Weeds and the soil environment - Ecological Management of Agricultural Weeds - page 217

weed species is strongly affected by water management, field preparation
methods, and rice seeding practices (Moody, 1991). For example, Sarkar &
Moody (1983) reported that weed biomass was 72-106 kg ha 1 when rice was
planted in puddled fields, but 1519-1582 kg ha 1 when it was planted in dry
fields. The relative abundance of different weed taxa was also affected by dry
versus puddled field preparation. Grass species comprised 7% to 35% of the
weed flora following puddling,but 56% to 64% of the weed flora following dry
field preparation; conversely,broadleaf and sedge species were relatively more
abundant following puddling than following dry field preparation.
Time of flooding and microtopography may also strongly affect the abun-
dance and growth of weeds in rice fields. In an experiment conducted with
irrigated rice during the dry season at Los Baños, Philippines, density and
biomass of the annual grass Leptochloa chinensis were nil when rice was flooded
five days after seeding, but increased progressively to 72 plants m 2 and 46 g
m 2 , respectively, as flooding was delayed until 20 days after planting (Moody
& Drost, 1983). Under rainfed conditions, establishment of L. chinensis seed-
lings began in higher areas of rice fields that dried during drought periods; as
water levels receded across the field, establishment of the weed followed
(Moody & Drost, 1983). Thus early, sustained, and uniform flooding can be
important for control of L.chinensis .
Although water-seeding and maintenance of flooded conditions may have
substantial value as components of weed management strategies for rice,
heavy reliance on these methods has led to a clear demonstration of how weed
floras can shift and adapt to the stresses placed upon them. Rice culture in
California using water-seeding and continuous flooding began in the 1920s as
a method to control severe infestations of Echinochloa crus-galli , which is well
adapted to dry-seeding (Seaman, 1983). However, the change to water-seeded
rice culture for control of E. crus-galli has increased the abundance of weed
species that were previously unimportant in dry-seeded fields. Echinochloa ory-
zoides and E. phyllopogon have become important weeds and have largely
replaced E. crus-galli .The former two species have large seeds which allow ger-
mination and emergence through flood water up to 30 cm deep; E. crus-galli
has smaller seeds and is unable to emerge through deep water (Barrett, 1983;
Seaman, 1983). Echinochloa oryzoides and E. phyllopogon are only partially con-
trolled by continuous flooding, and California farmers have become greatly
reliant on herbicides to control these and other species adapted to water-
seeding and flooded conditions (Seaman, 1983; Hill et al. , 1990). Reliance on
herbicides has resulted, in turn, in problems with water contamination
(Cornacchia et al ., 1984) and herbicide-resistant weed genotypes (Pappas-
Fader et al ., 1993). To improve weed control and prevent the development of