Use of Clearfield Rice Technology in Stale and Conventional Seedbed Systems and Clomazone in a Stale Seedbed System

Experiments were conducted during the summer of 2002 at the Rice Research and Extension Center, Stuttgart, AR, to evaluate: 1) various timings of sequential applications of imazethapyr at 0.063 lb/acre (4 oz Newpath/acre) in conventional and staleseedbed rice planting systems utilizing Clearfield ‘CL-161’ and 2) the timing of clomazone application at various rates and timings in conventional rice, ‘Wells’, in a stale seedbed planting system. In the imazethapyr study, conventional-seedbed plots were alternated with the stale-seedbed plots throughout the test area. In both studies, burndown treatments of glyphosate alone or in combination with various herbicides were applied 3 weeks prior to rice planting. Imazethapyr applications were made either 21 days prior to planting (DPP) followed by postemergence (POST) (stale seedbed only); preplant incorporated (PPI) fb POST (conventional seedbed only); preplant (PRE) fb POST (both planting systems); or sequential POST(both planting systems). Clomazone was also applied 21 DPP, delayed PRE, or early POST. Barnyardgrass (Echinochloa crusgalli L.), propanil-resistant and -susceptible biotypes (both seeded in rows) plus a natural population, and broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash] control were evaluated in both studies. Weed control did not differ between tillage systems, except at 14 DAE, when barnyardgrass control was only about 65% from the early preplant application of imazethapyr compared to 75 % from PPI or PRE treatments. With the very moist conditions early in the growing season, control of all weeds by 40 DAE was outstanding with all the imazethapyr sequential treatments in both cultural systems (100%). There was no crop injury observed with any treatment of imazethapyr and rice yields were good, especially in the stale-seedbed plots, 6640 to 8360 lb/acre. In AAES Research Series 504 170 the imazethapyr study, the stale-seedbed system yielded 15% more rice than the conventional seedbed system. Imazethapyr timing did not affect rice yield. An early preplant application of clomazone was less effective for in-crop barnyardgrass control than a DPRE application and the resulting rice yield was less. Clomazone was equivalent to pendimethalin plus quinclorac and superior to pendimethalin alone for in-crop barnyardgrass control, when applied DPRE. When mixed with propanil and applied early POST, clomazone, quinclorac, or the mixture of pendimethalin plus quinclorac were superior to pendimethalin for in-crop barnyardgrass control and gave excellent rice yields. In all cases, where grass weed control was superior, rice yield was high. There was excellent tolerance of Wells rice to even 0.8 lb/acre of clomazone with the stale seedbed system.