Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 283

been inconsistent. Although a majority of studies have shown that narrow
row spacing suppresses weeds, a large minority of studies have shown no
effect or an inconsistent effect across several experiments (Table 6.2).
Variability in the effect of row spacing on weeds in the literature largely con-
sists of variation among studies on a given species rather than variation
among species (Table 6.2).
Plants tend to distribute their leaves into areas of high light and away from
zones of high leaf area index, so that the horizontal distribution of leaves
tends to be more uniform than the planting arrangement. This effect could
explain some cases in which weed growth failed to respond to row spacing.
However, several studies have shown that percent canopy cover develops
faster, and light penetration through the canopy is less when rows are more
closely spaced (Teasdale & Frank, 1983; Murdock, Banks & Toler, 1986;
Yelverton & Coble, 1991; Murphy et al ., 1996). Decreased light penetration
with narrow rows results both from higher leaf area index (Murphy et al .,
1996), and from a more efficient arrangement of leaves that increases light
interception per unit leaf area (Flénet et al ., 1996).
The studies in Table 6.2 found that narrowing row spacing in the presence
of weeds increased crop yield in a slim majority of cases (27 out of 49).
However, several studies were unclear as to whether the increased yield was
due to weed control or would have occurred even in weed-free conditions.
Although many studies showed no yield response to row spacing, only three
observed a decrease in yield with narrower rows (Table 6.2).
Some cases in which weed biomass did not decrease with closer row spacing
may be related to nutrient management. Banding fertilizer next to the crop
row creates asymmetric competition for nutrients: on average, a unit of root
growth by the crop early in the season allows greater access to the fertilizer
than does a unit of root growth by the weeds because the fertilizer is closer to
the crop. However, as the rows are placed closer together, this relative advan-
tage declines.
Many cases in which weed biomass or crop yield in weedy conditions did
not respond to row spacing can probably be explained by the height of the
weed species relative to the crop. Note in this regard that the model of Fischer
& Miles (1973) concerned only the occupation of ground area and did not
include the relative height of the crop and weeds. In contrast, Schnieders et al .
(1999) used a model to explore effects of row spacing on competition between
two species that differed only in height. When the crop was potentially twice
as tall as the weed,closer row spacing increased crop yield.When stature of the
two species was equal, row spacing had a much smaller effect. Finally, when
the weed was taller than the crop, crop yield declined at closer row spacing