Cultivar and age of regrowth effects on physical, chemical and in sacco degradation kinetics of vegetative perennial ryegrass (Lolium perenne L.)

Perennial ryegrass is the principal component of diets fed to ruminants in New Zealand. Selection for ryegrass improvement should incorporate measures relating to feeding value (i.e. quality × intake) for sheep and cattle. This study was designed to measure the variation in characteristics of nutritive value of three ryegrass cultivars harvested at different stages of regrowth. The cultivars were Grasslands Greenstone (Greenstone; tetraploid), Grasslands Samson (Samson; diploid) and Quartet (tetraploid). They were grown as pure swards with leafy regrowth harvested after 2, 3, 4, 5, 6 and 7 weeks for measurement of chemical composition, physical parameters (i.e. shear force and energy required for mincing), degradation kinetics and distribution of constituents between the immediately degradable (A), potentially degradable (B) and undegradable (C) fractions. Analyses showed that age of regrowth resulted in larger differences in composition and degradation kinetics than cultivar, and emphasised the need to harvest material in a similar condition to that grazed by animals. Cultivar differences were best indicated by chemical composition and ratios of crude protein (CP):neutral detergent fibre (aNDF) in the dry matter (DM), the proportion of CP in the ‘A’ fraction as well as degradation of DM in the ‘B’ fraction. The CP:aNDF ratios averaged for the three cultivars at 2, 3, 4, 5, 6, and 7 weeks were 0.82, 0.76, 0.71, 0.55, 0.48 and 0.42, respectively, with averages at weeks 5 and 6 for Greenstone, Samson and Quartet of 0.59, 0.49 and 0.48, respectively. The ratio differed between ages (P<0.001) and across cultivars (P<0.05). The DM fractional degradation rates (k) tended to decline from 0.18 to 0.12/h with maturity (P<0.05) and after weeks 5 and 6 of regrowth averaged 0.17, 0.16 and 0.14/h for the respective cultivars. Both the concentration of lignin(sa) in DM and shear force were weakly associated with regrowth and cultivar and appeared to be less important than chemical composition for indicating nutritive value of leafy ryegrass.