Reduced stem length increases perennial ryegrass seed yield

The effect of plant growth regulators inducing severe stem shortening on the seed production of perennial ryegrass (Lolium perenne) was investigated over two seasons in Canterbury using the tetraploid, late season cultivars 'Halo' (2012-13) and 'Bealey' (2013-14). Stem shortening was achieved through the application of Moddus® (active ingredient 250 g/l trinexapac-ethyl, TE) either alone or in combination with Payback™ (active ingredient 250 g/l paclobutrazol, PB and 'Cycocel®' (active ingredient 750 g/l chlormequat-chloride, CCC). Trinexapac ethyl applied as a single application at Zadoks growth stage 32 increased seed yield by up to 44% as rates increased from the untreated (1720 kg/ha) to 3.2 l/ha (2470 kg/ha). Where TE was applied in sequences (Zadoks growth stages 30, 32 and 39) seed yield was increased by 59% to 2730 kg/ha. Combinations of TE, PB and CCC increased seed yields by up to 95% (3360 kg/ha) above the untreated control. Seed yield increase was achieved through an increased number of seeds/m2. Total stem length was reduced from 105 cm to 85 cm by applications of TE alone and further to 65 cm where applications of TE, PB and CCC were applied in combination. On average, each centimetre of stem length reduction increased seed yield by 45 kg/ha. Stem length reduction was associated with delayed onset of lodging and absolute lodging at harvest, where only crops shorter than 71 cm remained standing at harvest. These results suggest that growers should aim to shorten ryegrass stem lengths to approximately 70 cm to reduce lodging and maximise seed yields.