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A glasshouse study was undertaken to determine the priority within the perennial ryegrass (Lolium perenne L.) plant for leaf and root growth and daughter tiller initiation after defoliation, in relation to various levels of water‐soluble carbohydrate (WSC) reserves at defoliation. Individual plants were arranged in mini‐swards, and underwent varying defoliation frequencies and ambient temperatures before defoliation, and harvest heights at defoliation, to obtain a gradient of WSC content at H1, the date when all plants were defoliated. Defoliation interval consisted of defoliating either three times at the one new leaf tiller–1 stage (1‐leaf stage) of regrowth, or once only at the 3‐leaf stage, up to H1, while night temperature in the week prior to H1 was altered from 15°C to either 8 or 20°C. At H1, plants were defoliated to a stubble height of either 20 or 50 mm. Plants were subsequently destructively harvested at days 4, 6, 8, 12, 18 and 27. Leaf and root extension and tiller dynamics were also measured. On a regrowth timescale, tiller initiation was most sensitive, root regrowth moderately sensitive, and leaf regrowth relatively insensitive to a decrease in WSC. The time of daughter tiller initiation also coincided with replenishment of stubble WSC levels. In contrast to this sequence of regrowth events following defoliation, the quantitative effects on growth were different, with elongation and survival of roots most affected by reduced WSC levels. A 30‐fold difference in stubble WSC at H1 between high and low WSC plants (1·52 vs. 0·05 mg tiller–1) produced only a 4‐fold increase in leaf dry matter (DM) after 27 d (2·2 vs. 0·6 g plant−1), while tiller number plant−1 increased 6‐fold (138 vs. 23% increase in tiller number from H1). Root elongation rate was 59 times higher in the high than in the low WSC plants (1·18 vs. 0·02 mm d−1). From a pasture management perspective, the study confirms that defoliation, coinciding with the 3‐leaf stage of regrowth and around a stubble height of 50 mm, optimizes persistence and productivity of perennial ryegrass. By allowing more rapid replenishment of WSC reserves, this optimal defoliation strategy enables a greater proportion of WSC to be allocated to maintain a more active root system, and promotes tillering, compared with more frequent and close defoliation.

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This glasshouse study aimed to determine the relative importance of water-solubie carbohydrates (WSC) and current photosynthate on root and top regrowth of perennial ryegrass {Lolium perenne L.). Individual plants were arranged in one of two miniswards (Experiments 1 and 2) and underwent varying defoliation frequencies designed to obtain a gradient of WSC content at the final harvest of each treatment (H,). when all treatments were defoliated. In Experiment I. the plants were defoliated either three times at the one new leaf per tiller stage of regrowth (treatment 3X1). once at the two-leaf and again at the one-leaf stage (treatment 2. 1). once at the one-leaf and again at the two-leaf stage (treatment 1.2) or once only at the three-leaf stage (treatment 1 X 3). up to H,, Leaf and root growth and other parameters were assessed over 6 d after H, in sunlight, and over a 4-week, period in darkness, and related to initial plant WSC content. In Experiment 2, plant defoliation treatments were: 3 X I, 1, 2 or I X 3. Leaf regrowth was assessed for 36 d until the plants had three fully expanded new leaves per tiller.Leaf regrowth in both experiments was significantly related to stubble WSC (below 50 mm height).In Experiment 1. plants were almost fully reliant upon plant reserves for the first 3 d of regrowth, with reliance decreasing up to 6 d. When regrowth of plants was compared after I week in light or in darkness, it was estimated that one-third of leaf regrowth was due to plant WSC reserves and the remainder due to photosynthesis. However, the capacity to Correspondence: Dr D.J. Donaghy, Wollongbar Agricultural Inslilule. Wollongbar, Sew South Wales 2477, Australia. photosynthesize and to grow roots after H, was also significantly related to stubble WSC content.In Experiment 2, there was a significant difference (P < 0-01) between defoliation treatments on leaf dry matter (DM) yield at 12 d (=1 leaf tiller"') of regrowth, and this was, as in Experiment 1, significantly positively related to WSC content in the stubble. However, after 36 d of regrowth, DM yield of plants defoliated at 2 or 3 leaves tiller"' uptoH, were similar, and both were significantly higher (P < 0 001) than regrowth of plants defoliated at the one-leaf stage.After defoliation, the period of reliance on WSC reserves may be substantially increa.sed in situations of shading (canopy competition or cloud cover) or if the new regrowth shoot is removed by regrazing.