Tiller size/density compensation in perennial ryegrass miniature swards subject to differing defoliation heights and a proposed productivity index

The objective of this study was to investigate the effects of cutting heigh t on tiller population density, tiller weight and herbage harvested in pere nnial ryegrass swards. One hundred mini-swards of Lolium perenne were estab lished from seed on 5 May 1993 in 150 mm x 150 mm plastic pots in a greenho use. The experimental period lasted 6 months (May to November). On 4 June a ll pots were cut to 50 mm and randomly allocated to five treatments (20, 40 , 80, 120 and 160 mm sward surface height) with twenty replicates (four per tray). All pots were cut twice weekly to the specified surface height from 11 June, and recording began on 14 June. Every 4 weeks from 9 July to 5 No vember the twenty pots contained in one tray were withdrawn for destructive measurements of tiller population density and the weights of tiller compon ents. Tiller population density increased with reduction in defoliation hei ght, except for the 20-mm treatment where tiller density was initially rest ricted. The slope of the size/density compensation (SDC) line was close to -5/2 over the range of defoliation heights 40-120 mm, but was less than -1. 0 between 120 and 160 mm. These slopes are consistent with a recent theory, which proposes that variations from a slope of -3/2 will be linked to defo liation-induced changes in canopy leaf area and to change in tiller leaf ar ea:volume ratio, R. At defoliation heights below 120 mm, SDC slope was incr eased by reduction in canopy leaf area. Above 120 mm, increase in R forced a relatively rapid tiller population decline, resulting in a reduced SDC sl ope and decrease in canopy leaf area. Whereas traditional measures of leafi ness such as leaf:stem ratio or leaf:non-leaf ratio decreased with increasi ng height of defoliation, the leaf area:volume ratio, R, increased with def oliation height. Distance from an arbitrarily positioned -3/2 SDC line was correlated with sward productivity.