Cereal planting dates as a tool in the management of Galeopsis tetrahit and associated weed species in spring barley and oat

The effects of delayed planting and herbicide treatments on populations of Galeopsis tetrahit and associated weed species, and on mid-season above-ground dry weight and grain yield of spring barley and oat were measured in field experiments conducted from 1989 to 1991, in La Pocatière, Québec, Canada. Two crop planting dates were compared in 1989 and three dates in 1990–1991. The weed control factor included two post-emergence herbicide treatments (cyanazine/MCPA, thifensulfuron-methyl) and a weedy control treatment. By mid-season, post-planting cohorts of G. tetrahit generally had comparable densities and above-ground dry weight, regardless of crop planting date. Delayed planting reduced density but had no effect on dry weight of associated weeds. Both herbicide treatments effectively suppressed G. tetrahit and associated weeds. However, effects of weed control treatments were generally the same, regardless of planting date. Delayed planting most often did not compensate for the benefits from chemical weed control and affected both crops at mid-season by reducing crop height and dry weight. This effect carried over to oat but not to barley grain yields. Herbicide applications were beneficial to barley grain yields in two out of three years but not to oat yields. Overall, delayed planting was generally not favourable to early barley and oat growth and oat yields, and did not provide the expected suppression of G. tetrahit and associated weeds which could have alleviated the need for chemical weed control.