Utilisation of N in perennial ryegrass cultivars by stall-fed lactating dairy cows

In the summers of 2000 and 2001, the effect of six diploid perennial ryegrass (Lolium perenne L.) cultivars on the N utilisation by 12 high productive dairy cows was determined. Experiments were conducted according to a double 3 × 3 Latin square design; within each Latin square, three cultivars were fed to six cows during three periods of 2 weeks each. Two cultivars had a higher water-soluble carbohydrate (WSC) content than the other cultivars in both years. A higher WSC content was more compensated by a lower neutral detergent fibre (NDF) content than by a lower crude protein content. In both years, dry matter intake (DMI) and milk production (MP) varied only slightly among cultivars and no differences among cultivars were found in the excretion of N in milk (in g/day and as % of N intake). Furthermore, cows fed the two cultivars with elevated WSC content had a lower N intake and this was associated with a lower milk urea N content and urinary N excretion. The small differences in chemical composition among cultivars did not result in significant differences in DMI, MP and N utilisation in dairy cows fed indoors. The results suggest that there is little scope to improve N utilisation of dairy cows at high N fertilization levels by breeding new cultivars.