Improving the quality of perennial ryegrass ( Lolium perenne L.) for dairy cows by selecting for fast clearing and/or degradable neutral detergent fiber

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Neutral detergent fiber (NDF) fractional clearance rate (Kcl) and fractional degradation rate (kd) of six varieties of perennial ryegrass were measured to examine the possibility of selecting for varieties with fast degradable NDF. The experiment was conducted in 2000 and repeated in 2001. In each year, six multiparous rumen-cannulated dairy cows were stall-fed with six varieties of perennial ryegrass during three 2-week periods, using a double 3 × 3 Latin square design. The NDF fractional clearance rate (Kcl NDF) and the acid detergent lignin fractional clearance rate (Kcl ADL) were estimated using two consecutive rumen evacuations, separated by a 12-h period of feed deprivation assuming a first-order kinetics. The NDF fractional degradation rate (kd NDF) was calculated based on the assumption that NDF fractional passage rate (kp NDF) was equal to Kcl ADL. The Kcl NDF of the different grass varieties was in the range of 5 to 6%/h. The kd NDF was lower than reported in situ values and ranged from 2 to 3%/h. The Kcl NDF and kd NDF were not significantly different among grass varieties in both years. The difference between the fastest disappearing and degrading variety and the slowest one was less than 1%/h. This may indicate that within perennial ryegrass varieties, there is a narrow range of improvement with regard to clearance and degradation rates.