Seeding Recommendations for Irrigated Soybean and Dryland Corn in West-Central Nebraska

 

PART 1 – Irrigated soybean: planting date, row spacing, seeding rate and nitrogen management

 

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 (Stepanovic et al., 2018a)

 

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. Cover photo: Study investigating effects of row spacing, planting date, seeding rate and nitrogen management at Grant, NE (2019).

 

 

 

 

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.

 

Weather conditions including total monthly precipitation and maximum and minimum temperatures at Grant, NE 

Figure 1. Weather conditions including total monthly precipitation and maximum and minimum temperatures at Grant, NE (2018 vs 30-year average).

 

 

 

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 (Jasa, 2010).

 

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.

 

 

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:

 

 

* Correlation coefficient significant at 5% level. The sign of coefficient indicates the nature of relationship (either positive + or negative -) while the magnitude of coefficient (ranging from 0 to 1) represents the strength of the linear relationship.