Sunflower response to nitrification inhibitor application

Sunflower has become an important crop in the Northern Great Plains. Nitrogen (N) plays a pivotal role in sunflower yield and oil content. Excess supply of fertilizer N can increase yield but reduce the oil content due to dilution of oil in heavier seeds produced (Blamey and Chapman, 1980). However, shortage of N supply, particularly at an early growth stage, could reduce yield due to suppression of physiological attributes (plant height, total number of florets, and seed vigor) (Connor et al., 1993). For dryland and irrigated sunflower, Zubriski and Moraghan (1983) reported yields of 1,553 and 2,117 lb/ac with 25 lb N/ac and 2,369 and 3,266 lb/ac with 150 lb N/ac, respectively. Current fertilizer N recommendation for eastern conventional till oilseed sunflower N recommendation ranges between 47 and 150 lb N/ac based on the cost of fertilizer N and sunflower price (Franzen, 2016). Recently, Schultz et al. (2018) reported that 14 out of 21 sites responded to fertilizer N additions with the yield response curve in response to incremental fertilizer N application rates following a quadratic relationship in North Dakota and South Dakota.

Adding a nitrification inhibitor to fertilizer N has the potential to reduce fertilizer N loss through denitrification and leaching, and hence increase fertilizer N use efficiency (Thapa et al., 2016). Some researchers reported toxicity of 6-chloropicolinic acid, a principal metabolite of nitrapyrin on sunflower seedling growth; and the suppressing influence of nitrapyrin was controlled by the form of fertilizer N (Maftoun et al., 1982). The main goal of this research was to find out the potential of inhibitor addition to increase yield by increasing N use efficiency. On-farm trials were conducted to determine the effects of urea N rate and inhibitor addition on sunflower seed yield and soil N availability.

Research Experiment and Analyses

During 2018 growing season, two growers’ fields located at Buffalo and Chaffee in eastern North Dakota were selected for this experiment. Details of the experimental sites are presented in Table 1. Monthly average of maximum air temperature and rainfall during the growing season (April through October) from the Prosper, ND weather station of the North Dakota Agriculture Weather Network (https://ndawn.ndsu.nodak.edu/) close to the two sites are presented in Fig. 1. The early to middle part of the growing season (May through July) had lower rainfall than normal; and maximum air temperature was also higher than normal in May. Seven treatments consisted of three fertilizer N (in the form of urea) application rates: 100% (150 lb N/ac), 75% (110 lb N/ac), and 50% (75 lb N/ac) of recommended N, with and without nitrapyrin (Instinct HL, Corteva AgriSciences, 2.5 lb a.i./gal), at the rate of 24 fl oz/ac along with a control (no fertilizer N). Treatments were laid out in randomized block design with four replications. Plot size was 25 ft long and 11 ft wide. Fertilizer N was hand-broadcast just before planting and incorporated to surface soil using tillage equipment. Phosphorus and potassium fertilizers were not applied due to high soil test values according to the current recommendation (Franzen, 2016). Sunflower (oilseed-type cultivar) was planted and growers were responsible for in-season cultural operations. Surface soil samples 0 to 1 ft deep were collected from each plot two and four weeks after planting. Soil samples were analyzed for inorganic N (NH₄⁺ and NO₄^_) concentration by extracting with 2 M KCl and subsequently analyzed with a Timberline ammonia analyzer (TL-2800, Timberline Instruments, Boulder, CO). At maturity, sunflower heads of the middle two rows of each plot were harvested by hand by clipping the head from the stalk as close to the head as possible and putting it into burlap bags. Sunflower heads were oven-dried at 104°F to a moisture between 8 and 10% prior to being threshed. Threshing of sunflower heads was conducted using an Almaco low-profile plot thresher (Almaco, Nevada, IA). Sunflower seed grain yield was weighed; moisture and test weight were determined on a seed grain subsample using a Dickey-John GAX500XT moisture and test weight meter (Dickey-John, Auburn, IL). Yields were adjusted to 10% moisture. Statistical analysis of seed yield and soil inorganic N after two and four weeks was conducted using a randomized block design using PROC GLM procedure under SAS 9.4 software (SAS Institute Inc., Cary, NC). Mean separation of treatments was performed using the LSD at 95% significance level.

Seed Yield and Soil Inorganic N

Sunflower seed yields in response to fertilizer N application rate and inhibitor addition at two sites are presented in Table 2. At the Buffalo site, the highest seed yield (4,236 lb/ac) was observed with 100% of recommended N without nitrapyrin addition; whereas, the 75% of recommend N with nitrapyrin addition had the highest seed yield (3,841 lb/ac) at Wheatland. However, these treatments were not significantly different from the control. Schultz et al. (2018) also found that sunflower yield increased with N rate until reaching the critical level and the required N rate to maximize the yield at the conventional tillage site of eastern North Dakota was 200 lb N/ac. For this study, total soil available N (initial soil N + fertilizer N) for the 100% of recommended fertilizer N treatment was close to 190 lb N/ac. Application of fertilizer N additives, nitrification, and/or urease inhibitors could reduce the loss of N but had hardly any effect on yield and quality (Thapa and Chatterjee, 2017).

Soil inorganic N concentration was lowest for the control after two weeks for both sites (Table 2). At the Buffalo site, 100% of recommended N without nitrapyrin had the highest soil N availability (81.5 ppm) after the second week of planting, and it was significantly higher than 50% of recommended N with (17.2 ppm) and without (13.1 ppm) nitrapyrin and the control (8.06 ppm). At Wheatland, 75% of recommended N without nitrapyrin had the highest soil N availability (20.2 ppm) after the second week of planting, and it was significantly higher than 50% of recommended N with nitrapyrin (8.11 ppm) and the control (6.44 ppm). Application of 50% of recommended N without nitrapyrin (19.9 ppm) had higher soil N availability than 50% of recommended N with nitrapyrin (8.11 ppm) at Wheatland after the second week of planting. At Buffalo, 100% of recommended N with nitrapyrin had the highest soil N availability (32.1 ppm) after the fourth week; but it was not significantly different than without nitrapyrin (26.9 ppm). At Wheatland, soil N availability after the fourth week did not vary among treatments. Nitrapyrin effect on soil N availability probably varied with soil type irrespective of urea N rate. Only for Wheatland at the second week did 50% of the recommended N rate show significant difference between with and without inhibitor addition; however, the difference in soil N availability did not result in any difference in yield. Duration of nitrification inhibition by nitrapyrin varied with location and year due to differences in soil moisture, temperature, and precipitation (Omondoe and Vyn, 2013).

Conclusion

Optimizing the supply of N is critical for the sunflower seed yield, and the full recommended N rate is necessary to optimize the yield. The addition of an inhibitor did not influence yield or soil N availability. The growing season of 2018 was relatively dry and not an ideal environment for denitrification and leaching losses; hence, spring application of a nitrification inhibitor had hardly any effect on soil N dynamics under sunflower production.