Herbicides and Seeding Rate Effects on Sprinkler‐Irrigated Rice1

Abstract

AbstractTwo herbicide programs and four seeding rates were tested in 1981 and 1982 at Keiser, AR, to determine the optimum seeding rate that would be effective in maximizing yields in sprinkler‐irrigated rice (Oryza sativa L.). ‘Lebonnet’ rice was drill‐seeded into a Sharkey clay (Vertic Haplaquepts, 1.4% O.M.) at rates of 50, 100, 150, and 200 kg ha 1. The herbicide programs used were propanil [N−(3,4− dichloropheny1)propanamide] at 3.4 kg ha 1 or a tank mixture of propanil at 3.4 kg ha 1 + oxadiazon [3−[2,4−dichloro−5−(1−methylethoxy) phenyl]−5−(1,l−dimethylethyl)−1,3,4−oxadiazol−2−(3H)‐one] at 1.1 kg ha−1, each applied at the 2‐leaf stage of weeds. Each treatment was followed by 2,4,5−T [(2,4,5−trichlorophenoxy)acetic acid] at 1.1 kg ha 1 applied over‐the‐top at the onset of internode elongation of the rice stems. Both herbicide programs controlled barnyardgrass [Echinochloa crus‐galli (L.) Beauvois], broadleaf signalgrass [Brachiaria platyphylla (Grisebach) Nash], large crabgrass [Digitaria sanguinalis (L.) Scopoli], common cocklebur (Xanthium strumarium L.), and smooth pigweed (Amaranthus hybridus L.). Propanil + oxadiazon controlled pitted morningglory (Ipompoea lacunosa f. purpurata L.) and prickly sida (Sida spinosa L.), but propanil alone did not. A significant increase in rice plant population and panicle density with the increased seeding rates was offset by a corresponding decrease in number of seeds per panicle and rough rice yield, and an increase in tillering at the lower seeding rate. The results indicate that the present recommended seeding rate (140 kg ha 1) for sprinkler‐irrigated rice should be reduced to between 50 and 100 kg ha−1, but must be accompanied by timely and effective herbicide programs.