Determination and Manipulation of Leaf Area Index to Facilitate Site-Specific Management of Double-Crop Soybean in the Mid-Atlantic, U.S.A.

Double cropping soybean after small grain harvest does not always allow sufficient canopy growth to maximize photosynthesis and seed yield. This is due to a shorter growing season and moisture deficits common to the Mid-Atlantic USA. Leaf area index (LAI) is the ratio of unit leaf area of a crop to unit ground area and is a reliable indicator of leaf area development and crop biomass. An LAI of 3.5 to 4.0 by flowering is required to maximize yield potential. Soybean LAI will vary within and between fields due to soil differences, cultivar selection, and other cultural practices. Site-specific management strategies such as varying plant population may be used to manipulate LAI and increase yield in leaf area-limited systems. Furthermore, methods to remotely sense leaf area are in order to facilitate such management strategies in large fields. The objectives of this research were to: i) determine the effect of plant population density on soybean LAI and yield; ii) determine the relationship between LAI measured at different reproductive stages and yield; iii) investigate and validate relationships between LAI and yield for two cultivars in three crop rotations across varying soil moisture regimes; iv) validate relationships found in previous work between soybean LAI and yield across soil moisture regimes in grower fields; and v) determine if normalized difference vegetation index (NDVI) values obtained from aerial infrared images can be used to estimate LAI and soybean yield variability. Increasing plant population increased LAI for cultivars at Suffolk in 2000 and 2001, but LAI increased with plant populations on soils with lower plant available water holding capacity (PAWHC) at Port Royal in 2001. In 2000 at Suffolk, seed yield increased quadratically with increasing population and cultivar did not affect the response. In 2001, no relationship occurred between yield and plant population at either Suffolk or Port Royal, but the relationship of yield and LAI depended on soybean development stage at both sites. However, this relationship was not consistent between sites or years. In another study, crop rotation affected LAI and yield one out of two years. However, LAI and yield in both study years were negatively impacted on soil types with lower PAWHC. Where significant, a linear relationship was observed between yield and LAI for all soil types. Studies on grower fields showed similar linear relationships between yield and LAI. Remote sensing techniques showed promise for estimation of LAI and yield. When obtained at an appropriate development stage, vegetation indices correlated to both LAI and yield, and were observed to be effective as a predictor of LAI until plants achieved LAI levels of 3.5 to 4.0.