Effects of Environment and Management on Soybean Performance in the Tropics

Yield potential of soybeans [Glycine max (L.) Merr.] in tropical areas is similar to that in temperate regions. Yield variability among sites indicates that exploitation of this potential under tropical conditions will require a better understanding of how environment and management influence yield. This study was undertaken to investigate the effects of several environmental and management variables on among site variability in yield and other plant characteristics of soybeans in the tropics. Results from 10 cultivars common to International Soybean Program (INTSOY) experiments at 25 sites 10 countries were used in the study. Agronomic variables, expressed as site means, were: yield, seed weight, pod number, number of days to flower, number of days from flowering to maturity, and plant height. Environmental variables measured were: altitude, latitude, daylength, maximum temperature, and minimum temperature. Management variables measured were: amounts of N, P, and K fertilizer applied and mean site nodule number, nodule dry weight, and plant population. Mean site yield was not significantly correlated with any environmental variable measured. The only management variable significantly correlated with mean site yield was nodule dry weight. However, after adjustment for the environmental effects, all management variables together accounted for 56% of the variability in yield among sites. The environmental variables measured, after adjustment for management effects, accounted for only 27% of the variation in yield. Therefore, with respect to the 25 sites studied, the environmental variables were concluded to be less important than the management variables as determinants of yield. The mean site number of days to flower was influenced more by environmental variables than by management variables. Conversely. the mean site number of days from flowering to maturity was more affected by management variables than by environmental variables. Individual variables significantly associated with days to flower were altitude, daylength, maximum temperature, and minimum temperature. Only daylength was significantly correlated with number of days from flowering to maturity. Mean site plant height was positively correlated with latitude, daylength, nodule number, and nodule dry weight, and negatively correlated with applied K fertilizer. Environmental and management effects accounted for similar percentages of among site variability in plant height. Mean site plant height and mean site seed weight were the only agronomic characteristics significantly correlated with mean site yield. The high grand mean yield for the 25 sites studied (2,034 kg/ha) suggest that with adequate management at least some of the cultivars among those tested may perform well in tropical environments. Because of the importance of management in explaining among site variation in yield, considerable effort must be made to train farmers in the proper management of soybeans when they are introduced as a new crop.