Rate and Time of Nitrogen Application for Wheat following Different Crops

Abstract

Winter wheat ( Triticum aestivum L.) is often rotated with other crops in the eastern Great Plains to diversify cropping systems. Optimum management of N fertilizer is important for maximum profit potential and for avoiding environmental concerns. This study was conducted from 1986 through 1991 in southeastern Kansas to determine the influence of previous crop (grain sorghum [ Sorghum bicolor (L.) Moench], soybean [ Glycine max (L.) Merr.] and spring oat [ Avena sativa L.]) on the N requirement for hard red winter wheat and to determine the optimum time of N application (fall, late winter, or fall + late winter and fall + late winter + early spring). Doublecrop soybean followed wheat in a 2‐yr cropping rotation. Soil type was a Parsons silt loam soil (fine, mixed, thermic, Mollic Albaqualf) with 2.8% O.M. Influence of previous crop on wheat yield, grain protein, and plant N increased with time in the rotation sequence. Differences between previous crops were small after the first cropping cycle; however, after the third cycle, all values were significantly highest following spring oat and lowest following grain sorghum. Grain yield response to applied N was highest following grain sorghum, intermediate following soybean, and lowest following spring oat. Nitrogen response differences probably are due to a larger net mineralization of soil‐N and crop residue N following spring oat than soybean or grain sorghum and to immobilization of applied N following grain sorghum. Number of heads per sq ft was highly correlated (R 2 = 0.85) with grain yield response. Split N (fall + late winter) and split‐spring N (fall + late winter + early spring) applications generally showed no consistent yield advantage over single N treatments (fall preplan! or late winter topdress). Grain protein, however, was often higher when N was delayed until late winter or early spring. Results indicate that fertilizer N requirements for hard winter wheat are likely to be low following another small grain crop, intermediate following soybean, and high following a high residue crop, such as grain sorghum. For the climatic and soil conditions in this study, applying N fertilizer at the proper rate was more important than timing of application. Applying up to 1/4 of the total N in early spring, however, could be beneficial for increasing grain protein content in hard winter wheat areas. Research Question In the eastern Great Plains, winter wheat is often rotated with other crops such as soybean, feed grains, and small grains to diversify cropping systems. Optimum management of N fertilizer is important for maximum profit potential and for avoiding environmental concerns, such as nitrate leaching and N runoff from surface N applications. The objectives of this study were to determine the influence of previous crop on the N requirement of hard red winter wheat and the optimum N application timing. Literature Summary Because climate and soils influence N management for winter wheat, N recommendations vary considerably throughout the major wheat‐growing regions of the USA. In the hard winter wheat‐producing area of the Great Plains, where precipitation levels are low, N fertilizer typically is fall‐applied and often combined with fall tillage. In the soft winter wheat‐growing regions of the eastern USA, however, a major portion of the fertilizer N is applied as a topdressing in early spring as wheat breaks winter dormancy. Split N applications have been evaluated in climates where the potential is greater for N losses through leaching and denitrification. The influence of previous crop on wheat N management has been evaluated primarily in doublecropping systems of the southeastern USA. No information is available, however, concerning the effects of previous crop on N needs of winter wheat in the eastern Great Plains. Study Description Field studies were conducted at the Kansas State University Southeast Ag Research Center, located near Parsons, from 1986 through 1991. Soil: Parsons silt loam Experimental design: split‐plot Main plots: previous crop (2‐yr rotation of wheat following grain sorghum, soybean, or spring oat). Doublecrop soybean followed wheat in each previous crop system. Subplots: factorial arrangement of three N rates and four N application times N Rate: 40, 80, and 120 lb N/acre and control (no N) treatment N Time: all in the fall (preplant incorporated) all topdressed in late winter 1/2 in fall and 1/2 in late winter 1/4 in fall, 1/2 in late winter, and 1/4 in early spring N source: Urea (except for early spring N treatment, which was urea‐ammonium nitrate solution, 28% N). Applied Questions What effect did previous crop have on the N requirement for wheat? Fertilizer N requirements for hard winter wheat in the eastern Great Plains are likely to be low following another small grain crop, intermediate following soybean, and high following a high residue crop, such as grain sorghum. Grain yield declined with increasing N rate following spring oat; but, yields increased following grain sorghum or soybean, although the magnitude of response following grain sorghum was greater than for soybean. The highest N rate in this study, 120 lb N/acre, was not sufficient to optimize yield when wheat followed grain sorghum. Including doublecrop soybean in the rotation after wheat was effective in scavenging residual soil nitrate‐N. Grain protein, unlike grain yield, increased with increasing N rate, regardless of previous crop. Plant N results suggested that yield and protein response differences probably were due to a larger net mineralization of soil‐N and N in crop residue following spring oat than soybean and to immobilization of applied N following grain sorghum. When was the optimum time to apply N fertilizer? Grain yield and wheat plant N status suggest that N losses from leaching, denitrification, or ammonia volatilization were minimal in this study. Thus, applying N at the proper rate was more important than timing of application. Split N (fall and late winter) and split‐spring N (fall + late winter + early spring) applications generally showed no consistent yield advantage over single N treatments (fall preplant or late winter topdress). Number of heads per square foot, however, were highest when some N was fall‐applied and lowest for the late winter only topdress application. This suggests that some supplemental N is needed in the fall for good tiller development, especially when wheat follows a high‐residue crop. Grain protein was often higher when N was delayed until late winter or early spring compared with fall application. Applying up to 25% of the total N needs in early spring could be beneficial in hard winter areas if grain protein premiums existed. Further wheat research is needed to evaluate N management effects, such as N placement, in high‐residue no‐till cropping systems.