Changes in contribution of white clover to canopy structure in perennial ryegrass/white clover swards in response to N fertilizer

Twelve plots were laid down on an existing perennial ryegrass/white clover sward, one plot in each of six replicated blocks receiving 100 kg N ha(-1) (100N) and one plot receiving no N (0N). Biomass, canopy development (stratified cuts and point quadrat records at 2-4-week intervals) and changes in stolen population density were recorded during one 8-week regrowth period (25 JuIy-23 September) to investigate the likely causes of N effects on white clover in mixed swards. Over the period, N fertilizer resulted in an increase of 74% in perennial ryegrass biomass and a reduction of 24% in white clover biomass. There was also a reduction of 44% in stolen growing point density, mainly due to lower density of younger stolen-branches. White clover's contribution to the upper three leaf area index (LAI) units (taken as an estimate of the proportion of photosynthetically active radiation (PAR) intercepted) was, on average, 70% at 0N producing 74% of the sward biomass, compared with 46% contribution to interception and 37% contribution to biomass at 100N. While there was no evidence of overtopping, it is concluded that N fertilizer application increased the LAI of perennial ryegrass in the upper layers of the canopy thereby reducing the share of available PAR to white clover. This, coupled with a lower radiation use efficiency at high N and lower population density, results in white clover's reduced performance in mixed swards receiving N fertilizer.