Weed Control Effects of Different Preemergence Herbicides on Stevia rebaudiana

To screen for safe and efficient preemergence herbicides for stevia, treatments of 96% S-metolachlor EC 750 mL/hm², 33% pendimethalin EC 3 L/hm², and 20% napropamide EC 3 L/hm² were sprayed onto the ridge before film mulching. The phytotoxicity of each treatment was observed on the 5th, 15th, and 30th days after transplanting of stevia, and the weed control effect was investigated on the 30th day after transplanting. Plant heights were measured on the 30th, 60th, 80th, 100th, and 116th days after transplanting, and the dry leaf yield during the harvest period was determined. The results revealed that 33% pendimethalin EC 3 L/hm² and 96% S-metolachlor EC 750 mL/hm² were safe for stevia, whereas 20% napropamide EC 3 L/hm² had irreversible phytotoxicity to stevia. The fresh weight control rates of 20% napropamide EC 3 L/hm², 33% pendimethalin EC 3 L/hm², and 96% S-metolachlor EC 750 mL/hm² were 92.36%, 86.76%, and 83.34%, respectively. The 20% napropamide EC 3 L/hm² treatment strongly affected the growth of stevia, and the plant height 60 days after transplanting was significantly different from those treated with 33% pendimethalin EC 3 L/hm², 96% S-metolachlor EC 750 mL/hm², and the control treatment. The dry leaf yield of stevia for each treatment was: 33% pendimethalin EC 3 L/hm² (2.90 t/hm²), 96% S-metolachlor EC 750 mL/hm² (2.23 t/hm²), and 20% napropamide EC 3 L/hm² (1.57 t/hm²), which, compared with the control, were 82.39%, 40.25%, and − 1.26%, respectively. Our results suggested that the 33% pendimethalin EC 3 L/hm² treatment had the best effect, followed by the 96% S-metolachlor EC 750 mL/hm² treatment, and the 20% napropamide EC 3 L/hm² treatment.