Sunflower Response to Nitrogen and Phosphorus in Wheat‐Fallow Cropping Systems

Abstract

Sunflower ( Helianthus annuus L.) is often grown as an alternative crop in wheat ( Triticum aestivum L.)‐fallow areas of the northern High Plains. Optimum management of fertilizers is important to maximize profit potential and avoid environmental concerns. The objectives of this research were to determine the environmental and soil conditions where seed yield, oil content, and chlorophyll responses to added N and P fertilizers should be expected for sunflower grown in the northern High Plains. Sunflower was grown over 40 site‐years in the Nebraska Panhandle during 1993 and 1994 on producers’ fields. Fertilizer treatments were four rates of N (0,35,70, and 105 lb N/acre) with or without 69 lb P 2 O 5 /acre. All fertilizers were broadcast and incorporated shortly after planting. Sunflower was grown in a wheat‐sunflower‐fallow cropping system. Fertilizer effect was evaluated for three sunflower seed yield goals: high (>1000 lb/acre), medium (500–1000 lb/acre), and low (<500 lb/acre). Application of P did not influence sunflower seed yield and oil content. Regardless of the initial soil nitrate level or yield goals, application of N beyond 35 lb/acre did not influence seed yield in 80% of the site‐years. However, the percentage seed oil content was reduced significantly by increasing levels of N for most site‐years. The relationship between the N levels and oil content had an 13 value of 0.96 for the high, 0.93 for the medium, and 0.94 for the low yield goals. Residual nitrate level did not significantly influence either the seed yield or the seed oil content. Chlorophyll meter readings increased with application of N fertilizers at six site‐years; however, at these sites N did not Significantly affect seed yield. The results indicate that fertilizer N and P application in the region is not producing economic returns. The yields were highly variable and not significantly affected by the growing season, total precipitation, or soil moisture content at time of planting. The lack of response to N or P fertilizers regardless of the soil N and P level, indicate that other growth factors such as precipitation and temperature at critical growth stages and types of sunflower cultivars are the possible sunflower yield‐controlling factors for the region. Research Question In the northern High Plains, sunflower is often used as an alternative crop in wheat‐fallow rotations to diversify cropping systems and to increase profit by growing more than one crop in 2 yr. Optimum management of fertilizer is important for maximum profit potential and for avoiding environmental concerns such as nitrate leaching and N and P runoff from surface application. The objective of this study was to determine the N and P requirement of sunflower grown in a wheat‐sunflower‐fallow cropping system. Literature Summary Soil N status is critical for optimal production of oilseed crops. Lack of N results in low yields, while excess N promotes reduced oil contents. However, the effect may vary depending on soils and environmental conditions. In the High Plains, lack of moisture is often the factor that limits yield in many years and responses to fertilizer are not usually observed in this situation. Current recommendations for N in the region are based primarily on data developed in other sunflower producing regions. No database is available concerning N fertilization of sunflower in the northern High Plains when used as an alternative crop in a wheat‐fallow rotation. Study Description Forty site‐year field experiments were conducted in four western Nebraska counties by the University of Nebraska Panhandle Research and Extension Center, located at Scottsbluff, during 1993 and 1994. Soils: Alliance, Keith, Kuma, Rosebud, Satanta, Tripp loam, Duroc and Keith silt loam, and Bayard fine sandy loam are the major soils on which the experiments were conducted. Experimental Design: Factorial in a randomized complete block with three replications at each site‐year. Treatments: Four N rates (0, 35, 70, and 105 lb N/acre) applied as ammonium nitrate in a factorial combination with two rates of P fertilization (0 and 69 lb P 2 O 5 /acre) applied as triple super phosphate Applied Questions What effect did applied N have on sunflower yield? Nitrogen fertilization did not routinely increase seed yields of sunflower used as an alternative crop in wheat‐fallow rotations in the northern High Plains. Regardless of the residual nitrate N status of the soils, yields were not affected by N additions in most site‐years. A wide range of seed yield was obtained on soils with comparable residual nitrate N, indicating factors other than N, such as precipitation and temperature at critical growth stages, are limiting response to N in the region. What effect did applied N have on sunflower seed oil content? Sunflower seed oil content was significantly reduced by increasing addition of N. The decline in oil content due to increased rate of N fertilizer application was comparable across all yield goals. Did application of P influence seed yield or oil content? The influence of P on both seed yield and seed oil content was not significant in this experiment. This lack of response to applied P may be explained by the level of soil P that was medium to high available P in more than 80% of the site‐years. Should growers stop applying N and P fertilizers to sunflower grown as an alternate crop in wheat‐fallow rotation? The results of this study show no yield benefit from application of N and P fertilizers for the yield range obtained at 37 site‐years in the region. More importantly, at the yield level of up to 1900 lb obtained in this study, profit will be affected negatively by the declining oil content of the seed without any improved yield from the applied N fertilizers. The great variability in sunflower seed yield and lack of response to N and P fertilizers, regardless of the soil N and P status, indicate that growing condition factors other than N or P fertilization influence the yield and seed quality of sunflower. These factors may include plant population, moisture, temperature, early frost conditions at critical stages during the growing season, and type of sunflower cultivars.

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