Effects of Late-Season Nitrogen Fertilizer on Irrigated Soybean Yield And Composition

Abstract

Even though soybean [Glycine max (L.) Merr.] fixes N from the air, N demand at the R4 to R6 (pod-fill) growth stages of high-yielding, high-protein soybean may be greater than the amountsss of available N with normal farming practices. In Kansas, supplemental N has not been recommended on soybean regardless of yield level. Research was conducted at eight sites over a 2-yr period on irrigated soybean to evaluate effects of N rates (0, 20, 40 lb/acre) and sources (urea-ammonium nitrate solution [UAN], ammonium nitrate, urea, urea + N-[n-butyl] thiophosphoric triamide NBPT]) on leaf N concentrations, grain yield, and grain protein, and oil concentrations. The NBPT used is a commercially available urease inhibitor. Most N was broadcast but UAN was applied over the top of the canopy through flat-fan nozzles. All applications were made at the R3 growth stage. Nitrogen concentrations in leaf samples taken 2 to 3 wk after N application were unaffected by N fertilization. Soybean yields were increased significantly by late-season N application at six of eight sites; the average increase was 6.9 bu/acre or 11.8 %. Both of the nonresponsive sites had yields averaging under 50 bu/acre, whereas responsive sites yielded 56 bu/acre or more. Soybean plants at all locations were well nodulated. In most cases, 20 lb N/acre provided positive responses. Late-season N fertilization increased grain protein and oil concentrations at some sites, but overall combined analysis indicated nonsignificant effects. Nitrogen sources performed similarly. Application of UAN resulted in leaf burn, which probably reduced yields at the 40 lb N/acre rate. However, this would not be a problem for producers applying UAN through irrigation systems where the UAN would be much more diluted. Even when well nodulated, soybean with high yield potential may not be able to take in enough N to achieve maximum yield of high quality grain. Public and private soil test labs and crop consultants may have to reevaluate N recommendations for soybean with high yield potential. Supplemental N application at the R3 growth stage can provide positive economic returns for producers growing irrigated soybean with high yield potential. Research Question Can soybean symbiotically fix enough N to produce high yields under irrigation? Producers of irrigated soybeans are now routinely achieving yields in excess of 60 bu/acre. A soybean crop yielding 70 bu/acre requires nearly 250 lb N/acre. Late-season supplemental N may increase yields. In addition, future soybean marketing strategies may include protein and oil concentrations. Late-season supplemental N has potential to affect these seed quality considerations. The objective of this research was to evaluate the effects of late-season supplemental N on soybean grain yield and composition. Literature Summary Previous research on N fertilization of soybean has produced inconsistent results. Much of the earlier work evaluated N applied either preplant, at planting time, or early in the growing season. Results from previous work indicated both increased and decreased soybean yields. Few studies have evaluated the impact of late-season N application on irrigated soybean with high yield potential. Limited information exists on N rate and source effects. Study Description This research was conducted over a 2-yr period at four irrigated sites. Locations: Johnson County (JO94, JO95); Shawnee County (SN94, SN95); Reno County (RN94, RN95); Stafford County (SF94, SF95) N rates: 20 and 40 lb N/acre applied at R3 growth stage (first pods 0.25–0.5 in. long) and a 0 N check. N sources: urea-ammonium nitrate solution (UAN), urea, urea + N-(n-butyl) thiophosphoric triamide (NBPT), ammonium nitrate Applied Question Does late-season supplemental N increase yield and improve grain protein and oil concentrations of irrigated soybean with high yield potential? Application of late-season supplemental N increased soybean grain yield at six of eight locations in Kansas (Fig. 1). Nitrogen sources performed similarly, except the 40 lb N/acre rate of UAN, which resulted in lower yield than other N sources. The high rate of UAN caused severe leaf bum at most locations. The UAN was applied through flat-fan nozzles with a backpack sprayer in 40 GPA total volume. Leaf burn would not be a problem with more dilute UAN applied through an irrigation system. Figure 1Open in figure viewerPowerPoint Effects of late season N application on soybean yield. Although small increases in soybean protein and oil concentrations were noted at some sites, overall combined analysis indicated nonsignificant effects. Economic evaluation, based on yield, indicated that applying late-season supplemental N to irrigated soybean with high yield potential is a viable practice. Recommendation Late-season supplemental N at a rate of 20 lb/acre should be applied to irrigated soybean with high yield potential at the R3 growth stage.