Investigating the Effect of Management Operations on the Time of Redroot pigweed (Amaranthus retroflexus L.) Emergence in Soybean

Redroot pigweed is one of the most common dicotyledonous weeds in the world that is widely distributed in many agricultural areas. Thus, in order to evaluate the effect of different management practices on the emergence time of spotted spurge, a split-split plot experiment was conducted in a completely randomized block design with three replications in Dasht-e-Naz Sari in 2016 and 2017. Two tillage systems (Conventional tillage and no-till), three soybean densities (200,000; 300,000 and 400,000 plant ha-1), and three doses of imazethapyr (0, 50, and 100 g ai ha-1) were considered as experimental factors. The redroot pigweed emergence was significantly affected by management practices including tillage system, soybean seeding rate, and imazethapyr dose. Conventional tillage required a thermal time (TT) of 266.72 and 139.33 d °C to reach 50% emergence in 2016 and 2017, respectively, while for no-till, the respective TT requirements were 187.19 and 132.11 d °C. On increasing soyabean density from 200 000 to 400 000 seeds ha-1, the TT requirements for 50% emergence (T50) of redroot pigweed also increased. The T50 at the herbicide dose of 100 g a.i. ha-1 was 234.56 and 142.81 d °C in 2016 and 2017, respectively, while for the non-herbicide treatment, the respective TT requirements were 217.80 and 136.09 d °C. From an integrated weed management perspective, a combination of conventional tillage with a soyabean density of 400 000 seeds ha-1 and a 100 g a.i. ha-1 imazethapyr dose not only resulted in the lowest redroot pigweed seedling density m-2 but also caused the longest delay in the time to reach the T50. This condition provided more time for soybeans to establish better and improve their competitive ability. The results of our study could help to develop effective management strategies for this species.