evidence that ME of the diets is not a homogeneous entity, the balance of absorbed nutrients being a central issue to predicting the responses of the ruminants.