The effect of accumulation period and harvest date in spring on dry-matter yield and forage quality in mixed swards containing Lolium spp. and Trifolium subterraneum in Western Australia

The object of this study was to determine the effect of closing date and date of harvest for conservation (accumulation period), on dry-matter (DM) yield and forage quality of annual pasture in Western Australia. The field study comprised 48 plots, 2 m × 2 m, sown with either annual ryegrass (Lolium rigidum Gaud.) or Italian ryegrass (L. multiflorum Lam.), and mixed with subterranean clover (Trifolium subterraneum L.). Defoliation of swards until the end of winter was at the three leaves tiller–1 stage. In spring, once stem nodal development had commenced, swards were defoliated every 3–4 weeks. Swards were defoliated either twice with three leaves tiller–1 (accumulation period 1 commenced on 15 August); twice with three leaves tiller–1 and then once after 4 weeks (accumulation period 2 commenced on 11 September); twice with three leaves tiller–1 and then twice after 4-week intervals (accumulation period 3 commenced on 9 October) or; twice with 3 leaves tiller–1 and then twice after 4-week intervals and then once after 3 weeks (accumulation period 4 commenced on 30 October). From the commencement of the accumulation period, tiller density, DM yield and forage quality were determined weekly for up to 10 weeks. There was a positive quadratic association between DM yield and days after the commencement of the accumulation period. Yields were maximized from accumulation period 1 with 5·3, 6·6 and 9·5 t DM ha–1, and growth rates were 140, 128 and 145 kg DM ha–1 d–1, for Wimmera annual ryegrass and Richmond and Concord cultivars of Italian ryegrass respectively. In contrast, in vitro dry-matter digestibility (IVDMD) and crude protein (CP) content were negatively associated with days after the commencement of the accumulation period, and initial values were greater than 0·80 and 180 g kg DM–1 for IVDMD and CP content respectively. The rate of decline in IVDMD d–1 for Wimmera annual ryegrass was 0·005, 0·019 and 0·012 d–1 for accumulation periods 1, 2 and 3, respectively, while for Italian ryegrass cultivars Richmond was 0·015, 0·011, 0·02 and 0·012 d–1 and Concord was 0·014, 0·009, 0·013 and 0·01 d–1, for the 4 accumulation periods respectively. It is recommended that annual and Italian ryegrass pastures be harvested between 10% and 20% inflorescence emergence when IVDMD will exceed 0·70 regardless of cultivar and/or defoliation practice prior to the commencement of the accumulation period.