Objective 1: Conduct on-farm research to assess soybean seeding rates (N. Mueller)
Trials were conducted using large strip plots at two sites in southeastern South Dakota to evaluate the yield advantage of planting two soybean varieties according to landscape position (‘92Y51’ in upland areas and ‘92Y70’ in lowland areas) versus planting each line by itself across the field. Treatments of twin-row planting for the two lines, and twin-rows where each of the paired rows was a different line, were also included in the trial. There tended to be a yield advantage with planting twin-rows over single rows; however, planting one row of each line in a twin row system did not show a yield advantage over sole-planting the better of the two lines. On the other hand, with single rows, matching variety to landscape position showed about a 3 bu/ac yield benefit over planting either line by itself across the landscape. This data is from only one season with limited replications at one of the sites; therefore it should be interpreted with caution – nevertheless, these initial results suggest there is a yield benefit to be gained by appropriately matching soybean lines with good yield potential to landscape positions where they have a relative advantage. A portion of this work is summarized in Mueller et al. (2013).

Objective 2: Winter Rye seeded after Corn and Crop crop timing of sowing impacts in soybean. Results showed that there appears to be potential for growing winter rye after corn and ahead of soybeans within a corn-soybean rotation. However, growing cover crops largely depends on the available moisture. Under low moisture there would be a negative influence on soybean yield unless the rye is sprayed out early. In South Dakota, approaches to implementing cover crops into soybean production that benefit farming systems is difficult due to short growing seasons, limited planting time, and fall conditions that are often cold and dry. To be successful as a late season ground cover, interseeding a cover crop into soybeans has to allow a long enough establishment time under optimal environmental conditions. A 1-year field study was conducted in order to determine the effects and interactions among cover crop sowing times and biomass production and soybean yield and 100-seed weight. Three cover crop sowing times [second trifoliate (V2), full flower (R2), and first leaf drop ] and two interseeding techniques [drill (DRL) and broadcast (BRD)] were used. Cowpea (Vigna unguiulata) and grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] were sown at rates of 7.6 and 1.4 kg pure live seed ha-1. Cover crops emerged 4 to 13 d (days) after interseeding. The V2 interseeded cover crop mixture developed 93 to 99% more dry matter (DM) compared with the R2 and leaf drop, although these timings did have limited establishment. The V2 treatment reduced soybean yield and 100-seed weight by about 32 and 4% compared with the no cover crop, R2, and leaf drop treatments. These results indicate that cover crops can be established into South Dakota soybeans after the R2 growth stage with no adverse impacts on soybean yield.

Objective 3: On-farm studies (C.G. Carlson, D.E. Clay, R. Gelderman, and S.A. Clay)
Between 2009 and 2013 we have been tracking corn and soybean yields as influenced by high P rates in five on-farm studies. In these experiment very high P rates were applied in strips across the field. The soil test P level ranged from 6 to 10 ppm. Corn and soybean yield differences were not detected. This work will not be repeated in 2014.

Combines equipped with yield monitors are being used to collect on-farm data. The accuracy of this information is in question. The objective of this research was to use yield monitor data to validate a variable rate seeding recommendation, and determined the economic feasibility of variable rate seeding for corn and soybeans.
Yield monitor protocols were determined by comparing corn (Zea mays) hand-harvested and yield monitor data collected from 2 South Dakota corn fields. Analysis of variance (ANOVA) determined the yield monitor and hand-harvested yields were similar when the data was aggregated into plots 86 m long. On-farm research and yield monitor data was then used to validate two variable rate corn seeding recommendation at Webster, Winner, and Beresford, SD, and compare site-specific plant populations against the producer’s uniform plant populations at the same locations as well as Bruce, SD. At each site, five plant populations were planted in strips running perpendicular the field variability. Using yield monitor data, economic optimum plant populations were calculated at four landscape positions.
Profitability due to variable rate recommendations increased with said variability. This result suggests site-specific plant populations depend upon soil and climate variability. The economic optimum populations were calculated using two plant population models at four landscape positions. Each population model was developed using separate data sets. At Webster, SD the second equation calculated recommendations similar to the economic optimum seeding rates, while at Winner the first equation was most accurate. These results suggest different recommendations should be used in different corn growing conditions.

Similar studies using soybeans were performed at Hayti, SD and Beresford, SD in 2012 and 2013. Soybean plant populations were found to have no impact on yield at Hayti, SD in 2012. At Beresford the 432,750 seed/ha rate yielded higher than the other populations. Soybean variable rate plant populations at four landscapes followed a trend opposite of corn; however soybean grain yield did not vary based upon landscape position. This suggests variable rate seeding in soybeans can reduce cost; however yield benefits from the practice might be limited. Soybean twin rows and planting date experiments.

Peter Sexton: Yields in the trial area at Beresford averaged 52.1 bu/ac, while at Tripp the average yield was 48.9 bu/ac. The two varieties tested did not significantly differ from each other in yield at either site when planted in a single row configuration. Of the two soybean lines, ‘92Y51’ tended to have slightly higher yield, and it seemed to benefit more from a twin-row configuration than did ‘92Y70’, showing a yield advantage over ‘92Y70’ when planted in twin-rows. Seed treatment impact on soybean yields.