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Soybean (Glycine max L.) is one of the staple oil crops widely planted around the world. However, soybean production is not sufficient to meet the demand for vegetable oil. The major factor depressing soybean production is excessive vegetative growth due to much rain during the rainy season. Excessive vegetative growth during the flowering stage also causes lodging, fallen petals, fallen pods and yield reduction in soybean. This is common in most east Asian countries.
Excessive vegetative growth might be constrained by interspecific competition under intercropping system. This competition usually leads to a reduction in vegetative or reproductive growth of at least one species. But there is recovery of reduced crop growth after one crop harvest in intercropping system. Based on these relationships between two crops in intercropping system, we supposed that soybean's excessive vegetative growth could be constrained by interspecific competition during the intercropping period and reduction in the early vegetative growth can be compensated by vigorous vegetative growth during the rainy season.
Intercropping is a prevalent practice that can increase LER and decrease damage in repeated cultivation. Until now, several planting patterns and plant densities for intercropping system have been widely studied to decrease interspecific competition for environmental resources between intercrops and increase LER. The degree of interspecific competition that is inevitable in intercropping system is directly associated with plant density. Meanwhile, plant density is also related to LER. Accordingly, recent focuses have been mainly on increasing the distance between intercrops to decrease interspecific competition for environmental resources, and maintaining the same plant densities for each crops in intercropping as in sole cropping by using a smaller plant distance within each row in intercropping, which compensates for reduced number of rows. These approaches lead to poor interspecific competition and rich intraspecific competition in intercropping system. To increase plant density and interspecific competition and decrease intraspecific competition, adding pattern should be used as a planting pattern. We thought that ridge-furrow intercropping system is just that kind of system that can increase plant density and interspecific competition and decrease intraspecific competition. We selected flax for ridge-furrow intercropping with soybean.
Flax (Linum usitatissimum L.) is an oil crop acclimated to moderate climate. It is an early ripening crop that has a short growth period of 90-100 days from March to July. Flax seed is rich in protein, lipid, and dietary fiber and it contains plenty of lignan and linolenic acid. Thus, flax seed is widely used as an oil source, functional food and livestock feed. When soybean and flax are cultivated together, their co-growth period can be as long as nearly 2 months. As this period includes the reproductive stage of flax and the early vegetative growth stage of soybean, both flax reproduction and soybean's early vegetative growth are affected by interspecific competition between these species.
Soybean seeds were sown in ridges and flax seeds were in furrows between ridges. During the co-growth stage, the early growth of soybean intercropped with flax was depressed by interspecific competition. At the reproductive growth stage, the height of intercropped soybean plant was still shorter than sole cropped soybean, while other agronomic parameters, including leaf area index (LAI), dry matter per plant, photosynthetic rate and chlorophyll contents, were greater for intercropped soybean. This led to a higher yield of soybean (118.5%) and land equivalent ratio (LER) (203.5%) in ridge-furrow intercropping, as compared to sole cropping. Our results suggest that ridge-furrow intercropping system can be an effective way to constrain soybean's excessive vegetative growth during the rainy season and improve soybean seed yield and LER.
The results were published in the "Field Crops Research" under the title of "Growth, photosynthesis and yield of soybean in ridge-furrow intercropping system of soybean and flax" (https://doi.org/10.1016/j.fcr.2021.108329).