Delayed soybean emergence: Competition or compensation?

Farmers experience delayed crop emergence and question if it decreases plant growth or yield. The effects of delayed emergence within soybean populations have not been well studied. Studies of delayed emergence have addressed only populations and situations in which vegetative growth and reproductive plasticity is not limited. No studies have attempted to quantify sources of compensation beyond seeds per plant, seed size and crop growth rate. Our objectives were to determine if delayed emergence of soybean grown in 19-cm rows 1) alters the growth and the development of individual plants and the stand of plants and 2) causes competition or contribution to overall yield and individual yield forming factors. The delays were created by planting 38-cm rows and interplanting between rows at succeeding growth stages of the initial stand of soybean. Row-to-row delay was studied across three factors: 2 initial plant populations (247,000 and 123,500 plants ha-1), 3 interplanted percent delay (33, 67, and 100% of both initial populations), and 4 interplanted timings of delay (when initial rows were: Planted (None), VC, V2, and V4). Treatments were arranged in a randomized complete block design with four replications near West Lafayette, Indiana. Five flagged plants per row in initial and interplanted rows were measured for plant height, nodes per plant, and growth stage. Post-harvest measurements were taken on 2 m of harvested row and included branches per plant, biomass, pods and seeds per main stem, pods and seeds per branch, total seed weight per main stem, and total seed weight per branch. Delayed plants generally produced fewer nodes and less biomass per plant and per m 2 due to constrained vegetative growth and high competition from established neighbor plants. Initial plants decreased height to node ratio, increased mainstem nodes per plant, increased overall area nodes, increased biomass per branch, and generally increased area biomass (within a percent delay by timing of delay interaction) in response to prolonged timing of delay in 2010. High population or low percent delay increased initial plant competitiveness and contribution and decreased that of delayed plants. Overall yield was not affected by any treatment combination except in 2011 due to late season hail damage effects upon soybean compensatory ability. In general, harvest index, seeds per pod, seed size, pods, seeds, and yield of delayed plants decreased as timing of delay was prolonged. Harvest index, pods, seeds, and yield of initial plants generally increased to compensate proportionally. Initial plant branch and mainstem seed yield both increased drastically (when measured in 2010) as delay lengthened which increased area yield of initial plants. In 2010, initial plant area yield increased 230 and 434 g m-2 for VC and V2 timing of delay compared to no delay control. Total area mainstem yield decreased in importance (though still contributing a majority of yield) and total area branch yield increased in importance with prolonged timing of delay. Initial population and percent delay interacted with timing of delay to affect the magnitude of many of these changes. Though delayed emergence caused shifts in source of yield, soybean with uninterrupted growth compensated fully