Soybean Response to Postemergent Wheel Traffic 1

Considerable researchas demonstrated the yield advantage of soybean [ Glycine max (L.) Merrill] grown in narrow‐row spacings, but little information is available concerning postemergent wheel‐traffic effects where row spacing is too narrow to prevent mechanical traffic for conventional intrarow cultivation or spray application. The objective of this study was to determine yield and plant characteristics of soybean subjected to postemergent wheel traffic. Three indeterminate soybean cuitivars, Corsoy, Northrup King 1492, and Asgrow 3127, were planted in a 35‐cm row spacing in 1980 and 1981 on Aquic Hapludoils and Typic Haplaquolls, respectively. Treatments were applied by driving a tractor through the plots at one of six growth stages from VE (emergence) to R4 (full pod). A skip‐row treatment, where wheel track rows were not planted, was also included. Individual rows were harvested within each plot and grouped together as wheel‐track rows (rows driven over), border rows (rows adjacent to the wheel‐track rows), and nonborder rows (rows not bordering wheel track rows). The whole‐plot seed yields of Asgrow 3127 and Northrup King 1492 were not reduced by wheel traffic up to full flowering; whereas, yield of Corsoy was reduced (9‐19%) wheel traffic at vegetative stage V5 and reproductive stages of full flower and full pod. Yield losses ranged from 26% for wheel traffic at emergence to 92% at the full‐pod stage. Yield in borderows increased from an average of 5% for wheel traffic at emergence to 20% at the full‐flower stage in a compensatory response to yield losses in wheel track rows. Seed weight was increased in border rows associated with treatment at the full‐pod stage, and in general, border row plants set pods lower on the stem and had more branches, nodes, and pods per plant. Although cultivars responded differently, the yield advantage to narrow‐row spacing was not seriously reduced by postemergent wheel traffic.