Ryegrasses with increased water soluble carbohydrate : evaluating the potential for grazing dairy cows in New Zealand

An elevated concentration of water soluble carbohydrate (WSC) in grasses may increase the availability of readily degradable energy, increase milk production and improve the efficiency of nitrogen utilisation. Two ryegrasses with elevated WSC, a tetraploid Italian (IRG) and a diploid perennial selected for higher WSC (HSG), were compared against a standard diploid perennial (STG) during spring 2004 and 2005, and autumn 2006 and 2007. Groups of cows (n=20) grazed each grass type for periods of 6 weeks in spring and 2 weeks in autumn. During spring, the ‘high’ WSC grasses were 20-40 g/kg DM higher in concentration of WSC than the standard (P<0.01), whereas in autumn the difference was smaller and not significant. The production of milk and milksolids (MS) did not differ significantly among grasses in spring or in autumn 2006. In autumn 2007, MS production was higher for HSG than STG (1.09 and 0.99 kg MS/cow/day for HSG and STG, respectively, P=0.006). While the greater MS production cannot be attributed to higher WSC per se, it may be related to other differences between grass types, such as a lower fibre content of the high WSC grasses, or factors affecting intake and the partitioning of nutrients to production or body reserves by cows during late lactation.