The effect of starch supplementation of straw on microbial protein supply in sheep

Abstract

The effect on microbial N (MN) supply of supplementation of straw with a mixture of maize starch and ground barley was examined. Urinary excretion of purine derivatives (PD) was used as an index of microbial protein supply. Six wether sheep of 42–48 kg with rumen cannulae were offered ad libitum either barley straw with 5% molasses, or barley straw with 15.5 or 31.0% of a mixture of maize starch and ground barley (1.2:1.0), and 9 and 13% molasses respectively. All sheep received all three diets for periods of 21 days, excreta being collected during the last 7 days. Digestible organic matter intake (DOMI) increased with starch inclusion as 435, 750 and 1043 (SED 47) g day −1 for the three diets respectively. Total PD excretion was 7.0, 12.3 and 21.1 (SED 0.92) mmol day −1, and the estimated MN supply was 5.7, 10.5 and 18.2 (SED 0.84) g day −1. Expressed as per unit DOMI, MN supply was 12.8, 14.1 and 17.5 (SED 1.3) g N kg −1 DOMI. Rumen liquid and solid outflow rates (fraction of rumen volume h −1) were 0.047, 0.054 and 0.073 (SED 0.0074) and 0.078, 0.105 and 0.130 (SED 0.0083) respectively. MN supply per unit DOMI increased probably due to a greater rumen outflow rate. The practical implication is that with low quality straw, supplementation of starch concentrate up to 31% of the whole diet increased both voluntary food intake and the supply of microbial protein per unit of DOMI.