Effect of replacement ratios on plant traits and seed quality properties in intercropping of soybeans (Glycine max L. Merr) and niger (Guizotia abyssinica Cass.)

A field experiment was conducted to evaluate the competition of soybean intercropped with niger in different planting ratios using randomized complete block design with four replications at Sari agricultural sciences and natural resources university in 2018 and 2019. The intercropping ratios were 0:100, 25:75, 50:50, 75:25, and 100:0 (soybean:niger, respectively) using the replacement series. Results showed that intercropping significantly affected plant height, total dry matter accumulation at different growth stages, seed yield, oil, and crude protein yield of both crops. The highest plant height of soybean and niger was obtained in sole cropping of soybean and 75:25 planting ratio, respectively. The impact of competition on total dry matter accumulation up to 90 days after planting was positive and complementarity effect that changed to mutual cooperation after this stage. In addition, oil yield and crude protein yield of soybean and niger seeds in planting ratios were higher than niger (100: 0) and less than soybean in the sole cropping of soybean (100: 0). The maximum amounts of LER (1.28 and 1.23) were obtained in 50:50 and 75:25 intercropping ratios, respectively. Results showed that the complementarity effect between niger and soybean on seed yield in all intercropping ratios was greater than the selection effect. The increases in seed yield in intercropping ratio 50:50 and 25:75 was due to the largest share of the complementarity effect (76.2% and 59.7%, respectively) in seed yield. Therefore, selecting niger as a companion species in intercropping with soybean would lead to mutual cooperation relationship and mitigate the effect of competition.