Carbohydrate and crude protein fractions in perennial ryegrass as affected by defoliation frequency and nitrogen application rate
Citation
Loaiza, P.A., Balocchi, O., Bertrand, A. (2017). Carbohydrate and crude protein fractions in perennial ryegrass as affected by defoliation frequency and nitrogen application rate. Grass and Forage Science, [online] 72(3), 556-567. http://dx.doi.org/10.1111/gfs.12258
Plain language summary
Perennial ryegrass is the most widely used forage grass in temperate pasture-base dairy production system. We investigated the effects of frequent defoliation and nitrogen application in the field in Chile on the ratio of carbohydrate to crude protein of perennial ryegrass. It is well known that the imbalance between energy and protein in forages adversely affects ruminant performance due to a large supply of proteins with insufficient energy to be processed by rumen microflora. In addition to increase the risk of nitrate toxicity to the animals, this imbalance contributes to environmental pollution by nitrogen losses in urine. This problem is exacerbated by untimely nitrogen fertilization. We tested the effect of different seasonal nitrogen application rates and of different defoliation frequency to mimic animal grazing, on the protein/energy ratio of perennial ryegrass forage. Our results showed that N fertilization rate should be reduced in autumn and early spring when plant growth is reduced to minimize protein accumulation. Defoliation at early stage of plant development is another approach to reduce protein accumulation in forage.
Abstract
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.