Two soybean trials were conducted in 2020 at the NDSU Carrington Research Extension Center on dryland and under irrigation. Composite soil samples were taken in spring from the two sites at 0-6 and 6-12 inches deep. From these, subsamples were oven dried at 72°C, ground, and sent to the lab for routine nutrient analysis (Table 1). Distillers grains (WDG and CDS) used for the trials were obtained from the Tharaldson Ethanol Plant at Casselton, ND. The CDS was well mixed to homogenize the content and a subsample analyzed for elemental content and dry matter content (Table 2). Trial plots were measured at 25 feet long by 10 feet wide for treatments to be applied following the randomized complete block design with four replicates. The four treatments consisted of CDS, WDG, and TSP (triple super phosphate) fertilizer applied at the rate of 40 lbs P, plus a control (no P applied). To compensate for the 30 lbs N from CDS when applied at 40 lbs P, 30 lbs of N was added as urea to the control and TSP plots. Meanwhile, WDG supplied 66 lbs N at 40 lbs P/ac. After the treatments were applied (May 28th) on the surface of respective plots, they were incorporated the next day on half of the plot (5 x 25 feet), and then seeded. At the 5th trifoliolate growth stage, aboveground parts were collected from 16 random plants per plot and then separated into leaf axes (stems and petioles) and leaf blades. The axes were ground and analyzed for ureide-N by the method of Goos et al. (2015. The ureide-N concentration is indicative of nodule N fixation activity in soybean growing under stress free conditions. Yield data were collected and seed protein and oil concentration were determined.
Table 1. Preplant soil analysis from the dryland and irrigated fields
Site Depth pH SOM N P K S Zn
inches % lb/ac -------------ppm--------------
Dryland 0 - 6 8.4 2.7 4 8 220 6 0.89
6 - 24 6 5
Irrigated 0 - 6 8.4 2.7 16 8 131 6 0.64
6 - 24 18 7
Results
Soybean seed yield and quality
Phosphorus fertilization did not significantly impact yields and seed quality, even though yields improved on by four bushels on average for all P treatments on dryland (Figure 1). The lack of P impact was probably due to adequate soil available P and ability for soybean to take up soil P. Average yield was 60 bushels on dryland, which was five bushels greater than under irrigation. This was probably due to an early frost on September 8th and 9th when the irrigated crop was at R5 while the dryland crop was at R8 and already maturing. Normally before R7, soybean yields are still susceptible to adverse impact of frost. There were no significant treatment effects on seed test weight, protein, and oil. Mean seed protein on dryland was lowest at 35.4% for the control surface treatment and highest at 36.1% for surface applied WDG. Mean protein under irrigation ranged from 37.8 for the control surface application to 38.6%.for the CDS incorporated treatment.
Leaf ureide-N content
The rates of N applied with distillers grains or as urea did not result in differences in leaf ureide-N concentration, implying N fixation may have been similar among treatments (Figure 2), despite application of 66 lbs N with WDG versus 30 lbs N supplied with CDS, or added to the control and the TSP treatments. It was anticipated that 66 lbs N would likely result in lower activity of nodules (lower N fixation) and therefore, lower leaf ureide-N compared to 30 lbs N if the applied N was readily available to the crop. Beard et al. (1971) had reported a reduction in nodule formation of soybeans from preplant application of 50 lbs N/ac, while in a later study, Diebert et al. (1979) reported a 26 to 48% reduction in N fixation when over 40 lbs N was applied at planting. But the observed lack of ureide-N differences between WDG versus the rest of the treatments was probably due to slow release of N from WDG (Moore et al., 2010). The ureide-N content for every treatment was probably low compared to reported critical range of 1200-1500 ppm at flowering (Goos, 2015). Ureide-N ranged between 962 ppm for the check to 1102 ppm for WDG under irrigation, and 906 ppm for TSP to 966 ppm for the check on dryland. In view of average to above average seed protein results, these low ureide-N values could probably have resulted from increased N fixation after a slow start, following leaf sampling at 5th trifoliolate stage, or higher available N mineralized from soil organic matter, or the critical ureide range may be lower at 5th trifoliolate than at values reported at flowering.
Phosphorus removal with seed
Phosphorus treatments had no effects on P removed with seeds at either site. On dryland, P removal was not significantly different between the methods of application (Figure 3). Under irrigation, surface application inexplicably resulted in greater grain P removal than the incorporated treatment (Figure 4) by at least 5 lbs/ac on average. The amount of P removed was greater on dryland. Even though P removed was consistently greater for the incorporated than the surface applied treatments on dryland, the differences were not significant (Figure 3). Grain P removal had a strong correlation with yields on dryland, but not as strong under irrigation (Figure 5). Yields explained about 72% of the variation in P uptake on dryland, and just 52% under irrigation.
Conclusions
Failure to observe yield impact from P application means soybean is less likely to respond to soil available P when the soil P test is medium. Application of distillers grains did not produce any detrimental effects on soybean yields, seed quality, or N fixation, even at rate over 60 lbs/ac as WDG, suggesting a slow release of the N may only have a negative impact on N fixation at much higher rates than have been observed with urea. Any application of distillers grains as fertilizer sources should be done based on the nutrient analysis of the type to be applied, in order to determine the quantity to apply. Further studies at higher N rates from distillers grains will help determine thresholds of potential impact of its N supply on N fixation.