Critical Period of Weed Control in Cotton (Gossypium hirsutum L.) at Three Nitrogen Levels*

Abstract

The critical period for weed control is that part of the crop
growth cycle during which the crop must be kept weed-free to prevent
unacceptable yield losses. Field studies were conducted for two
consecutive years, 2004 and 2005, in the Thessaly plain in central Greece
to determine the critical period for weed control in cotton at three nitrogen
levels (50,100, and 150 kg N ha-1 ).Treatments of increasing duration of
weed interference and weed-free period were imposed at biweekly
intervals from 0 to 10 weeks after crop emergence (WAE). Based on an
arbitrarily acceptable yield loss level of 5% and 10%, the beginning and
end of the critical period were determined by fitting the polynomial
regression to relative yield data representing increasing duration of weed
interference and weed-free period. Plant height, shoot and root dry
weights and yield of cotton were reduced by prolonged delays in weed
removal at all nitrogen levels in 2004 and 2005. The results showed that
weeds reduced cotton shoot and root dry weights, respectively, by 55%
and 38 % at 50 kg N ha-1 , 50% and 42 % at 100 kg N ha-1 and 40% and 46
% at 150 kg N ha -1 , when allowed to compete for two weeks. At 10%
acceptable yield loss level, the critical period was 8.2 weeks, starting 0.4
WAE and ending 8.6 WAE at 50 kg N ha -1 , and was decreased to 7
weeks, starting 1 WAE and ending 8 WAE at 100 kg N ha-1 . The critical
period decreased to 6.1 weeks, at 150 kg N ha -1 , starting 1.3 WAE and
ending 7.4 WAE. These findings showed that the supply of nitrogen to the
crop and weeds significantly influenced the crop-weed interference
duration. This information will be useful for providing weed control
recommendation to cotton producers.