The effect of defoliation practice in Western Australia on tiller development of annual ryegrass (Lolium rigidum) and Italian ryegrass (Lolium multiflorum) and its association with forage quality

The effect of defoliation on the vegetative, early reproductive and inflore scence stages of tiller development, changes in the dry-matter yield of lea f, stem and inflorescence and the associated changes in forage quality was determined on plants of annual ryegrass (Lolium rigidum Gaud.) and Italian ryegrass (L, multiflorum Lam.). The field study comprised seventy-two plots of 1 m x 2 m, sown with one annual ryegrass and seven Italian ryegrass cul tivars with a range of heading dares from early to late: defoliation commen ced 6 weeks after germination. During the vegetative stage of growth, plots were defoliated when the tillers had three fully expanded leaves (three-le af stage). During the early reproductive stage of growth, to simulate a cut for silage, plots were defoliated 6-7 weeks after 0.10 of the, tillers dis played nodal development. The subsequent regrowth was defoliated every 3 we eks. Assessments of changes in tiller density, yield and quality were made in th e growth cycle that followed three contrasting cutting treatments during th e winter-spring period (from 10 July). In treatment 1, this,growth cycle (f ollowing closing-up before a subsequent conservation cut) commenced on 7 Au gust following two defoliations each taken when the tillers were at the thr ee-leaf stage. In treatment 2, the growth cycle commenced on 16 October fol lowing: for early-maturing cultivars. two cuts at the three-leaf stage, a c ut for silage and an additional regrowth cut; for medium-maturing cultivars three cuts at the three-leaf stage and a cut for silage; and late-maturing cultivars, five cuts at the three-leaf stage. In treatment 3, defoliation up to 16 October was as for treatment 2, but the growth cycle studied starr ed on 27 November following two additional regrowth cuts for early- and med ium-maturing cultivars and cut for silage for the late-maturing cultivars. Tiller development for all cultivars was classified into three stages; vege tative, early reproductive and inflorescene. In treatment 1, in vitro dry-m atter digestibility (IVDMD) and crude protein (CP) content were negatively associated with maturation of tillers. IVDMD ranged from 0.85 to 0.60 and C P ranged from 200 to less than 100 g kg(-1) dry matter (DM) during the vege tative and inflorescence stages respectively. This large reduction in forag e quality was due to an increase in the proportion of stem, inflorescence a nd dead material, combined with a reduction in the IVDMD and CP content of the stem. A high level of forage quality was retained for longer with later -maturing cultivars, and/ or when vegetative tillers were initiated from th e defoliation of early reproductive tillers (treatments 2 and 3). However, 15 weeks after the closing-up date in treatment I, defoliation significantl y reduced the density of inflorescences with means (+/- pooled s.e.m.) of 1 560, 1178 and 299 +/- 108 tillers m(-2), and DM yield of inflorescence with means of 3.0, 0.6 and 0.1 +/- 0.15 t ha(-1) for treatments 1, 2 and 3 resp ectively. This study supports the recommendation that annual and Italian ryegrass cul tivars should be classified according to maturity date based on the onset o f inflorescence emergence, and that the judicious defoliation of early repr oductive tillers can be used to promote the initiation of new vegetative ti llers which in turn will retain forage quality for longer.