LEAF NUMBER AS A CRITERION FOR DETERMINING DEFOLIATION TIME FOR LOLIUM-PERENNE .1. EFFECT OF WATER-SOLUBLE CARBOHYDRATES AND SENESCENCE

This study sought to define the optimum defoliation interval for Lolium perenne, with the maximum interval being determined by the onset of senescence, as reflected by the number of fully expanded leaves, and the minimum interval set by the replenishment of water-soluble carbohydrates (WSCs). In a glasshouse set at 13-degrees-C/23-degrees-C night/day temperature and at a plant spacing of 94 m-2 (3290 tillers m-2), the accumulation of dry matter against leaf number and days was exponential to the four-leaf stage of the regrowth cycle. Senescence commenced at the 3.5-leaf stage. WSCs in roots, stubble and leaf showed a very significant linear relationship with leaf number. Levels of WSCs in leaves were restored to predefoliation values by the one-leaf stage, after which WSCs accumulated preferentially in the stubble, rising to 22% at the four-leaf stage. Regrowth after 6 d was significantly greater for plants defoliated at the three-leaf stage than at the one-leaf stage, but this difference disappeared at the end of the regrowth cycle. This questions the importance of WSCs in determining the yield of ryegrass under normal rotational grazing management. Regrowth at 6 d was related positively to stubble WSCs (r2 = 0.66) and to stubble DM per tiller (r2 = 0.71).