Rumen degradation characteristics of perennial ryegrass cultivars during the growing season

The objective of this experiment was to evaluate the effects of sward characteristics and chemical composition of eight diploid perennial ryegrass cultivars during the growing season on degradation characteristics in the rumen of dairy cows. As part of an indoor-feeding experiment with dairy cows, sward characteristics were measured, and samples of cultivars were collected on two consecutive days during seven periods throughout the summer. ars did not differ (P>0.05) in sward characteristics. Two cultivars had higher concentrations of water-soluble carbohydrates (WSC, P<0.001), intermediate to low concentration of crude protein (CP = N × 6.25, P<0.01), and lower concentrations of neutral detergent fibre (aNDFom, P<0.05). Among cultivars, only small differences in proportions of different degradation fractions, and in fractional degradation rates (kd) of organic matter (OM), CP and aNDFom (<0.01 h−1) were found. One cultivar with higher concentrations of non-fiber carbohydrates (NFC) and WSC, had a higher extent of effective degradation (ED) of OM (P<0.05), and of NFC (P<0.001), and lower ratios between ED of N and OM, and N and carbohydrates (CHO, summation of aNDFom and NFC) (P<0.05). Seasonal changes occurred: during the first two periods (June 21–22 and July 5–6) lower proportions of leaf blades (<0.71 g/g), a higher lignin (sa) (10 g/kg DM; P<0.01) in period 1, or a slightly higher concentration of aNDFom in period 2 (P<0.05) were found. In the first period, this was related with a slightly lower kd,aNDFom (0.004–0.008 h−1; P<0.05). The concentration of CP differed more among periods (>25 g/kg DM) than among cultivars (<15 g/kg DM). Any increase in concentration of CP was related with a decrease in concentration of NFC and WSC, an increase in the proportion of potentially degradable (D) CP, and in kd,CP, resulting in a higher EDCP, and higher hourly ratios between degraded N and OM, and N and CHO in the rumen. The possible effects of differences in (hourly) ED on the efficiency of nitrogen utilisation by dairy cows are discussed. The results of this study indicated that changes in weather conditions and grassland management have more effect on chemical composition and rumen degradation of perennial ryegrass than genetic differences among cultivars.