Understanding the effect of a triazole with anti-gibberellin activity on the growth and yield of oilseed rape (Brassica napus)

The effects of the anti-gibberellin triazole metconazole on yield, lodging resistance, seeds/m(2) (sink size) and rooting in the presence of negligible amounts of disease were investigated using five multisite experiments carried out in the UK between 1999 and 2007. The green area index (GAI) of the crop at the time of application had the greatest influence on the yield response across 173 comparisons between metconazole-treated and untreated crops. Crops with a GAI in March (growth stage (GS) 3.3-3.5 'green bud') of 0.8-2.8, which represented moderate to large sized UK commercial crops, had an average response of +0.21 t/ha to metconazole applied at the full recommended rate of 1.2 litres/ha. Crops with a GAI of less than 0.8, which represented small commercial crops, had an average response of -0.14 t/ha. Applications between green bud and early flowering gave the greatest yield increase. Positive yield responses were associated with a reduction in lodging, or more seeds/m(2). ora combination of both. The proportional area lodged was reduced by 0.1 for each 50 mm reduction in crop height (R-2=0-60; P<0.001). Across six field trials. metconazole applied in spring increased root length density between soil depths of 0.40 and 1.00 in from an average of 0.478 to 0.601 cm/cm(2) (P<0-05). It was estimated that this could increase yield by 0.2-0.3 t/ha in a dry season. The results show that the growth regulatory properties of metconazole can be used to improve several key aspects of growth that are believed to restrict current farm yields.