EFFECTS OF TIME OF WEED REMOVAL ON GROWTH AND YIELD OF SUGAR-BEET

Field experiments at Sutton Bonington between 1970 and 1974 tested how crop yields were affected by hand weeding at different stages in the life of early- and late-sown crops. Losses where weeds were never controlled ranged from 95 where tall-growing Ohenopodium album L. predominated to 50 when Stellaria media (L.) Vill. and Tripleurospermum maritimum L.) Koch ssp. inodorum Hyl ex. Vaarama (T. inodorum) were most involved. In five of the seven crops examined the latest date (X) at which weeding had to commence to prevent irreversible effects on growth and yield and the earliest date at which weeding could cease without yield loss (Y) coincided. For late March/early April sowings this occurred 6 weeks after crop emergence, while for late April/early May sowings in which events were more rapid the same stage was reached 4 weeks after emergence. For an early-sown crop in 1973, when rainfall was frequent and heavy and C. album prolifiC. X preceded Y with an interval of 4 weeks between the two dates (4-8 weeks after crop emergence, mid-May to mid-June). In complete contrast for a late-sown crop in 1974, when the weather was exceptionally dry and G. album virtually absent, Y preceded X and a single weeding at any time between 2 and 8 weeks after emergence would have prevented yield loss. Sugar beet had generally reached the late singling stage (4-6 true leaves) by the time weeding must have commenced. During the next 6 weeks, final yield was depressed by 120-150kg/ha (1.5) with each day that weeds were allowed to remain. However, leaf production continued throughout the season and the crop was capable of recovering from early checks caused by weed competition. It proved possible for crops whose weights were depressed by 25-30 when first weeded to recover completely by harvest. © 1979, Cambridge University Press. All rights reserved.