Agronomic responses of soybean cultivars to narrow intra-row spacing in a cool region of northern Japan

Abstract

Combinations of cultivar selection and management practices are used to maximize soybean yields in Japan. High plant density could increase the amount of solar radiation intercepted by the canopy and thereby increase growth and yield, but both cultivar and environment affect the response to plant density. The mechanisms underlying responses of soybean to high plant density are not fully understood. This study tested the effects of high plant density by narrow intra-row spacing on agronomic traits in 2 years using three cultivars (Ryuhou, Okushirome, and Fukuibuki) in Morioka, Iwate, in the Tohoku region of Japan. Averaged across years and cultivars, narrow intra-row spacing increased seed yield. Interestingly, there was a significant year × intra-row spacing × cultivar interaction: In the first year, narrow intra-row spacing increased yield of Ryuhou and Fukuibuki but not of Okushirome; in the second year, however, it increased yield of all cultivars similarly. The lack of yield response of Okushirome to narrow intra-row spacing in the first year presumably resulted from an excessive leaf area index (LAI) and a higher lodging score. The results suggest that high plant density by narrow intra-row spacing is an effective option for increasing soybean yields in the Tohoku region, although agronomic traits such as LAI and susceptibility to lodging should be considered.