Weed control in glyphosate tolerant sugar beet.

Abstract

Between 1997 and 1999 weed control trials were carried out with sugar beet tolerant to glyphosate. Glyphosate was applied at a total dose of 1620.0, 2160.0, 3240.0 and 4320.0 g a.i. ha-1 in two and three applications. These were compared with a standard and double standard three spray sugar beet herbicide programme. In all seasons application of the lowest dose of glyphosate, 1620.0 g a.i. ha-1 gave marginally better control of weeds than the standard herbicide programme. Herbicide timing was more flexible with glyphosate and only two weeds, Polygonum convolvulus and Lamium purpureum required more than one application to kill all the weeds. In most cases no significant difference in weed control was observed between the glyphosate treatments after the second and third applications but the three spray programmes were marginally better than the two spray in 1997 and 1998. In 1997 the sugar beet strain was not totally tolerant to glyphosate and a reduction in plant numbers was recorded after the initial glyphosate application. The strain used in subsequent years was fully tolerant and no plant loss occurred even at the highest glyphosate dose. At harvest most of the root yields in the glyphosate treatments were significantly higher than the yields from the standard herbicide comparison treatments. Crop vigour was not affected by any of the treatments in 1997 but in 1998 and 1999 the two standard herbicides reduced crop vigour by 10 and 20 per cent respectively.