Effect of sowing date on forage rape seed quality

New Zealand produced forage rape (Brassica napus L.) seed lots usually have a germination of ≥ 90% but can differ markedly in seed vigour. Abiotic stress during seed development can reduce seed vigour, and changing sowing date has been proposed as a means of reducing the impact of this stress. Seeds of forage rape cv. Greenland were sown on 25 March and 11 April 2011 at Lincoln, Canterbury. Plant reproductive growth was monitored and a seed development study conducted. Seeds were hand harvested at 50% seed moisture content (SMC), 25% SMC and 14% SMC and their germination, seed vigour and thousand seed weight (TSW) determined. Seeds reached physiological maturity (PM) at 48% SMC after the plants had accumulated a total of 1279 (March) and 1229 (April) growing degree days (Tbase = 5°C). Seed development was similar for both sowing dates. Sowing date had no effect on germination, but both seed vigour and TSW were lower for the March than the April sowing. In the three weeks between PM and harvest maturity (14% SMC), seed quality was reduced significantly for both sowing dates. Because climate data were obtained from a weather station one kilometre from the trial site, it was not possible to directly relate seed quality differences between sowing dates to abiotic stress during seed development, and this requires a further study. For this late flowering forage rape cultivar, Greenland, an April rather than a March sowing is likely to produce a greater yield of higher quality seed