INFLUENCE OF INTERFERENCE FROM A MIXED WEED SPECIES STAND ON SOYBEAN (GLYCINE-MAX (L) MERR) GROWTH

Field experiments were conducted to examine soybean-weed interactions under Ontario conditions. Trials were conducted at Flora. Ontario in 1 990 and 1991 using comparative growth analysis to investigate the abov eground response of soybean to weed interference from a mixed weed spe cies stand. Although season-long weed interference caused a significan t decrease in soybean grain yield, weeds could remain in the crop up t o 40 d after emergence, between R1 and beginning pod development stage (R3), in 1990 and up to 16 d after emergence, 3rd node development st age (V3), in 1991 without causing more than a 5 % yield loss. These cr itical times of weed removal coincided with the timing of significant effects of weed interference on soybean LAI. Weed interference also si gnificantly reduced total aboveground soybean dry weight and crop grow th rate. Pods m-2 was the yield component most affected by season-long weed interference. Although there was no consistent effect of weed in terference upon net assimilation rate and leaf weight ratio, weed inte rference resulted in a decrease in dry weight distribution to branch s tems, causing as much as a fivefold reduction in allocation to branch stems. It may be useful to develop early branching soybean varieties, or use narrower row spacing, to enhance soybean competitiveness agains t weeds.