Cropping Intensity and Nitrogen Management Impact of Dryland No-Till Rotations in the Semi-Arid Western Great Plains

Crop N needs are not usually predicted based on cropping intensity or on tillage practice. However, N fertilizer requirements may increase dramatically as less fallow and less tillage are used in semi-arid regions of the Great Plains where summer fallow cropping is common. This long-term experiment was conducted to study the influence of N fertilizer rate, source/placement/timing (NSP), and crop rotation factors on the production of winter wheat (Triticum aestivum L.), corn (Zea mays L.), and grain sorghum (Sorghum bicolor L.), as well as their fertilizer N use efficiency (FNUE) for the initial years of conversion to no-till dryland farming. Research was conducted from 1987 through 1992 on two soils (Keith clay loam, a fine-silty, mixed, mesic Aridic Argiustoll and Weld loam, a fine-silty, mixed, mesic, Aridic Argiustoll) in eastern Colorado. Rotations included winter wheat-fallow (WF) and winter wheat-corn or grain sorghum-fallow (WCF). Wheat yields were similar between WF and WCF with adequate N application. Response to N fertilizer at lower rates was greater in WCF than WF because of its greater depletion of soil N. Corn production averaged 72 bu/acre with adequate N and required 1 lb/acre of N uptake to produce 1 bulacre of grain. Current N fertilizer recommendations for wheat and corn were not adequate to insure maximum production under no-till management. Fertilizer placement significantly affected average annual rotational yield (40 to 70 lb/acre per yr difference) but application rate was more important economically. Grain biomass produced in each rotation per pound of total plant N uptake (GNUE) was 17 lb/acre per yr in WF compared with 29 lb/acre per yr for WCF. This 70% increase in average annual grain production of WCF over WF was accomplished with a 44% annual increase in fertilizer N application. Research QuestionThe traditional dryland wheat-fallow (WF) rotation under conventional tillage in the western Great Plains is not economically and environmentally sustainable because of low soil water storage and high soil erosion potential. Addition of corn into a WF rotation under no-till management can increase production potential through greater soil water conservation while controlling soil erosion. Nitrogen cycling under no-till differs from conventional-till management. The objectives of this study were to determine N rate and placement requirements of winter wheat and corn (or grain sorghum) in a more intensive rotation (wheat-corn-fallow, WCF) relative to WF, and to identify N fertilizer rates and placements that give the highest grain production per unit of N uptake by the plant. The study was conducted under no-till management at two locations in the western Great Plains. Literature SummaryPrevious studies have shown that no- or reduced-till systems increase soil water storage efficiency and grain production potential compared with conventional tillage systems. Annual grain production increased by 72% under more intensive cropping compared with a WF rotation. Greater production potential means that N fertilizer requirements will also increase. During the initial years of no-till management, crop residues begin to accumulate on the soil surface. As a result, residue decay is slower, with a subsequent delay in the rem of residue N to the soil until the rate of decay equals the rate of accumulation. Consequently the transitional years from conventional tillage to no-till may require greater N inputs. Also, as cropping intensity increases, surface residues will accumulate even faster, with an eventual increase in soil organic matter. Accumulation of surface residues will also affect placement of N fertilizer. Studies have shown that broadcast applications of N fertilizer are less available than knifed or surface-banded applications. Sidedress applications of N have also increased crop response compared with N applied at planting. Study DescriptionA 6-yr field study (1987 to 1992) was conducted at two locations in the western Great Plains (northeastern Colorado near Sterling on a Weld loam and east-central Colorado near Stratton on a Keith loam). We evaluated the effect of N rate and placement for winter wheat in two rotations, WF and WCF, on grain production and N-use-efficiency under dryland no-till management. Each phase of each rotation was present every year; e.g. wheat in WF, fallow in WF, wheat in WCF etc., which allowed evaluation under the same weather conditions. Grain sorghum was grown at Stratton from 1987 to 1989 but corn was grown thereafter because of insufficient growing degree days for sorghum. From 1987 to 1990, N rates were 0, 25, 50, and 75 lb N/acre for wheat and sorghum and 0, 30, 60, and 90 lb N/acre for corn. Rates were increased in 1990 to 0, 30, 60, and 90 lb N/acre for wheat and 0, 35, 70, and 105 lb N/acre for corn. Nitrogen fertilizer placement treatments were (i) preplant broadcast of UAN at planting (UANPPB) (ii) 30% UAN banded below the seed at planting and 70% applied sidedress after breaking dormancy in wheat and at the 4 to 6 leaf stage in corn (UANST) (iii) 30% UAN banded below the seed and 70% dribbled over the seed at planting (UANSP) (iv) granular urea preplant broadcast at planting (UREA). Grain yields and total biomass were measured as well as N content of grain, straw and stover in order to calculate N fertilizer-use-efficiency. Residual soil nitrate to a 6 ft. depth was also determined. Applied QuestionsWhich fertilizer placements resulted in the best grain production? For wheat production, UANSP and UREA consistently produced the highest yields. The UANST treatment produced variable results that appeared dependent on weather conditions at the time of the sidedress application. UANPPB consistently produced the lowest yields. For corn, all four placements performed the same. The banded treatments showed greater uptake of N at the lower N rates (0 to 70 lb N/acre), which would translate into potentially higher yields if more precipitation were to occur during grain fill. What are the differences in annualized N fertilizer requirements and production potential between the WF and WCF cropping systems? The WCF cropping system required 44% more N fertilizer input than the WF. In terms of grain (wheat and corn) produced per year, WCF was 72% higher than WF. Higher production in the WCF cropping system and the associated increase in N inputs will not pose an environmental threat since use of the N is more efficient in WCF than the WF. Does the greater production in the WCF rotation translate into more economic return than for the WF rotation? Our results suggest that wheat production would be 20 bu/acre per yr in the WF rotation (average of the two sites) compared with 13 bu/acre per yr of wheat and 25 bu/acre per yr of corn in the WCF rotation. If it is more profitable to produce 25 bu/acre per yr of corn at the expense of producing 7 bu/acre per yr less of wheat, then the WCF rotation would be better. Another study at these sites showed a 35% increase in economic return with more intensive crop rotation. RecommendationNo-till management increases soil water storage efficiency and provides the opportunity to grow more crops over a number of years than does the traditional WF rotation. Consequently, N fertilizer demands also increase. Producers should not assume that N recommendations for WF or for dryland corn will be accurate for a more intensive cropping system such as WCF. It is important to base N fertilizer rates on a soil test recommendation that has been correlated to intensive cropping systems. Nitrogen rate is the most critical aspect of N fertilizer management, however N placement can also enhance productivity. Broadcast application of urea or banded application of liquid UAN at planting provides the best placement of N for wheat. Banded application of N in corn provides the best opportunity for maximum yields in most years.