A field study was carried out to identify phenological parameters in corn indicative of the onset of weed interference and the moment to start weed control. The treatments consisted of different periods of weed/crop coexistence: 0 to 7, 0 to 14, 0 to 21, 0 to 28, 0 to 35, 0 to 42, 0 to 49 and 0 to 56 days after corn emergence (DAE), and checks with and without weeds. Plant height, stem diameter and leaf number were evaluated at every interval of seven days until 56 DAE and at tasseling (75 DAE). Yield was also evaluated. The effect of weed on the three parameters evaluated and on yield reduction started simultaneously at 28 days after corn emergence. The effects on plants were irreversible. The plants were unable to recover after weed competition was removed; therefore, the use of phenological parameters as indicators of the start of weed control was shown to be inappropriate.
Zea mays; competition; plant height; stem diameter; leaf number
