Row Spacing (30″ vs. 7.5″ drilled beans)

Evaluation of Row Spacing in Soybeans in the 2015 and 2016 Growing Seasons at the Southeast Farm

Peter Sexton* , Garold Williamson, Doug Johnson, and Brad Rops

INTRODUCTION

This report summarizes data from two recent row spacing studies done with soybeans at the Southeast Farm, one in 2016 and another larger study from 2015. In 2016, we had a very wet spring which delayed soybean planting. Under these conditions, one would expect a benefit from narrower row spacing both in terms of yield and in weed control. To observe the actual impact of narrower rows on yield and weed incidence, a large-plot trial was established comparing soybeans planted in 30” rows to soybeans drilled in 7.5” row spacing. In the previous season (2015), a trial was conducted comparing twin row and single row seeding of soybeans on 30” centers across a range of populations (35,000 to 210,000 seeds per acre) and a drilled treatment was also included at a single population (140,000 seeds per acre) for further comparison. Each of these plots was split with half receiving a fungicide application at R3 and half not – so the 2015 trial included seed rate, row spacing, and fungicide treatments as variables.

METHODS

The 2016 trial was planted on 20 June 2016, and consisted of the following treatments: 30” row width seeded at 160, 000 seeds/acre; drilled soybeans seeded at a rate of 160,000 seeds/acre; and drilled soybeans seeded at a rate of 200,000 seeds/acre. Plots were 30 feet wide and ran for a length of 500 feet. Plots were visually rated for percent control of grass and broadleaf weeds just before harvest. This was done by two people and the scores averaged and the data arc-sine transformed before being subject to statistical analysis. Yield samples were taken by combining the center 20’ from each plot and weighing the seed for each plot in a weighwagon. The 2015 trial was planted on 1 June 2015.

Main plots were 15 feet (6 rows) wide by 200 feet in length and were laid out in a randomized complete block design with four replications. A foliar application of ‘Headline’ fungicide was made at the R3 growth stage in 90 strips across each replication in a strip-split plot design. Plots were end trimmed 10’ at harvest, and the middle four rows of each plot were combined for whole plot yield. All data were analyzed with standard ANOVA using the Proc GLM routine in SAS statistical software. Yield was regressed against stand at harvest using individual plot data, rather than treatment means, in the 2015 study because of large plot to plot variability in stand in that particular field.

RESULTS

In the 2016 trial soybean yields were 4 bu/ac higher when drilled in 7.5” rows versus when they were planted in 30” rows (Table 1). There was no difference between the high and low seed rates for the drilled soybeans. There was a trend for slightly greater weed incidence in the 30” rows, but differences in weed control between treatments were not statistically significant. The field was worked before planting. The only herbicide applied in this trial was glyphosate applied as a single postemergence application.

In the 2015 trial with a range of seed rates in both single and twin rows, seed rate showed a strong effect on yield as expected, while use of twin rows and fungicide application both showed trends for about a 3 bu/ac greater yield but differences were not statistically significant for these latter two effects (Table 2). Regressing yield data from individual plots against stand at maturity showed that yield increased up to a final stand of approximately 114,000 plants per acre and then plateaued at stand density greater than that. Assuming a value of $10 per bushel price for soybeans, and a cost of $50 a bag for seed and 20 % seed mortality, the predicted economic optimum final stand in this study would be at 107,000 plants per acre at maturity – which would mean an initial seed rate of 134,000 seeds per acre.

The common theme between these two trials was the trend for slightly greater yield (about 3 to 4 bu/ac) with narrower row spacing, and the lack of yield response to population above 114,000 plants per acre. This is consistent with previous work on soybean seed rates conducted at the Southeast Farm in 2013 where yield plateaued at approximately 100,000 plants per acre. Assuming a target final stand of 110,000 plants per acre and that 80 % of the seeds planted survive to make a plant, a seed rate of 138,000 seeds per acre would provide enough of a stand to maximize yield in this environment.

Modified from: https://www.sdstate.edu/sites/default/files/2016%20SERF%20AR%20FINAL.C.pdf

By Peter Sexton* , Garold Williamson, Doug Johnson, and Brad Rops

ACKNOWLEDGEMENTS

The authors appreciate the contributions of the South Dakota Soybean Research and Promotion Council and the South Dakota Agricultural Experiment Station to support this project.