Sunflower response to nitrogen and phosphorus in wheat-fallow cropping systems
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Sunflower (helianthus annuus L.) is often grown as an alternative crop in wheat (Triticum awstivum 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 P2O5/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 (5001000 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 r2 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.
