Continuous corn is the most common irrigated crop sequence in southwest Nebraska. Although rotating to other crops, such as soybeans, can mitigate some production issues of continuous corn and often boost the next year’s corn yield, larger adoption of soybean has not readily occurred in this area. According to USDA Farm Service Agency planted acreage data, on average southwest Nebraska farmers plant irrigated soybean every fifth year.

The culture of farming in southwest Nebraska evolves around corn, which often prevents growers from raising soybeans under more ideal conditions. For example, priority is often given to planting corn first, soybeans are planted strip-till in 30-inch rows, and seeding rates of 160,000 seeds/ac are common. In addition, late season chemigation with nitrogen (N) is widespread without a full understanding of when and where it’s warranted. (See results from Nebraska on-farm research on late-season nitrogen management in soybean.)

The objective of this study was to investigate the impact of planting date, row spacing, seeding rates, and N management on yield and yield components of irrigated soybean in southwest Nebraska.

Characteristics of the Two Research Sites

The study was conducted at two locations in Perkins County (the Kemling and Stumpf farms) in 2018. The predominant soil type at the Kemling Farm was Rosebud loam; at the Stumpf farm it was Kuma silt loam. At the Kemling Farm, the whole field was disked prior to planting; at the Stumpf farm, soybeans were seeded no-till. At both locations the previous crop was corn. Besides study treatments, soybeans were grown following UNL agronomic and irrigation recommendations.

The 2018 seasonal precipitation (May-Oct) was 6.5 inches higher than the 30-year average, especially early in the season (Figure 1), leading to issues with crusting and soybean germination. In addition, two hail events occurred at both sites. The first hail event occurred May 25, causing stand reduction in early planted soybeans. The second hail event occurred in mid-August, causing 20% hail injury at the Stumpf Farm and 5% at the Kemling Farm.

Data We Collected

The study evaluated four practices, each at two levels, for a total of 16 treatments:

• Planting dates (May 1 vs June 5)
• Row spacing (15-inch vs 30-inch rows)
• Seeding rates (90,000 vs 140,000 live seeds/ac)
• N management:
  • Stumpf Farm – control vs chemigation with 50 lbs of N/ac at R5 (beginning seed)
  • Kemling Farm – control vs pre-plant compost at 5 tons/ac

Each treatment was replicated four times and each replication was divided into blocks by N management (fertility regime). Seeding practices (planting date, row spacing, and seeding rates) were randomized within each fertility N management block. The study treatments were planted into strips 40 ft by 180 ft. The middle 30 ft of each strip was harvested for yield using a John Deere 6000 series combine.

In addition, harvest population (plants/ac) was counted in each strip right before the harvest and five plant subsamples were taken to evaluate yield components, including nodes/plant, branches/plant, pods/plant, seeds/pod, and seed weight.

Grain Yield Results

Overall, grain yield was lower at the Stumpf Farm compared to the Kemling Farm, mostly due to greater impact of soil compaction and hail injury. A cool wet spring in combination with direct seeding (no-till) of soybean at the Stumpf farm caused issues with sidewall compaction, soil crusting, and early season growth and development. Disked soil at the Kemling Farm dried out quicker, creating better seeding conditions, less sidewall compaction, and consequently fewer issues with crusting and early season plant growth. (For information on avoiding sidewall compaction, see UNL recommendations.)

At both locations the best soybean yields were observed at the early planting date (May 1) and in narrower row spacing (15 inches), while higher seeding rates did not have any measurable yield increase regardless of location and practices used.

At the Kemling Farm, early planted soybeans benefited from pre-plant application of compost at 5 ton/ac, yielding as much as 107 bu/ac. This trend, however, was not observed at late planting dates as yields dropped to 28-41 bu/ac. At the Stumpf Farm, chemigation of 50 lbs of N/ac at R5 (beginning seed) did not result in a yield increase.

What are Soybean Yield Components and Why do They Matter?

Grain yield is comprised of several components that, when analyzed separately, can allow us to better understand their individual contribution to overall grain yield. Despite differences in grain yield, the relationship between grain yield and yield components was similar at the two sites. Table 1 summarizes correlation coefficients averaged across sites. The sign of correlation coefficient (r) indicates the nature of the relationship (either positive or negative) while the magnitude of coefficient (ranging from 0 to 1) represents the strength of the linear relationship.

Correlation between grain yield and plants/ac, seeds/pod, and seed weight was not significant (Table 1), suggesting that:

1. changes of plant population had no impact on grain yield, and
2. differences observed in grain yield had no impact on seeds/pod and/or seed weight.

On the other hand, significant positive correlation was observed between grain yield and nodes/plant (r=0.58), branches/plant (r=0.50) and pods/plant (r=0.42) suggesting that the best seeding and N management practices are those that facilitate node, branch, and pod development.

Why Planting Date Matters

Previous UNL research on soybean in eastern Nebraska has demonstrated that for each day that soybean planting is delayed after May 1, yield penalties of 0.25-0.63 bu/ac can occur, depending on the year. (See Why Planting Soybean Early Improves Yield Potential and Soybean Planting Date — When and Why.) In our one-year study in southwest Nebraska, we found much larger daily yield penalties of 1.40 bu/ac/day at the Kemling Farm and 0.64 bu/ac/day at the Stumpf Farm (Figure 3).

Among yield components, nodes/plant, branches/plant, and pods/plant were all negatively correlated with planting date (Table 1), suggesting that each soybean plant produced fewer nodes, branches, and pods as planting date was delayed (Figure 3).

Why Row Spacing Matters

Overall, soybeans yielded better when planted in narrower rows. At the Kemling Farm a yield advantage of 8 bu/ac was observed with 15-inch rows at early planting, while there was no yield advantage with narrower rows at late planting date (Figure 4). At the Stumpf Farm, there was a yield advantage of 11 and 6 bu/ac with narrower rows at early and late planting, respectively. This is largely in agreement with our previous on-farm research studies that showed 3-13 bu/ac increases with 15-inch as compared to 30-inch rows.

Narrower seeding did not influence soybean node development; however, we did observe enhanced branching and consequently a greater number of pods per plant. The additional pods located on the side branches contributed greatly to the yield increase in narrower rows (data not show).

Why Seeding Rate Matters Less than Other Factors

Soybean yield at both the Kemling and Stumpf  farms did not respond to changes in plant populations. Although soybeans were seeded at 90,000 and 140,000 live seeds/ac, actual harvest population (plants/ac) ranged between 30,000 and 120,000 plants/ac at the Kemling Farm and between 20,000 and 110,000 plants/ac at the Stumpf Farm. The stand reduction at both sites was due to early season crusting issues and hail injury.

Lack of soybean yield response to increasing populations may be explained by increased competition among the soybean plants themselves. Increasing plant population causes individual soybean plants to produce fewer branches, pods, and seeds, and consequently less yield (Figure 5).

It’s All About Being More Profitable

In summary, soybean yield potential is increased when the crop is seeded earlier (0.64-1.40 bu/ac/day) and in narrower rows (up to 11 bu/ac yield advantage). This yield potential was achievable at lower seeding rates and without late season N supplementation.

It is not uncommon in western Nebraska to see soybean seeding delayed until after irrigated corn is planted, and to do it in 30-inch rows and at 160,000 seeds/ac. Assuming that yield penalties for late planting are lower for corn than for soybean, that typically there are fewer soybean acres to plant, and that market prices of soybean ($8.00/bu) are higher than corn ($3.30/bu), we outline potential savings from incorporating the following practices:

• Seeding soybeans 10 days earlier than the traditional practice and before corn – $48 to $112/ac;
• Seeding soybean in 15-inch rather than 30-inch rows with modest 3 bu/ac yield increase – $24/ac;
• Reducing seeding rates from 160,000 to 120,000 seeds/acre – $15/ac; and
• Eliminating late season chemigation with 50 lbs of N/ac – $20/ac.

Among these four production factors, early planting is the one factor that soybean growers in the region most often overlook and therefore lose the opportunity to increase their profit margins substantially. Therefore, the real question is what should we plant first in southwest Nebraska: corn or soybeans? The answer is: soybeans. We can look to Iowa State University research for supporting data. Corn planted between April 20 and May 5 achieved 100% yield potential. Depending on year-to-year variability 99% of yield potential could still be achieved with corn planted before May 20. In the three-year study, significant yield reductions occurred only once and that was when corn planting dates were extended to late May or June. In southwest Nebraska research in 2018, we observed daily yield penalties of 0.5-1.0 bu/ac/day for corn planted after May 1 (one year data).

We strongly recommend soybean farmers in western Nebraska evaluate their seeding and fertility practices and consider implementing changes that could lead to a more profitable crop.

Acknowledgements

I would like to thank my interns Nemanja Arsenijevic and Zaim Ugljic as well as our part-time technician Justin Richardson for their hard work on this project. We also thank Jim and Troy Kemling who allowed us to do this research on their farm. Lastly, we thank the Nebraska Soybean Board. Without their financial support, this project would not have been possible.