Weed Population Dynamics and Weed Control in the Peruvian Amazon

AbstractIn traditional slush‐and‐burn agriculture, forest fallow is a primary agent in weed control. Stable continuous‐cropping systems could be expected to require a comprehensive program of weed management, probably including the use of herbicides. Weed control measures in a sequence of rice (Oryza sativa L.)‐corn (Zea mays L.)‐soybean (Glycine max L.)‐rice‐corn‐soybean were studied for 2 yr in Yurimaguas, Peru, with the following objectives to: (i) identify weed species resistant to the herbicide program in a continuous cropping system, and (ii) develop effective weed management practices for intensively managed cropping systems in the humid tropics. First‐crop weeds were 60% grass, 25% sedges, and 15% broadleaf weeds. The grass was composed of two species: large crabgrass [Digitaria sanguinalis (L.) Scop.] and goosegrass (Eleusine indica (L.) Gaertn]. Sixth‐crop weed population was 80% grass, 13% broadleafs and 7% spreading dayflower (Commelina diffusa Burm. f.); and itchgrass (Rottboelia exaltata L.f.) comprised 85% of the grasses. Metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2‐methoxy‐l‐methylethyl) acetamide] controlled crabgrass, goosegrass, and most broadleafs in corn and soybean; but it did not control itchgrass. Metolachlor alone resulted in a weed population that was 97% itchgrass in the sixth crop. Sethoxydim {2‐[(1‐ethoxyimino)butyl]‐5‐[2‐(ethylthio)propyl]‐3‐hydroxy‐2‐cyclohexen‐1‐one}‐plus bentazon [3‐(1‐methylethyl)‐(1.H)‐2,1,3‐benzothiadiazin‐4(3H)‐one 2.2‐dioxide] on soybean controlled grasses including itchgrass, but broadleaf weeds and dayflower increased. Propanil [N‐(3,4‐dichlorophenyl) propanamide]‐plus‐oxadiazon {3‐[2,4‐dichloro‐5‐(1‐methylethoxy)phenyl]‐5‐(1,1‐dimethyl)‐1,3,4‐oxadiazol‐2‐(3H)‐one} on rice resulted in a mixed grass population. Rice was more vulnerable to weed pressure than either corn or soybean and appears inappropriate for this high‐input rotation because of the high cost of weed control.