Soybean production is increasing in South Dakota with many different tillage systems. Row spacing varies from wide (30") to narrow rows (6-7"). Therefore a long-term research project was initiated to evaluate the influence of row spacing, tillage system, and P treatment on three soybean varieties representing three maturity groups at Brookings SD.

A research site was established in 1995 on the Crop Improvement farm near Aurora SD. A split/split randomized complete block design with 4 replications, tillage system as the main plot, row spacing as the first split, P treatment as the second split plot, and variety as the sub plot was used. No-till and conventional tillage systems were established. A chisel plow was used for tillage on the conventional till blocks. Row spacing were 7 and 28 inch. Phosphorus (P) treatments were a check with no added P and a P treatment where P would be non-limiting to crop growth. In May of 1995, 75 Ibs/a P₂O₅ was applied as 10-34-0 with a spoke applicator set at 8.5-inch spacing and soybeans were planted. In 1996, corn was planted on this site with no P application. Tillage systems were maintained in 1996. Prior to planting in 1997, 75 lbs/a P₂O₅ was applied with the drill parallel to plot rows. In 1998 soybeans were planted on this site with no P fertilizer application. Corn was planted in 1999 with no P fertilizer application. After corn emergence in 1999 soil samples from all replications were obtained and analyzed for P analysis. Prior to planting soybeans in 2000, tillage systems were maintained as in prior years, and Roundup Ultra (1 qt/a) was applied to no-till plots for burndown control of emerged weeds. Hendricks (maturity group 0), Granite (maturity group I), and Sturdy (maturity group II) were planted at 200,000 pure live seeds/a on June 6, 2000. After planting 1 qt/a Boundary herbicide was applied for residual grass and broadleaf weed control. Early bloom (EB) shoot samples were removed and number of plants counted from a 10ft² area of each plot on July 19, 2000. The EB samples were dried, weighed, and ground for nutrient analysis. Bean leaf beetle pod feeding was observed and resulted in a application of Lorsban (12 0z/a) August 22, 2000. Grain was harvested with a plot combine on October 12, 1999. Statistics were computed using SAS.

Soil test showed extractable Olsen P of 12 ppm for the P treated blocks and 7 ppm for the check P blocks. P levels for both the treated and untreated plots are considered to be in the high and medium categories, respectively. Response to applied P at this location is possible. The ANOVA showed that tillage significantly influenced EB dry weight per plant (Table 1). Variety and P treatment significantly influenced plant population (Table 1). Variety, tillage, and row spacing significantly influenced grain test weight (Table 1). Variety, tillage, P treatment, and row spacing significantly influenced grain yield (Table 1).

The EB dry weight per plant was significantly higher for conventional tillage (Table 2). Plant population was significantly higher with the Granite variety and the check P treatment (Table 2). Grain test weight was significantly lower for the Hendricks variety, and higher for 7 inch rows and conventional tillage (Table 2). Grain yield was significantly highest for the Hendricks variety, 7" rows, conventional tillage, and the P check treatment (Table 2). P treatment significantly influenced EB dry weight per plot at the .01 level (Table 1). P treated plots had higher EB dry weights and this was probably induced by a probable response to higher P soil test or significantly lower plant population. Moisture stress at pod fill stages might have stressed the larger plant. Larger plants in the P treated plots were visually observed at pod fill. Visual moisture stress occurred at pod fill. This combination of effects could explain the reduction in grain yield with the P treated plots (Table 2).

The growing season was long enough for later maturing varieties to become fully mature, but late season moisture stress could have limited grain yield. The Hendricks (maturity group 0) had significantly higher yields when compared to the maturity group I and II varieties. Plant row spacing was a factor in increasing grain yield since there was less competition in the narrow spaced rows than in the wider rows. Low precipitation could have enhanced moisture competition between plants in wide versus narrow rows.

This project was partially funded by the South Dakota Soybean Research and Promotion Council and the SD Ag. Experiment Station.