The composition of high sugar ryegrasses.

(Nocek & Russell 1988). Such grasses are promoted to the pastoral industry for their ability to increase milk production or liveweight gain and, particularly relevant to the dairy industry, their ability to reduce the excretion of urinary nitrogen (N). This excretion is an important environmental problem, as well as being wasteful in terms of utilisation of plant nutrients. The animal production responses from grasses with higher concentrations of WSC have been inconsistent (Miller et al. 2001b; Moorby et al. 2006; Cosgrove et al. 2007a; Tas et al. 2005). There does not appear to be a direct relationship between a higher concentration of WSC and an increase in animal performance. In some studies, increases in WSC of less than 40 g WSC/ kg DM have resulted in animal responses (Miller et al. 2001b; Cosgrove et al. 2007a), whereas greater increases of up to 80 g WSC/kg DM have not (Moorby et al. 2006). It is apparent that chemical constituents of the plant other than just WSC are intricately involved in the animal response. For example, Lee et al. (2001) concluded that the higher liveweight gain of lambs grazing a high sugar (WSC) grass may have been due as much to an increase in dry matter intake because of the reduced concentration of structural fibre, as it was to the increase in WSC. Similarly, the lower fibre concentration of high WSC grasses may allow higher DM intake by zero-grazed dairy cows, but this does not always translate to higher milk production (Moorby et al. 2006), and was observed with cows at pasture (Miller et al. 2001a). These UK studies with dairy cows also demonstrated improvements in N utilisation, albeit with the grasses having atypically low concentrations of protein (92 – 145 g crude protein/kg DM). Where a higher concentration of WSC has resulted in a lower concentration of protein it has been argued that maintaining even the same level of milk production on a low protein grass is substantive evidence of benefit from the higher WSC (Miller et al. 2000; Miller et al. 2001a). However, in New Zealand, with higher concentrations of protein (e.g. a mean of 220 g/kg DM in 6300 pastures samples; Litherland & Lambert 2007), benefits from improved protein nutrition are not as likely. And, while reducing N excretion is important, the currently available high sugar grasses may not have enough WSC to markedly improve the utilisation of N Abstract Enhancing the concentration of water soluble carbohydrate (WSC) in ryegrass by management or breeding affects other chemical constituents, which may also contribute to the animal response. A large data set (n=220) of chemical composition of 3 ryegrasses was examined to determine the relationship between elevated WSC and other constituents. Samples of pasture were collected at 8:00 am and 4:00 pm, 2-3 times per week during measurement periods in spring of 4 consecutive years and during autumn of 3 consecutive years, freezedried and concentrations of major constituents predicted by NIRS. Using a two-step statistical procedure effects due to year and day within season were removed, and then within season the concentration of WSC regressed, by six groups (3 cultivars x two times of day), against crude protein (CP), neutral detergent fibre (NDF), lipid and ash. For each grass a unit increase in WSC (slope) was offset by a decrease in CP of 0.62 units in spring (R2=43%) and 0.44 units in autumn (R2=47%), and a decrease in NDF of 0.30 units in spring (R2=23%), and 0.17 units in autumn (R2=42%). The Y-axis intercepts differed for cultivar and time of day. For protein the decrease was greater than expected from the effect of dilution alone, and for NDF it was less than dilution. While the physiological basis for these relationships was not determined, they indicate that the reduction in protein and fibre should enhance the effect of greater WSC on the nutritional value of ryegrasses and the efficiency of nutrient utilisation in the animal.