Carbohydrate and crude protein fractions in perennial ryegrass as affected by defoliation frequency and nitrogen application rate

The objective of this study was to evaluate the effects of defoliation frequency (either at two-or three-leaf stage) and nitrogen (N) application rate (0, 75, 150, 300, 450 kg N ha(-1) year(-1)) on herbage carbohydrate and crude protein (CP) fractions, and the water-soluble carbohydrate-to-protein ratio (WSC: CP) in perennial ryegrass swards. Crude protein fractions were analysed according to the Cornell carbohydrate and protein system. Carbohydrate fractions were analysed by ultra-high-performance liquid chromatography. Sward defoliation at two-leaf stage increased the total CP, reduced the buffer-soluble CP fractions and decreased carbohydrate fractions of herbage (P < 0.001). The effect of defoliation frequency was less marked during early spring and autumn (P < 0.001) than for the rest of the seasons. An increase in N application rate was negatively associated with WSC, fructans and neutral detergent fibre (P < 0.001), and positively associated with CP and nitrate (N-NO3) contents of herbage. Nitrogen application rate did not affect CP fractions of herbage (P > 0.05). The fluctuations in CP and WSC contents of herbage resulted in lower WSC: CP ratios during early spring and autumn (0.45: 1 and 0.75: 1 respectively) than in late spring (1.11: 1). The herbage WSC: CP ratio was greater (P < 0.001) at the three-leaf than the two-leaf defoliation stage and declined as the N application increased in all seasons (P < 0.001). The results of this study indicate that CP and carbohydrate fractions of herbage can be manipulated by sward defoliation frequency and N application rate. The magnitude of these effects, however, may vary with the season.