In an environment of high production costs and low grain prices, efficient use of inputs is essential for maintaining economic viability of farming operations. Application of excessive amounts of nitrogen (N) fertilizer may reduce profitability and create groundwater pollution hazards. On the other hand, yield responses to N frequently make this technology cost effective, but determination of the optimum level of fertilizer is necessary to maximize profit.

Current North Dakota fertility recommendations for sunflower suggest 5 pounds N/cwt. yield goal (Dahnke, 1994). Thus, a 2,000 pound yield goal would require 100 pounds of total N (soil test nitrate-N + credit for a previous legume crop + applied fertilizer). However, experiment station trials sometimes show no yield benefit from N fertilization of sunflower and usage under production conditions varies greatly. The objectives of this study were to determine the effects of N fertilizer on sunflower yield and soil nitrate-N levels and the influence of tillage practices on these effects.

Sunflower data were analyzed from the Spring Wheat – Sunflower – Barley – Fallow rotation in a long-term cropping systems experiment, which was initiated in 1987. Fertilizer N levels (see below) and tillage practices (conventional, reduced, no-till) were arranged in perpendicular strips. Sunflowers were planted in 30" rows and managed to maximize yield by optimizing P fertility and pest control. Fall soil samples from two replicates were analyzed for nitrate-N concentration. Yield was measured on all three replicates.

Averaged across years, a nearly linear increase in fall soil nitrate-N concentration was observed with increases in the level of spring N fertilizer (Table 1). Over the nine years covered by data, N fertilization not only increased yield (see below), but improved the N-status of soil for the next crop in the rotation.

Tillage practice had essentially no effect on fall soil nitrate-N level within a given year (data not shown). Averaged across years, significantly lower nitrate concentration in the no-till plots was likely due to uptake by weeds, which were more prevalent in this system. Higher water accumulation in these high-residue plots may have contributed to increased N losses by leaching.

Comparing the average fall nitrate-N values for years 88-90 (relatively low rainfall) with those for 93-95 (relatively high rainfall) points out the susceptibility of soil N to leaching and denitrification. The range of 4.3 to 47.3 pounds/acre stresses the importance of annual soil sampling to quantify the N-status of fields.

In most years, sunflower yield increased slightly with increases in N fertilization, but differences were not statistically significant (Table 2). However, the average across years shows an incremental increase in yield with applied N and the differences among treatments are highly significant.

In some years, significant yield reductions were observed in the no-till treatment (Table 3). These differences are attributed to inferior weed control in these plots, which will become less of a limiting factor as more post-emergence herbicides are labeled for use in sunflower. Yields under minimum tillage and conventional tillage were statistically similar in all nine years, which indicates the possibility of reducing tillage costs without affecting yield.