Abstract

Barnyardgrass (Echinochloa crus-galli) is one of most important weeds in rice field in Vietnam. Seventy-eight barnyardgrass seed samples were collected from rice fields in seven provinces in Mekong Delta of Vietnam. The seedlings grown from collected seed samples were evaluated to determine the level of resistance to bispyribac, penoxsulam and quinclorac. Farmers who managed the fields from which samples collected were interviewed about weed management practices and the cost of weed control per season. The average cost for weed management was 1, 980, 000 VND per hectare, the total cost included 650, 000 VND for herbicide and 1, 330, 000 VND for hand-weeding. Echinochloa crus-galli biotypes resistant to bispyribac, penoxsulam and quinclorac were found and biotypes with resistance to multiple herbicides were detected. The average resistance-score of three tested herbicides was 3.4 in the 78 samples collected. There was a correlation between field size, hand-weeding practice, and the herbicide-resistance level, herbicideresistant weeds were less common in smaller fields where hand-weeding was commonly used. The high cost and limited labour pool for hand-weeding in this region of Vietnam limits the long-term viability of this approach.