The major contributor to global warming is considered to be the high levels of greenhouse gas emissions, especially carbon dioxide (CO 2

No abstract

Typically, N fertilizer is fall‐applied to dryland winter wheat (Triticum aestivum L.) in the Great Plains, because cheaper N sources (anhydrous ammonia) can be applied in the fall and application can be combined with seedbed preparation tillage. Spring‐ or split‐applied N provides increased N management flexibility and potentially less N loss due to leaching or denitrifi‐cation over the winter. This study was conducted to compare winter wheat grain yields, protein, and economic returns associated with fall‐, spring‐, and split‐applied N at rates of 0, 20, 40, and 60 lb N/acre at 19 sites over 3 yr (1985–1987) in eastern Colorado. Grain yield response to N fertilizer was dependent on precipitation or residual soil NO3 levels, while grain protein response was not. Spring‐applied N increased grain yields and protein more than fall‐ and split‐applied N. Fall‐applied N required 20 and 18% (mean) more spring‐applied N to achieve the same grain yield and protein content, respectively. Spring‐applied N was more profitable than fall‐ or split‐applied N. Without application cost (“free ride”), N applied to solely increase grain protein was profitable. In semiarid climates, spring‐applied N is more available to winter wheat and more profitable than fall or split‐applied N.

Nitrogen fertilizer recommendations for dryland hard red winter wheat (Triticum aestivum L.) are more reliable when based on spring rather than fall soil and plant measurements. Objectives of this research were to develop spring N recommendations based on soil available N, plant tissue N, and/or climatic variables measured in the spring. Grain yield variation was described by simple and multiple regression analysis with various levels of complexity. Soil NH4‐N levels in the spring explained more grain yield variation than did NO3‐N. Spring soil sampling to depths >60 cm explained no additional grain yield variation. Percent sand in the soil surface (0‐30 cm) explained more grain yield variation between experimental sites than did other site variables. Simple Cate‐Nelson III models were more accurate than complex multiple regression equations. Cate‐Nelson III models, for available soil NH4‐N plus NO3‐N (0‐30 and 0‐60 cm) and Feekes 5 leaf N, predicted required N rates in the spring within ±22 kg N ha−1, 87 to 95% of the time. Spring N recommendations were improved when spring N rates were based on both available soil N and Feekes 5 leaf N. These results indicate (i) that analysis of available soil N in the spring must include soil NH4‐N to prevent over‐fertilization, and (ii) that simple Cate‐Nelson models are sufficient for calibration of spring N rates to available soil and plant N levels measured in the spring.