Defoliation affects soybean yield depending on time and level of light interception reduction

Abstract

Light interception (mainly due to defoliation) during the critical growth periods of soybean may be a suitable index for calculating the economic injury level (ELI) rather than measuring the insect population or percentage of defoliation. Our aim was to simulate the effects of time and levels of defoliation on soybean seed yield, its numerical components and light interception during the critical growth periods. We evaluated a combination of defoliation levels (0, 33, 66 and 100%) and two times of defoliation during soybean development (pod initiation and beginning of seed filling, i.e. R3 and R5, respectively). We measured the effects of radiation interception on seed yield and its components during the linear seed filling period (R5.5). The results showed that the total defoliation performed at R3 significantly reduced the seed yield compared to defoliation at R5 (P<0.0001) (90% and 21% yield reduction, respectively, as compared to controls). Similarly, total defoliation performed at R3 reduced seed number by 85%, whereas that performed at R5 reduced seed number only by 3%, as compared to controls (P<0.0001). Seed yield and its components were significantly reduced when defoliation was applied at R3, because the radiation interception at R5.5 was reduced (P<0.001). The photosynthetically active radiation (PAR) interception during the critical period (R5.5) was reduced only by total defoliation irrespective to the time of defoliation, evidencing the high vegetative plasticity of soybean. Our results provide a useful base for the development of economic injury levels (EILs) based on light interception during the growth critical periods.