Abstract:
Difficulties in chemically controlling large crabgrass in corn in South Africa have recently been
attributed to the occurrence of naked crabgrass. In contrast to large crabgrass, naked crabgrass is not
easily controlled with acetanilide herbicides. Critical periods of weed control (CPWC) for naked
crabgrass in corn was determined in field studies during the 2009/2010 and 2010/2011 growing
seasons at two separate localities for an early and late planting date of corn. Weed-free and weed–crop
interference treatments of increasing duration were maintained at various crop growth stages in the
presence of naked crabgrass. Biomass of naked crabgrass was determined as dry weight per square
meter quadrant, which yielded 428 g m22 at Potchefstroom and 594 g m22 at Wesselsbron. An
exponential regression model was used to determine the CPWC expressed as growing degree days
after crop emergence, on the basis of an estimated 10% relative yield loss in corn. The onset and
ending, as well as the duration of the CPWC, differed between seasons and localities. At 10% relative
yield loss, the onset of the CPWC ranged between the two-leaf (V2) and six-leaf (V6) stages, and the
ending between the 12-leaf (V12) stage and 2 wk after tasseling (T + 2). The duration of the CPWC
ranged between 22 and 80 d for the respective planting dates, years, and localities. Yield losses ranged
from 28 to 82% in the season-long weedy plots. The shifting of planting dates alone did not reduce
yield losses since the effect of late infestations of naked crabgrass is significant. Naked crabgrass
control from crop emergence is essential, followed by POST herbicide application during the critical
period of weed control to lower the risk of corn yield losses.
