Changes in water‐soluble carbohydrates relative to crude protein in perennial ryegrass in response to defoliation frequency

Abstract

AbstractThe water‐soluble carbohydrate relative to crude protein (WSC:CP) ratio in perennial ryegrass (Lolium perenne L.) has been postulated as a useful indicator of nitrogen (N) use efficiency in grazing dairy cows. A mini‐sward study was conducted to quantify the effects of defoliation frequency based on leaf stage on the CP fractions and the WSC:CP ratio in perennial ryegrass. Defoliation frequency corresponded to the time taken by ryegrass to develop 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 leaves per tiller. Plant growth dynamics and plant nutritive parameters related to N use efficiency were measured. The results showed that perennial ryegrass plants that were defoliated more frequently had reduced herbage mass accumulation, leaf elongation rate and total lamina length than plants defoliated less frequently in both growth cycles. Extending the defoliation frequency from the one‐ to five‐leaf stage resulted in a decline in WSC content, an increase in CP and buffer soluble CP and a decline in the WSC:CP ratio. Given the defoliation frequencies tested here, we concluded that plants defoliated too frequently have a potential risk of low N efficiency use by grazing animals, due to a low WSC:CP ratio. The best nutritional quality balance occurs when plants were defoliated at the 3.0‐leaf stage during the first growth cycle and at the 2.0‐leaf stage during the second growth cycle.