THE EFFECT OF STARCH SUPPLEMENTATION OF STRAW ON MICROBIAL PROTEIN SUPPLY IN SHEEP

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.