HERBICIDES in Asian rice - IRRI books - International Rice ...

Test 2. The efficacy of 38 rice accessions or germplasm lines identified as allelopathic
for aquatic weeds in dry-seeded and water-seeded culture was evaluated in the
field, in 1990 and 1991 for dry-seeded rice and in 1992 and 1993 for water-seeded
rice. Four cultivars (Gulfmont, Mercury, Palmyra, and Rexmont) that have no apparent
allelopathic activity were the controls.
The dry-seeded cultivars were seeded in 2.1- × 1.1-m plots, in three rows spaced
53 cm apart, with three replications. Unwanted weeds were controlled with herbicides
(propanil, fenoxaprop, and bentazon). Aquatic weeds, including ducksalad, purple
ammannia ( Ammannia coccinea ), and disc waterhyssop ( Bacopa rotundifolia ) that
germinated after the plots were flooded were not affected by the herbicides.
When the rice was ready for harvest, the 53-cm-wide space on each side of the
center row was evaluated for weed suppression. Weeds between the outside rice rows
were pulled, dried, and weighed. In 1990, a core soil sample (6.5 cm diameter by 2.5
cm deep) was taken on each side of the center row of eight test accessions and three
checks to determine root biomass. Soil samples were washed through a 20-mesh screen,
and the roots dried and weighed.
In the wet-seeded experiments, seeds of the same 38 accessions and four check
cultivars were placed in cloth bags, soaked in water for 24 h, and drained. The soaked
seeds were water-seeded in 2.1- × 1.1 -m plots in three 2-cm-deep furrows spaced
53 cm apart. Plots were surrounded with metal levees to prevent movement of the rice
seedlings.
Aquatic weeds ducksalad, purple ammannia, and disc waterhyssop germinated
when the rice germinated. Unwanted weeds were controlled by hand weeding. Visual
ratings were made 7 wk after flooding. At rice maturity, weeds between the outside
rice rows were harvested, dried, and weighed.
Test 3. In 1992-93, we compared the allelopathic activity of rice in dry- and
water-seeded cultures, seeded in rows or broadcast. Five allelopathic rice lines-T65/
2 × TN1, IR644-1-63-1-1, IR782- 131, IR643-75-1-1, and IR788-16-1-1-1 and
Rexmont, a standard Arkansas cultivar without allelopathic activity, were evaluated
in a split-split plot design with four replications. Dry- and water-seeded cultures were
main plots, broadcast and row seeding were subplots, and rice lines were subsubplots
(2 × 1 m). Rows were spaced 53 cm apart.
In the dry-seeded culture, rice seeds were broadcast on the soil surface or seeded
in 2.5-cm-deep furrows and covered with soil. In the water-seeded culture, rice seed
pregerminated for 36 h was broadcast or row-seeded onto soil flooded 5 cm deep. All
water-seeded plots were surrounded by a metal levee to prevent movement of rice
seedlings. A bird netting protected the plots from immediately after seeding until the
rice plants were 15 cm tall.
In dry-seeded rice, aquatic weeds germinated 37 d after seeding (7 d after flooding);
in water-seeded rice, 5-7 d after seeding. In dry-seeded rice, unwanted weeds
were controlled by propanil, bentazon, and fenoxaprop; in water-seeded rice, by hand
weeding. Aquatic weeds included ducksalad, purple ammannia, and disc waterhyssop.
Identifying allelopathy in rice germplasm 221