Weed succession as affected by repeated applications of the same herbicide in direct-seeded rice field

M. Azmi

Abstract

A long term study on the effect of repeated use of the same herbicide on the changes in weed population was carried out at MARDI Station, Seberang Perai. Eight herbicide treatments were repeatedly applied for ten consecutive seasons commencing in off-season 1996 to off-season 2001. Only a single application of herbicide at the recommended crop stage was carried out in each season. Residual weeds were sampled for counting and dry weight determination at 60 days after sowing. Summed dominance ratio (SDR) for each treatment was calculated by comparing weed species dominance and composition between seasons. The repeated used of a particular herbicide greatly influenced weed species dominance and composition. A noxious weed, Echinochloa crus-galli was dominant in plots repeatedly applied with 2,4-D amine. Whilst, a broad-leaved weed, Monochoria vaginalis became dominant when molinate/propanil, benthiocarb/propanil, pretilachlor, quinclorac, propanil and fenoxaprop-ethyl were used repeatedly. A perennial sedge, Scirpus grossus was among the dominant weed in plots repeatedly applied with quinclorac, propanil, pretilachlor, benthiocarb/propanil, fenoxaprop-ethyl and bensulfuron. However, this weed has become the most dominant weed in certain seasons in plots repeatedly applied with bensulfuron. Generally, all herbicide applications significantly showed an increase in a perennial grass, Paspalum vaginatum population. Furthermore, Bacopa rotundifolia was observed to increase in its population during season 9 and 10 in plots repeatedly applied with bensulfuron. The long-term result from continuous use of a particular herbicide showed an increase in certain weed species population. However, the majority of herbicide treated plots produced higher grain yields than untreated plots for ten consecutive seasons. The differences in yields arising from the use of alternative herbicides were recorded in most seasons. These results suggested that successive applications of the same herbicides in some cases should be limited to only two or three seasons. In the case of further successive applications, other methods should be used to control the prevalent weed species.

Full Text (377 KB)