Variable-rate technology (VRT) has been developed to variably apply crop inputs to manage in-field variability. Although growers have begun to adopt VRT, its profitability is uncertain in N ...management. The objective of this study was to assess the economics of uniform vs. variable-rate N fertilizer application under two N application scenarios (farmer vs. custom applications). On-farm studies were conducted on two continuous corn (Zea mays L.) fields in northeastern Colorado under furrow and center-pivot irrigation during the 2000 and 2001 growing seasons. The N management strategies were uniform, grid-based, site-specific management zone-constant yield goal (SSMZ-CYG) and site-specific management zone-variable yield goal (SSMZ-VYG). "Profit and loss" software was used to analyze the economics of each N management strategy and determine which N strategy was most profitable. Results from three site-years consistently indicated that less total N fertilizer (6-46%) was used with the SSMZ-VYG N management strategy when compared with uniform N management. Net returns from the SSMZ-VYG N management strategy were $18.21 to $29.57 ha(-1) more than uniform N management. Results of this study suggest variable-rate N application utilizing site-specific management zones are more economically feasible than conventional uniform N application.
Advances in precision agriculture technology have led to the development of ground-based active remote sensors that can determine normalized difference vegetation index (NDVI). Studies have shown ...that NDVI is highly related to leaf nitrogen (N) content in maize (Zea mays L.). Remotely sensed NDVI can provide valuable information regarding in-field N variability and significant relationships between sensor NDVI and maize grain yield have been reported. While numerous studies have been conducted using active sensors, none have focused on the comparative effectiveness of these sensors in maize under semi-arid irrigated field conditions. Therefore, the objectives of this study were (1) to determine the performance of two active remote sensors by determining each sensor’s NDVI relationship with maize N status and grain yield as driven by different N rates in a semi-arid irrigated environment and, (2) to determine if inclusion of ancillary soil or plant data (soil NO3 concentration, leaf N concentration, SPAD chlorophyll and plant height) would affect these relationships. Results indicated that NDVI readings from both sensors had high r 2 values with applied N rate and grain yield at the V12 and V14 maize growth stages. However, no single or multiple regression using soil or plant variables substantially increased the r 2 over using NDVI alone. Overall, both sensors performed well in the determination of N variability in irrigated maize at the V12 and V14 growth stages and either sensor could be an important tool to aid precision N management.
Water capture and precipitation use efficiency are of great importance in dryland cropping systems because the world's dependence on food produced in dryland areas continues to increase. Growing ...season evapotranspiration potential greatly exceeds growing season precipitation rates in dryland areas, creating a water deficit for crops. Management practices that positively impact soil physical properties increase the potential for soils to capture water. One way to assess the ability of soils to capture water is through the measurement of sorptivity. Sorptivity is defined as the cumulative infiltration proportionality constant and is governed by surface soil physical properties such as texture, degree of aggregation and aggregate stability. A study was conducted to determine how crop residue accumulation after 12years of no-till management affects surface soil sorptivity under semi-arid dryland conditions and how sorptivity is related to surface soil physical properties known to be related to crop residue accumulation. Surface soil sorptivity, bulk density, porosity (total and effective) and aggregation measurements were made across cropping systems and soil positions representing a wide gradient of crop residue accumulation at 3 sites in eastern Colorado. Results show that increasing crop residue accumulation will have the indirect effect of increased sorptivity via improvements in soil aggregation, bulk density, and porosity that are conducive to water infiltration. Management practices that result in greater amounts of crop residue returned to the soil system lead to beneficial soil physical properties that increase water sorptivity, greatly reducing the potential for runoff and erosion, and thereby increase the precipitation use efficiency of the system.
► Water capture and precipitation use efficiency are important in dryland cropping. ► Sorptivity is defined as the cumulative water infiltration proportionality constant. ► We study the effects of crop residue accumulation on soil sorptivity. ► Increased crop residue accumulation indirectly increased sorptivity.
Precision agriculture research has been directed toward enhancing the efficiency of N inputs by quantifying in-field variability. Remotely sensed indices such as normalized difference vegetation ...index (NDVI) can determine in-field N variability in maize (Zea mays L.). One method of determining NDVI is through the use of ground-based active crop canopy sensors. Several crop canopy sensors determine NDVI, however, climatic and management variables may affect NDVI readings. Our objectives were to compare two ground-based active crop canopy sensors (Crop Circle amber and GreenSeeker red) across plant growth stage, wind, crop row spacing, sensor movement speed, and N fertilizer rate under greenhouse conditions. Results show that wind had no effect on the NDVI readings of either sensor. Nitrogen rate and growth stage did affect the NDVI of both sensors with NDVI values generally increasing with increased N rate and advancing growth stage. For both sensors the V8 NDVI r 2 with N rate were lower than those observed at V10 and V12. However, the GreenSeeker (red sensor) had much lower r 2 values at V8 than the Crop Circle (amber sensor). Sensor speed had an effect on red sensor NDVI values while the amber sensor was not affected by sensor movement speed. The amber and red sensors distinguish plant N status and growth stage differences in maize in a greenhouse environment. However, the red sensor had more variability in NDVI readings and was affected by movement speed. The amber sensor shows no such limitations and therefore performed best under greenhouse conditions.
Numerous techniques of management zone delineation have been studied; however, few comparisons between techniques exist in the literature. The objectives of this study were: (i) to determine how ...consistently two management zone delineation techniques (a soil-color-based management zone SCMZ technique and a yield-based management zone YBMZ technique) characterize regions of high, medium, and low grain yield; and (ii) to compare the relative accuracies with which the two management zone delineation techniques characterize the grain yield within low, medium, and high productivity potential management zones. This study was conducted for three site years in northeastern Colorado. Management zones were delineated before planting. The SCMZ technique used: (i) bare-soil imagery, (ii) topography, and (iii) farmer's experience. The YBMZ relied on: (i) bare-soil imagery, (ii) soil organic matter, (iii) cation exchange capacity, (iv) soil texture, and (v) the previous season's yield map. Grain yields ranged from 6.9 to 15.5 Mg ha(-1) across all site years. Grain yields were significantly different between SCMZ zones for all site years. Grain yield in the SCMZ high zones were up to 1.88 Mg ha(-1) higher than YBMZ high zones. Areal agreements for the SCMZ technique were 37, 41, and 45% for Site Years I, II, and III. Based on the approaches used in this study to classify grain yield patterns, the SCMZ technique was found to be relatively better than the YBMZ technique.
Previous studies of no-till management in the Great Plains have shown that increased cropping intensity increased soil organic carbon (SOC). The objectives of this study were to (i) determine which ...soil C pools (active, slow, and passive) were impacted by cropping intensity after 12 yr of no-till across potential evapotranspiration (PET) and slope position gradients; (ii) relate C pool sizes to the levels found in total SOC; and (iii) determine C pool sizes relative to C levels found in a grass treatment (G). Cropping systems were wheat (Triticum aestivum)-fallow (WF), wheat-corn (Zea mays L.)-fallow (WCF), wheat-corn-millet (Panicum miliaceum)-fallow (WCMF), and continuous cropping (CC) at three PET sites in Colorado. Active C (Soil microbial biomass C SMBC); and slow pool C (particulate organic matter C; POM-C) increased as cropping intensity increased, dependent on PET. Passive C (mineral associated organic C MAOC) was strongly influenced by a site-by-slope position interaction but not by cropping system. Toeslope soils had 35% higher POM-C compared with summits and sideslopes. All C pools were strongly correlated with total SOC, with the variability decreasing as C pool turnover time increased. Carbon pool sizes in cropping systems relative to levels found in G were independently influenced by cropping system. The highest were found in the CC system, which had 91, 78, and 90% of the amounts of C found in the perennial G system in the active, slow, and passive C pools, respectively.
Soil organic C (SOC) has decreased under cultivated wheat (Triticum aestivum)‐fallow (WF) in the central Great Plains. We evaluated the effect of no‐till systems of WF, wheat–corn (Zea Mays)‐fallow ...(WCF), wheat–corn–millet (Panicum miliaceum)‐fallow, continuous cropping (CC) without monoculture, and perennial grass (G) on SOC and total N (TN) levels after 12 yr at three eastern Colorado locations. Locations have long‐term precipitation averages of 420 mm but increase in potential evapotranspiration (PET) going from north to south. Within each PET location, cropping systems were imposed across a topographic sequence of summit, sideslope, and toeslope. Cropping intensity, slope position, and PET gradient (location) independently impacted SOC and TN to a 5‐cm soil depth. Continuous cropping had 35 and 17% more SOC and TN, respectively, than the WF system. Cropping intensity still impacted SOC and TN when summed to 10 cm with CC > than WF. Soil organic C and TN increased 20% in the CC system compared with WF in the 0‐ to 10‐cm depth. The greatest impact was found in the 0‐ to 2.5‐cm layer, and decreased with depth. Soil organic C and TN levels at the high PET site were 50% less than at the low and medium PET sites, and toeslope soils were 30% greater than summit and sideslopes. Annualized stover biomass explained 80% of the variation in SOC and TN in the 0‐ to 10‐cm soil profile. Cropping systems that eliminate summer fallowing are maximizing the amount of SOC and TN sequestered.
Water is the principle limiting factor in dryland cropping systems. Surface soil physical properties influence infiltration and cropping systems under no‐till management may affect these properties ...through residue addition. The objectives of this study were: (i) to determine how cropping intensity and topographic position affect soil bulk density, porosity, sorptivity, and aggregate stability in the surface 2.5 cm of soils at three eastern Colorado sites; and (ii) to relate these properties to crop residue returned to the soil surface. No‐till cropping systems had been in place on three slope positions, at three sites, for 12 yr prior to this study. Wheat (Triticum aestivum L.)‐corn (Zea mays L.)‐fallow (WCF) and continuous cropping (CC) systems were compared with wheat‐fallow (WF) on summit and toeslope positions at two sites (Sterling and Stratton), and at the third site (Walsh) wheat‐sorghum Sorghum bicolor (L.) Moench‐fallow (WSF) replaced WCF. Cropping systems (CC and WCF or WSF) that returned more crop residue decreased bulk density and increased total and effective porosities compared with WF. Site and slope positions that produced more crop residue also improved these properties. However, sorptivity developed no significant differences as a result of cropping system. Macroaggregates made up a higher percentage of total aggregates in CC and WCF or WSF compared with WF in proportion to residue added and were also a function of clay content of the soil at different sites and slope positions. These factors enhance the potential for greater infiltration and hence greater water availability for crops.
Development of improved fertilizer management practices has the potential to increase fertilizer use efficiency and improve environmental quality. The objectives of this study were (i) to ...characterize the within field spatial variability of N uptake across irrigated corn production fields, (ii) to quantify and compare N uptake and grain yield across three site specific management zones (SSMZs), and (iii) to compare grain yield response to applied N between management zones. This study was conducted on continuous corn (Zea mays L.) in irrigated fields in northeastern Colorado. Fields were classified into high, medium, and low site specific management zones. Treatments consisted of a control and two uniform N application rates over 2 site-years (one field over 2 consecutive yr and another field over 1 yr). Nitrogen fertilizer rates varied with site-year and ranged from 56 to 268 kg N ha(-1). Aboveground biomass samples were collected at physiological maturity and analyzed for total N. Between management zones, N uptake, grain yield, and grain yield response to applied N were found to be statistically different (p < 0.05). Management zones were found to be less spatially variable than the whole field. The SSMZs accurately characterized variability in N uptake as well as grain yield response to applied N. Thus, variation in N uptake and grain yield can potentially be managed using SSMZs.
Precision manure management is a relatively new concept that merges the best agronomic and manure management practices along with precision agricultural techniques, such as site‐specific management ...zones (MZs), for agricultural productivity and environmental quality. The objective of the study was to assess the influence and compare the economic efficiency of variable‐rate applications of animal manure on grain yield in maize (Zea mays L.) fields across MZs in limited irrigation cropping systems. The study was conducted on furrow‐irrigated maize fields in northeastern Colorado, USA. Fields were classified into low, medium, and high yielding MZs, based on soil color, elevation, and yield history. Experimental strips were 4.5 m wide and 540 m long spanning across all MZs with manure and N fertilizer management strategies nested within MZs. Variable‐rate manure applications of 22, 44, and 67 Mg ha–1 were considered for variable yield goal (VYG) and constant yield goal (CYG) manure management strategies. The results of this study indicates that maize grain yield was significantly different across MZs a majority of times, however, not always consistent with the MZ productivity level. For instance, the low MZ showed a significantly (P ≤ 0.05) higher grain yield under a CYG manure management strategy. The enterprise budget analysis indicated that application of animal manure alone was economically inefficient for maize grain production. The study suggests that variable‐rates of manure can be used in conjunction with synthetic N fertilizer to ensure that crop N requirements are met at early growth stages of maize.
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