Enhancing the competitive ability of crops - Ecological Management of Agricultural Weeds - page 287

which N-S orientation is advantageous increases from six months to nine
months (Mutsaers, 1980). At the equator, N-S orientation is advantageous
except in January and December, and again during June and July.
Effects of row orientation on weed management have received little direct
attention, but several studies provide relevant information. In a seven-year
study in Illinois, Pendleton & Dungan (1958) found that oat consistently
yielded more when planted in N-S rather than E-W rows (mean increase of
75-276 kg ha 1 , depending on row spacing). Light levels in N-S-oriented
inter-rows were lower than in E-W inter-rows early and late in the day when
light levels were most likely to be limiting, and clover interseeded into the
oats had lower density in the N-S-oriented plots, especially in the crop rows.
In a study of interseeding cover crops into maize, Larson & Willis (1957) dem-
onstrated that with E-W-oriented rows, a zone of high average light intensity
on the south side of the row extended to near the base of the crop. With
N-S-oriented rows,shading was more symmetrical.This is critical since weeds
near the crop row are more competitively damaging and more difficult to
control by cultivation than weeds in the inter-row (see Chapter 4). The model
of Schnieders et al .(1999) found little effect of row orientation on competition
between two species planted in parallel rows. However, their two species were
given similar characteristics so this result may not represent systems in which
the crop and weed differ in initial size and growth rate (see Chapter 2).
Although the effects of crop orientation on weeds remains to be verified,
and are likely to be small relative to the benefits of other cultural practices,use
of a N-S row orientation may increase the efficiency of the crop and suppress
weeds with no extra expense to the grower.However, the benefits are unlikely
to compensate for potential problems if field shape and topography are not
conducive to N-S planting.
Crop genotype
Crop genotype affects interaction of crops with weeds
The role of crop genotype in weed management has received growing
attention over the past 30 years. Callaway (1992) reviewed literature on crop
varietal tolerance to weeds, and Callaway & Forcella (1993) assessed the pros-
pects for breeding improved weed tolerance.
Following the lead of plant ecologists working on problems of competitive
interaction between plants (Goldberg & Landa, 1991), Callaway (1992) distin-
guished between competitive effect , or the ability of the crop to suppress weeds,
and competitive response , or the ability of the crop to avoid being suppressed.