Effect of Rice Cultural Practices on Rotational Crop Herbicide Carryover and Imazethapyr Efficacy.

A two-year study was conducted to evaluate the possible interaction between rotational crop herbicides and herbicides used in water-seeded rice culture. Rice growth and yield reduction with rotational crop herbicides were not affected by the rice herbicides. However, the combination of molinate or thiobencarb with fluometuron, imazethapyr, metolachlor, or norflurazon at low residue levels further reduced rice heading compared with the corresponding rotational crop herbicide alone. Fluometuron and metolachlor residues at 1 to 2 half-lives have potential to injure rice and reduce rice yield in a water-seeded culture. Norflurazon injured rice and reduced rice yield at a residue level as low as 4 half-lives, thus has greater carryover potential to water-seeded rice. Compared with the other rotational crop herbicides, imazethapyr exhibited the lowest carryover potential to rice in a water-seeded culture. Greenhouse and field studies were used to compare the response of drill- and water-seeded rice to simulated carryover of the rotational crop herbicides. In the greenhouse study, greater reduction in emergence and shoot dry weight was observed with metolachlor and imazethapyr in drill- compared with water-seeded rice. In the field study, rice was injured with imazethapyr residue at 1 half-life in drill-seeded culture. Metolachlor at most residue levels caused much greater injury and yield reduction in drill- compared with water-seeded rice. However, norflurazon residue at 2 or 3 half-lives caused greater yield reduction in water-seeded rice. A greenhouse study was conducted to evaluate the effect of soil moisture on activity of imazethapyr applied preplant incorporated (PPI) or postemergence (POST) at 35 and 53 g ha-1. Response of barnyardgrass and red rice to imazethapyr PPI was affected by soil moisture. A reduced activity of imazethapyr PPI on barnyardgrass and red rice was observed at 50% soil moisture. Imazethapyr PPI had little activity on hemp sesbania. Imazethapyr activity on all three weeds was increased when applied POST compared with PPI. Activity of imazethapyr POST on barnyardgrass and red rice was generally not affected by soil moisture or application rates. Greater activity of imazethapyr POST on hemp sesbania was observed at soil moisture of 19 and 25%