Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 341

specified set of growing conditions and genotypes, these normal density sole
crops are expected to produce the highest yields from the least amount of
seed. An “additive mixture” containing 250 wheat plus 50 fava bean plants
m 2 could also be sown and would represent 200% of the normal population
densities used for the sole crops. In contrast, a “replacement series mixture”
containing equal proportions of the two crops at their normal densities would
contain 125 wheat plus 25 fava bean plants m 2 . Such a mixture would repre-
sent the same total plant density as the two normal density sole crops, and
comparisons of yield and weed suppression characteristics could be made
between the intercrop and sole crops without confounding changes in crop
diversity with changes in density.
Sharaiha & Gliessman (1992) found that yield advantages and superior
weed control resulted directly from increases in crop diversity. The investiga-
tors grew sole crops and two-species mixtures of fava bean, pea, and lettuce in
constant-density replacement series. The two legumes were direct-seeded;
lettuce was transplanted. Intercrops were formed by substituting rows of
other species for rows in the sole crops. All plots were weeded until 34 days
after planting; thereafter, weeds were allowed to grow. Intercropping yield
advantages were evident in LER values for fava bean/pea, fava bean/lettuce,
and pea/lettuce intercrops that averaged 1.55, 1.48, and 1.26, respectively.
Mean weed biomass in the fava bean/pea intercrop was 48% less than in the
fava bean sole crop,and 40% less than in the pea sole crop.Mean weed biomass
in the fava bean/lettuce intercrop was 30% and 32% less than in the respective
sole crops; in the pea/lettuce intercrop, mean weed biomass was 6% and 23%
less than in the respective sole crops.
Weed suppression advantages of intercrops over sole crops in constant-
density replacement series were also observed by Fleck, Machado & De Souza
(1984) in experiments with bean, sunflower, and maize. Intercrops were
formed by substituting rows of one crop for another. Bean/sunflower and
maize/sunflower intercrops were most successful in suppressing weed growth
(Figure 7.6),an effect attributed to the greater canopy cover of those two inter-
crops, and to the ability of maize and sunflower to grow taller than the asso-
ciated weeds.
In other studies, intercropping has failed to suppress weed growth more
than sole cropping unless total crop density has been elevated above levels
used for sole crops. Experiments with intercrops and sole crops of wheat and
fava bean (Table 7.1) (Bulson, Snaydon & Stopes, 1997), barley and pea
(Mohler & Liebman, 1987), sorghum and pigeonpea (Shetty & Rao, 1981), and
pearl millet and peanut (Shetty & Rao, 1981) indicated that at a given
overall crop density, weed biomass in the intercrop was always more than in