Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 218

adapted sets of weed species,rice growers in California are now advised to con-
sider alternating between water-seeding and dry-seeding practices (Williams
et al ., 1992).

Spatial variation in moisture availability
Localized placement of water using drip irrigation technology offers
considerable opportunities for weed management in arid environments.
Access to water can be largely restricted to the crop root zone, thus minimiz-
ing water application to weeds growing between crop rows. Figure 5.2 illus-
trates how selective placement of irrigation water affected weed management
in tomato production systems in California (Grattan, Schwankl & Lanini,
1988).The site used for this experiment is characterized by little or no precip-
itation during the summer months when many vegetable crops, including
tomato,are produced.Three irrigation systems were compared: sprinkler irri-
gation, which spread water uniformly over the entire plot; furrow irrigation,
which concentrated water between crop rows; and buried drip irrigation,
which concentrated water directly beneath the crop (Figure 5.2a, b). Seeds of
two annual weed species, Amaranthus retroflexus and Echinochloa crus-galli , were
sown on all plots before irrigation treatments began. Weed growth and crop
yield were measured in plots not treated with herbicides (Figure 5.2c,e) and in
those treated with napropamide and pebulate (Figure 5.2d, f).
In the absence of herbicides, weed biomass production between crop rows
in the sprinkler and furrow irrigation systems was

3.5 Mg ha 1 (Figure
5.2c), and tomato yield was reduced by weed competition (Figure 5.2e, f). In
contrast, when buried drip irrigation was used, weed biomass was 0.2 Mg

ha 1 , even in the absence of herbicides (Figure 5.2c), and crop yield was unaf-
fected by weed competition (Figures 5.2e, f). Grattan, Schwankl & Lanini
(1988) noted that initial costs of materials and installation for drip irrigation
are high, but are offset in subsequent years by reduced traffic demands in the
field, labor savings, higher water-use efficiency, and excellent control of many
annual weed species.
Seed placement can affect a seed's access to soil moisture, the timing of ger-
mination and seedling emergence, and the outcome of crop-weed interac-
tions.In a review of weed management tactics for rain-fed agronomic crops in
Nebraska, Bender (1994, p. 37) identified planting conditions as optimum
when enough moisture is present below the soil surface to germinate the crop,
but when the surface is sufficiently dry to prevent germination of weeds until
the next rain. In this case, seed placement in deeper, moister soil provides a
competitive advantage to the crop by permitting it to emerge before the
weeds.Asimilar approach can be used in irrigated systems,where water can be