Fertilizing Sunflower

Sunflower (Helianthus annuus L.) is a New World crop, domesticated in eastern North America about 4,000 years ago (Blackman et al., 2011). Perhaps due to the development of sunflower relatively recently in human history and its breeding as an oil seed for yield, the efficiency of nitrogen (N) utilization by sunflower is relatively high. Nitrogen application increases plant height but may or may not increase yield.

Oilseed sunflower is grown, as the name indicates, for oil content. Increasing the N rate increases seed protein and decreases oil concentration (Darby et al., 2013). Confection sunflower is grown for human consumption as a snack food. It is dehulled and the kernel is used on salads, and in sunflower butter and bakery goods. It is also incorporated into snack foods and other food recipes.

Nitrogen recommendations in North Dakota are now based on a yield-independent economic production function, while for most other states, including South Dakota, Nebraska and Kansas, N recommendations are based on expected-yield formulas. Formulas include reducing N recommendations due to soil test nitrate (Nebraska, South Dakota, North Dakota, Kansas), organic matter (Kansas, Nebraska) and all states by manure and previous crop credits. In contrast to the implied N response formula, which most often is constructed as [(yield expected (pounds per acre) multiplied by 0.05) less credits], recent N rate experiments in other states have achieved less yield response to N than expected (Darby et al., 2013; Scheiner et al., 2002).

This publication of North Dakota sunflower N recommendations is based on the results from 52 total experiments: four North Dakota N rate experiments in 2012-13, 40 North Dakota N and phosphorus (P) rate experiments in 2014-15, and eight N and P rate experiments in South Dakota in 2014-15, with 48 total experiments taken to yield.

The results from these studies indicate that yield is independent of N rate, meaning that any formula of (Yield X (a factor) = N rate) is incorrect and nonpredictive in North Dakota. Nitrogen rate is related to relative yield within an environment, not actual yield. In addition, the recommendations account for the greater susceptibility to lodging with higher N rates and the relationship between higher N rates and increased sunflower disease. Although in the absence of high wind and disease, higher yield might be possible with higher N rates than those in the recommendation tables or in the future N calculator for sunflower, sunflower growers assume a greater lodging and disease risk if they choose to use higher rates.

For the interactive North Dakota Sunflower Nitrogen Calculator, go to https://www.ag.ndsu.edu/temp/cnc/.

The sunflower recommendations are regionally-based and tillage-based. Perhaps due to the deeper rooting nature of sunflower, soil texture had less effect on yield compared with clay texture effect on corn N use efficiency in previous studies. However, at most sites that were planted to sunflowers for the first time in more than 30 years or did not have a deep-rooted crop such as sugar beets or sunflowers within the past five years, yield response was very slight, if any increase occurred. Oil content tended to be near or below 40%, even with zero added N. This would be possible only if these locations have some unaccounted-for deep N. The western North Dakota site that was not planted to sunflower for at least five years experienced a major drought in 2012, followed by relatively wet seasons in 2013-2015, which may have moved N downward in the soil. In the sunflower crop of 2015, the deeper N in this field likely was tapped, resulting in no yield increase with higher N rates and relatively low oil content in the seed. The sites in eastern North Dakota without sugar beet or sunflower in recent memory likewise did not have increased yield with higher N rates and had oil content that, even at the zero N rate, was below 40%.

If practical, new fields intended for sunflower should be sampled to a 4-foot depth to determine if deep N should be considered in N fertility recommendations. About 30 pounds of N from 2 to 4 feet in depth are assumed but not subtracted from N recommendations. If the 2- to 4-foot soil nitrate test is greater than 30 pounds N per acre, then the extra N should be subtracted from the total N recommendation.

If deeper soil sampling is not practical, an N nonlimiting area of a full rate of N based on the N calculator or the N recommendation tables should be applied, and about half the N calculator rate should be applied to the rest of the field. Using an active-optical sensor or “best-guess” assessment of color differences and vigor of the N nonlimiting area, compared with the rest of the field, a decision whether or not to apply additional N in-season could be made or not made between stage V-8 to R1 (approximately V12 to V14) using a side-dress applicator or high-clearance applicator (Figure 1). In fields without a history of sunflower, sugar beet or another deep-rooted crop such as safflower, no additional N likely needs to be applied.

Photo Credit:

Don Lilliboe

Figure 1. Side-dressing ammonia on sunflowers about V8.

Soil sampling to 2 feet in depth is very important for N recommendations in this region. Including soil test nitrate-N into analysis of N rate vs sunflower yield trials greatly increased the relationship of total known available N to relative yield in all regions.

There are differences in the response of sunflower in eastern North Dakota compared to those in western North Dakota (Figure 2). In addition, the Langdon region is separated from the rest of eastern North Dakota due to the small shale pieces the soil in this region contains. These shale pieces are the source of mineralizable, plant-available ammonium, causing these soils to act as a natural slow-release N fertilizer. The N recommended in the Langdon Region is less than in the rest of the eastern region.

Figure 2. Regions of eastern North Dakota, the Langdon area and western North Dakota important to North Dakota sunflower N recommendations. Soils in the Langdon area
contain small pieces of shale that release N during the growing season, making the N rate recommendations for Langdon less than those in the rest of eastern North
Dakota.

In the state, long-term no-till sites, those sites with continuous no-till for at least six years, required less N to achieve maximum economic yield compared to sites under conventional tillage. Similar results have also been recorded in spring wheat/durum and in corn, whose recommendations also contain reduced N rates for long-term no-till management.

Some harvest loss is likely with lodging, so the upper values of N rate were reduced to limit lodging loss. Although white mold and downy mildew seemed to be related to N rate in experiment observation, only one site was rated for disease in each treatment. This site, north of Dickinson, North Dakota, was rated at harvest for sunflower rust, which is caused by the fungus Puccinia helianthi. The results of the rating are available in Table 1.

* Ratings made on images obtained by retired agronomist Roger Ashley and rated by Samuel Markel, NDSU Extension plant pathologist.

Excessive wind resulting in sunflower root lodging or stalk breakage does not happen in every sunflower field every year, but lodging is a concern of most sunflower growers. Several research sites were affected by wind each year, and lodging severity was directly related to N rate (Figure 3).

Figure 3. Percent sunflower lodging with increased available N at Belfield and Bottineau sites, 2015.

Although in the N rate experiments all heads were included in yield, sunflower growers would suffer a decrease in harvest efficiency when sunflowers are on or near the ground. Therefore, the N rate is capped, even though small increases in yield and marginal profit might be possible with greater N rates in nonwind-affected growing seasons.

Seed oil content of oil-seed sunflowers always decreased with N rate, regardless of yield response. The economic impact of available N on yield response, seed oil response and N cost is factored into each N recommendation. Highest N rates are moderated due to possible harvestable yield reduction due to lodging. The N rate recommendations are available in Tables 2-13.