Rumen metabolism and nitrogen flow to the small intestine in steers offered forage diets bred for elevated levels of water-soluble carbohydrates

Eight Hereford × Friesian steers were used to investigate the effect of feeding Lolium perenne (L) forage containing elevated levels of water-soluble carbohydrate (WSC) on rumen metabolism and nitrogen (N) absorption from the small intestine. The steers were offered ad libitum access to one of two varieties with matched heading dates (Ba11353, high WSC, HS; AberElan, intermediate WSC, control) cut at different times of the day to accentuate WSC differentials, zero-grazed for 21 days. This was followed by a 14-day period where the animals were on grass silage to provide a covariate intake. Although the total N concentration was similar for the two grasses, all other measured values were significantly different. The dry matter (DM) concentration of HS was greater than that of the control (202 v. 167 g DM per kg; P <0•01). WSC and in-vitro dry matter digestibility (IVDMD) were 243 and 161 g/kg DM, and 0•61 and 0•56 for HS and control, respectively. In contrast, acid- and neutral-detergent fibre were 251 and 296 g/kg DM and 480 and 563 g/kg DM for HS compared with control, respectively. DM intake was increased (9•3 v. 6•7 kg/day; P <0•001) for HS animals and this contributed significantly towards higher flows of non-ammonia N to the duodenum as well as increased absorption of amino acids from the small intestine. This DM intake response was partly due to the elevation in DM concentration of HS. However fresh weight intake was increased proportionately by ca. 0•15 (P <0•05) in animals on HS compared with control. Rumen ammonia levels were lower (14•0 and 26•4 mg N per l; P <0•001) and concentrations of rumen propionate higher (P <0•01) and acetate lower (P <0•01; increasing the glucogenic : lipogenic volatile fatty acid ratio) in animals on HS compared with control. However, the efficiency of microbial protein synthesis (15•9 and 17•8 g microbial nitrogen per kg organic matter apparently digested) and flow of N to the duodenum per unit N intake (0•84 and 0•93) for HS and control, respectively, were similar across both diets