１９８６～１９８８年研究表明．大豆田混合一年生杂草从株高上可分为三个种群：以稗、苋为主的混合一年生杂草的春大豆田，无杂草期与大豆产量呈高度正相关，表现为Ｓ型曲线，方程为，认为无杂草临界期为大豆出苗后５周。杂草竞争持续期与大豆产量呈高度负相关，回归分析两年平均产量为倒Ｓ型曲线，方程为，杂草竞争持续临界期为大豆出苗后４周。结果表明大豆田除草的临界期为大豆出苗后４～５周，此期要保持大豆田基本无草状态，才能获得较高的产量。认为大豆田除草的最佳时期为大豆出苗后３～５周。 1986 ~ 1988 research shows. Soybean field annual weeds can be divided into three groups from the plant height: Echinochloa crusgalli, amaranth mixed annual perennial spring soybean field, no weed period and the soybean yield was highly positive correlation, the performance of S-curve, equation The critical period of no weed is 5 weeks after emergence of soybean. The duration of weed competition was highly negatively correlated with soybean yield. The regression analysis showed that the average yield of two years was inverted S curve. The equation was that the weed competition continued for the critical period of 4 weeks after soybean emergence. The results showed that the critical stage of weeding in soybean field was 4 ~ 5 weeks after the emergence of soybean, in order to maintain the basic soybean-free state in this period, higher yield could be obtained. The best time for weeding in soybean field is 3 to 5 weeks after emergence of soybean.