Effects of feeding perennial ryegrass with an elevated concentration of water-soluble carbohydrates on intake, rumen function and performance of dairy cows

This study investigated effects of feeding perennial ryegrass with an elevated concentration of water-soluble carbohydrates (WSC) on dry matter intake (DMI), rumen function, milk production and composition of dairy cows. Twelve Holstein-Friesian dairy cows in mid lactation were stall-fed with fresh herbage of six varieties of perennial ryegrass during three 2-week periods, using a double 3 × 3 Latin square design. In each Latin square, three varieties were fed, one of which was characterised by a higher level of WSC (HSV) of 24 and 31 g/kg DM, respectively, than the lower WSC varieties (LSV). Other chemical components were similar or differed marginally and digestibility was similar. The results prove that it is possible to select for grass varieties with an elevated level of WSC. The DMI was not positively influenced by the increased WSC content, as cows fed HSV had similar DMI to cows fed the LSV (16.2 kg/day versus 16.6 kg/day). Rumen pH, neutral detergent fibre fractional clearance (KclNDF) and fractional degradation (kdNDF) rates were not reduced as a result of feeding HSV. Total VFA concentrations were not changed, nor were the proportions of acetate:propionate:butyrate. Milk urea concentration was reduced (P < 0.05) as a result of feeding HSV. Milk yield and concentrations of milk fat, crude protein and lactose were not influenced. At the level of difference in WSC between grasses in this experiment, feeding HSV was not beneficial in improving DMI and milk production, or in altering the composition of milk. The hypothesis that feeding high WSC grasses increases DMI and milk production was, therefore not confirmed.