Effect of flooding depth and pretilachlor rate on emergence and growth of three rice weeds : junglerice ( Echinochloa colona ) , smallflower umbrella sedge ( Cyperus difformis ) , and ludwigia ( Ludwigia hyssopifolia )

The effects of intermittent flooding and pretilachlor rates on emergence and early growth of three common weeds of lowland rice were assessed in a screenhouse study. These species were junglerice (Echinochloa colona), smallflower umbrella sedge (Cyperus difformis), and ludwigia (Ludwigia hyssopifolia). The weeds were treated with pretilachlor at 0.075 to 0.3 kg a.i. ha -1 at 3 d after seeding, and subjected to alternate flooding (0, 2, 4, and 6 cm water depth) and draining that commenced 2 d after herbicide treatment. Intermittent flooding to 6 cm decreased the emergence of all three weed species. Smallflower umbrella sedge growth was not affected by flooding alone, but was completely suppressed by pretilachlor application. Growth of junglerice was reduced more by pretilachlor when application was followed by flooding. Our results suggest that emergence and seedling growth of these three weeds can be suppressed with pretilachlor and, further, the effect was increased when combined with flooding. Pretilachlor appears to be a useful management option on tropical lowland rice farms and particularly for the control of junglerice, for which farmers can combine an appropriate application rate with subsequent flooding of fields.