Center-pivot Irrigated Fields Research Articles

Optimal Timing of Carrot Crop Monitoring and Yield Assessment Using Sentinel-2 Images: A Machine-Learning Approach

Remotely sensed images provide effective sources for monitoring crop growth and the early prediction of crop productivity. To monitor carrot crop growth and yield estimation, three 27 ha center-pivot irrigated fields were studied to develop yield prediction models using crop biophysical parameters and vegetation indices (VIs) extracted from Sentinel-2A (S2) multi-temporal satellite data. A machine learning (ML)-based image classification technique, the random forest (RF) algorithm, was used for carrot crop monitoring and yield analysis. The VIs (NDVI, RDVI, GNDVI, SIPI, and GLI), extracted from S2 satellite data for the crop ages of 30, 45, 60, 75, 90, 105, and 120 days after plantation (DAP), and the chlorophyll content, SPAD (Soil Plant Analysis Development) meter readings, were incorporated as predictors for the RF algorithm. The RMSE of the five RF scenarios studied ranged from 7.8 t ha−1 (R2 ≥ 0.82 with Scenario 5) to 26.2 t ha−1 (R2 ≤ 0.46 with Scenario 1). The optimal window for monitoring the carrot crop for yield prediction with the use of S2 images could be achieved between the 60 DAP and 75 DAP with an RMSE of 8.6 t ha−1 (i.e., 12.4%) and 11.4 t ha−1 (16.2%), respectively. The developed RF algorithm can be utilized in carrot crop yield monitoring and decision-making processes for the self-sustainability of carrot production.

Satellite-Based Determination of the Water Footprint of Carrots and Onions Grown in the Arid Climate of Saudi Arabia

Increasing demand for food, climate change, and other human interventions are leading to significant increases in water consumption by the agricultural sector. This requires rationalizing the water used for the production of agricultural crops through improved irrigation management practices. Therefore, this study aimed to estimate the water footprint (WF) of onion (Allium cepa L.) and carrot (Daucus carota) crops using the CROPWAT model and the SSEB (Simplified Surface Energy Balance) algorithm. Experiments were carried out at two center-pivot irrigated fields belonging to Tawdeehiya Commercial Farms in the southeastern region of the Riyadh governorate, Saudi Arabia. Individual bands and vegetation indices (VIs) were retrieved from Sentinel-2 satellite data, including the normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), optimized soil adjusted vegetation index (OSAVI), renormalized difference vegetation index (RDVI), and enhanced vegetation index (EVI), and the land surface temperatures (LST) extracted from Landsat-8 data were used to estimate crop productivity (CP), crop water use (CWU) (i.e., evapotranspiration—ETa), and crop WF. Crop growth/phenology stages and georeferenced biophysical parameters were recorded during the growth period, and crop yield samples were collected randomly from predetermined sampling locations. It was found that the NIR band was appropriate for predicting onion yield (R2 = 0.68; p > F = 0.02) and carrot yield (R2 = 0.77; p > F = 0.02). The results also showed the feasibility of using the RDVI and EVI to estimate the yields of onion and carrot crops, with bias values of 15% and –17%, respectively. The CWU has also been successfully estimated using the SSEB algorithm, with an overall accuracy of 89%. The SSEB-estimated CWU was relatively high compared to the applied amounts by 10.6% (onions) and 12.6% (carrots). Finally, the crop WF was successfully estimated at 312 m3 t−1 and 230 m3 t−1 for carrots and onions, respectively, with an overall accuracy of 71.11%. The outcomes of this study can serve as a reference for crop irrigation management practices in the study region and areas with similar environmental conditions.

Comparison of Satellite Driven Surface Energy Balance Models in Estimating Crop Evapotranspiration in Semi-Arid to Arid Inter-Mountain Region

The regional-scale estimation of crop evapotranspiration (ETc) over a heterogeneous surface is an important tool for the decision-makers in managing and allocating water resources. This is especially critical in the arid to semi-arid regions that require supplemental water due to insufficient precipitation, soil moisture, or groundwater. Over the years, various remote sensing-based surface energy balance (SEB) models have been developed to accurately estimate ETc over a regional scale. However, it is important to carry out the SEB model assessment for a particular geographical setting to ensure the suitability of a model. Thus, in this study, four commonly used and contrasting remote sensing models viz. METRIC (mapping evapotranspiration at high resolution with internalized calibration), SEBAL (surface energy balance algorithm for land), S-SEBI (simplified surface energy balance index), and SEBS (surface energy balance system) were compared and used to quantify and map the spatio-temporal variation of ETc in the semi-arid to arid inter-mountain region of Big Horn Basin, Wyoming (Landsat Path/Row: 37/29). Model estimates from 19 cloud-free Landsat 7 and 8 images were compared with the Bowen ratio energy balance system (BREBS) flux stationed in a center pivot irrigated field during 2017 (sugar beet), 2018 (dry bean), and 2019 (barley) growing seasons. The results indicated that all SEB models are effective in capturing the variation of ETc with R2 ranging in between 0.06 to 0.95 and RMSD between 0.07 to 0.15 mm h−1. Pooled data over three vegetative surfaces for three years under irrigated conditions revealed that METRIC (NSE = 0.9) performed better across all land cover types, followed by SEBS (NSE = 0.76), S-SEBI (NSE = 0.73), and SEBAL (NSE = 0.65). In general, all SEB models substantially overestimated ETc and underestimated sensible heat (H) fluxes under dry conditions when only crop residue was available at the surface. A mid-season density plot and absolute difference maps at image scale between the models showed that models involving METRIC, SEBAL, and S-SEBI are close in their estimates of daily crop evapotranspiration (ET24) with pixel-wise RMSD ranged from 0.54 to 0.76 mm d−1 and an average absolute difference across the study area ranged from 0.47 to 0.56 mm d−1. Likewise, all the SEB models underestimated the seasonal ETc, except SEBS.

Variable Rate Irrigation of Maize and Soybean in West-Central Nebraska Under Full and Deficit Irrigation

Variable rate irrigation (VRI) may improve center pivot irrigation management, including deficit irrigation. A remote-sensing-based evapotranspiration model was implemented with Landsat imagery to manage irrigations for a VRI equipped center pivot irrigated field located in West-Central Nebraska planted to maize in 2017 and soybean in 2018. In 2017, the study included VRI using the model, and uniform irrigation using neutron attenuation for full irrigation with no intended water stress (VRI-Full and Uniform-Full treatments, respectively). In 2018, two deficit irrigation treatments were added (VRI-Deficit and Uniform-Deficit, respectively) and the model was modified in an attempt to reduce water balance drift; model performance was promising, as it was executed unaided by measurements of soil water content throughout the season. VRI prescriptions did not correlate well with available water capacity (R2 < 0.4); however, they correlated better with modeled ET in 2018 (R2 = 0. 69, VRI-Full; R2 = 0.55, VRI-Deficit). No significant differences were observed in total intended gross irrigation depth in 2017 (VRI-Full = 351 mm, Uniform Full = 344). However, in 2018, VRI resulted in lower mean prescribed gross irrigation than the corresponding uniform treatments (VRI-Full = 265 mm, Uniform Full = 282 mm, VRI-Deficit = 234 mm, and Uniform Deficit = 267 mm). Notwithstanding the differences in prescribed irrigation (in 2018), VRI did not affect dry grain yield, with no statistically significant differences being found between any treatments in either year (F = 0.03, p = 0.87 in 2017; F = 0.00, p = 0.96 for VRI/Uniform and F = 0.01, p = 0.93 for Full/Deficit in 2018). Likewise, any reduction in irrigation application apparently did not result in detectable reductions in deep percolation potential or actual evapotranspiration. Additional research is needed to further vet the model as a deficit irrigation management tool. Suggested model improvements include a continuous function for water stress and an optimization routine in computing the basal crop coefficient.

Soil water content monitoring for irrigation management: A geostatistical analysis

With the increasing attention to site-specific or variable rate irrigation management, it is helpful to reconsider the quantity and placement of soil water monitoring locations in this context. Volumetric soil water content (θv) was monitored using a neutron probe (NP) at 72 locations in a center pivot irrigated field in eastern Nebraska. Variance reduction and temporal stability analyses were performed on θv from shallow (∼top 46cm) and full profile (∼122cm) readings for four monitoring cycles in the 2015 growing season and 2016 preseason. Eleven additional cycles were included for a subset of the data for the temporal stability analysis. The spatial correlation scale for θv was found to be less than the closest spacing of monitoring locations in the study (i.e. <37m). For this field site, approximately three neutron probe monitoring locations were required to determine mean soil water depletion (±2cm) for the field or for a management zone. Little economy would be gained in variance reduction for areal mean θv from using a stratified network for management areas of reasonable size in a center pivot irrigated field. Temporally stable monitoring locations were identified. However, relatively low-cost spatial predictor variables, including elevation, deviation from mean elevation, apparent electrical conductivity, and mean relative difference of interpolated cosmic ray neutron probe surveys, were not consistent predictors of NP mean relative difference. The small range of variability of θv within the study field is thought to be a contributing factor. It is possible that for fields with similar variability, or for site-specific irrigation where zones have been selected to reduce within-zone variance, that sensor quantity is more important than sensor placement in quantifying the areal mean θv for irrigation management.

Energy Conservation Using Variable-Frequency Drives for Center-Pivot Irrigation: Standard Systems

<abstract> Energy use efficiency and energy consumption rates by center-pivot irrigation systems have been of utmost importance to the scientific community. Numerous studies have addressed different approaches to achieve the greatest energy use efficiency, but each encountered problems due to lack of detailed field information. This study introduces an approach to reduce energy consumption for center-pivot irrigation by varying the pump power supply speed using variable-frequency drive (VFD) technology. By varying the frequency of the power supply, the minimum required pivot point pressure can be provided at any required flow rate and position in a field. The model was developed using high-resolution digital elevation model (DEM) datasets and geographic information system (GIS) to simulate the minimum pressure required by 100 standard equipped and randomly selected pivots located in ten Nebraska counties. The variable topography within an individual field played an important role in determining the required pressure and thus energy consumption. The resulting energy reduction using a variable-speed motor approach was calculated for each center-pivot irrigated field. On average, the pivots in Cedar County showed the largest potential for energy reduction (9.6%) and also contained the largest elevation difference (13.6 m) between the pivot point and the highest point traversed by a pivot tower. In contrast, the pivots in Hamilton County showed low potential for energy reduction and contained the smallest average elevation difference (3.5Â m). To determine the potential economic benefit from the variable-speed motor approach, annual monetary savings were calculated for each installation and county. The pivots in Custer County would gain the greatest annual returns ($457 average) due to relatively long hours of operation, whereas the pivots in Hamilton County would achieve negligible savings due to gentle topography and few hours of operation. The county-average payback period for the capital cost of a VFD was found to exceed the economic life of the VFD in all ten counties.

Impact of Soil Firmness and Tillage Depth on Irrigated Maize Silage Performance

Abstract. A field experiment was conducted to investigate the response of maize silage ( L.) to tillage depth under different soil firmness levels. The study was carried out on a 16 ha center-pivot irrigated field in a commercial farm located in the eastern region of Saudi Arabia. A soil firmness map was generated and used as a management map. This map was divided into three soil firmness zones based on soil cone index (low: 617 to 1270 kPa for a 0 to 15 cm depth in undisturbed soil, medium: 1271 to 1652 kPa and high: 1653 to 2306 kPa). Three tillage depth treatments (10, 20, and 25 cm) were imposed on each of the three soil firmness zones, using a tandem disc harrow. Maize growth parameters [plant population, plant height, and Normalized Difference Vegetation Index (NDVI)] and maize silage yield were used to evaluate the response of the maize crop to tillage depth. The results revealed that soil firmness and tillage depth at both early (25 days after sowing) and late (60 days after sowing) growth stages did not introduce significant effects on maize plant population. However, the plant height measured at 60 days after sowing showed a significant response to soil firmness. The lowest mean value of plant height (114.4 cm) was recorded at the high soil firmness level, while the greatest mean value (136.3 cm) was recorded under low soil firmness level. Also, significant differences in maize silage yield were recorded under different soil firmness levels and tillage depths. For maize silage production, a tillage depth of 10 cm was observed to be optimum for areas of low and medium soil firmness. For areas of high soil firmness, the optimum tillage depth was 20 cm. Keywords: Disc harrow, Kriging, Maize, Maps, Penetrometer, Tillage depths.

Northern Bobwhite Demographic and Population Response Following an Intensive Habitat Modification to an Agricultural Landscape

Northern Bobwhite (Colinus virginianus) populations have been declining throughout most of their endemic range due to numerous factors (e.g., increased urbanization, predators); however, changing land-use practices have proved most detrimental to bobwhites. In parts of the southeastern USA, small-scale farming has been replaced by large-scale center-pivot irrigated fields and this has exacerbated habitat loss. Despite these trends, bobwhite populations in the Southeast have remained stable or increased on many areas employing intensive habitat management regimes, substantiating the importance of appropriate habitat management for long-term bobwhite persistence. In effort to reverse one such decline, we intensively modified a center-pivot, agriculture dominated landscape to benefit bobwhites by creating new habitat and improving existing habitat. Techniques utilized to modify this landscape were: establishment of linear habitats (field borders and buffer strips); planting longleaf pines; and management of existing habitat via prescribed burning and timber management. During 1998-2001, we monitored bobwhite (n = 498) demographics and population response following annual habitat restoration and management using radio-telemetry and fall abundance estimation (i.e., covey call-counts). Average survival during over-winter (0.4698, SE = 0.0721), breeding (0.3561, SE = 0 .0667) and annual (0.1673, SE = 0.0411) time-periods were higher than those reported for other agriculture studies and similar to those of intensively managed, “plantation” habitats. Bobwhite coveys and broods used newly developed longleaf pine, linear habitats (e.g. field borders/hedgerows), and managed woodlands. Further, nest site selection was commonly associated with these novel habitat types. As a result of the positive demographic response to habitat modification, bobwhite abundance also improved during the study. Consequently, we surmised that modification of agricultural landscapes may improve habitat quality and quantity for bobwhites and subsequently help to increase demographic rates and bobwhite abundance.

Land’s complex role in climate change

To mitigate climate change at local, regional, and global scales, we must begin to think beyond greenhouse gases.

Pumpage Reduction by Using Variable-Rate Irrigation to Mine Undepleted Soil Water

Conventional irrigation schedules are typically based on portions of the field where root zones hold the least available soil water. This leaves undepleted available water in areas with larger water holding capacities. The undepleted water could be used through variable-rate irrigation (VRI) management; however, the benefits of VRI without in-field mapping are unexamined. In this research, the field-averaged amount of undepleted available soil water in the root zone was calculated from the NRCS Soil Survey Geographic database for 49,224 center-pivot irrigated fields in Nebraska. Potential reductions in pumpage from mining undepleted available water were then estimated. Results of the analysis show that widespread adoption of zone control VRI technology based only on the pumping savings from mining undepleted available water may be unwarranted for current VRI costs and average pumping energy expenses in the Central Plains ($0.0026 m -3 to $0.0947 m -3 ). Pumpage reductions exceeded 51 mm year -1 for only 2% of the fields and exceeded 25 mm year -1 for 13% of the fields; thus, reductions may be small compared to annual pumpage requirements. If VRI were implemented on all fields with a potential pumpage reduction greater than 51 or 25 mm year -1 , the volume of pumpage reduction would be approximately 0.35% or 1.3%, respectively, of the total irrigation pumpage in Nebraska. These data may be a conservative estimate of pumpage reduction in fields where the measured variability in soil properties exceeds that described by the NRCS Soil Survey, or if undepleted water is mined early in the season and the soil water profile is refilled by precipitation, allowing undepleted water to be mined again. Adoption of zone control VRI for mining undepleted available water is most feasible for fields where the pumpage reduction from VRI is large and pumping costs are above normal. Pivot fields with high undepleted water were sparsely distributed across Nebraska and were often located along streams and or in associated alluvial areas. The prevalence of fields with large quantities of undepleted water differed among and within soil associations. We were unable to assign feasibility of VRI based on the soil association, as the occurrence of undepleted water varied significantly within a soil association. These findings should assist producers and other entities interested in VRI technology; however, pumpage reduction through use of undepleted soil water is only one benefit of VRI technology and management. Producers are encouraged to consider all potential benefits when analyzing VRI investments.

Prediction of Potato Crop Yield Using Precision Agriculture Techniques.

Crop growth and yield monitoring over agricultural fields is an essential procedure for food security and agricultural economic return prediction. The advances in remote sensing have enhanced the process of monitoring the development of agricultural crops and estimating their yields. Therefore, remote sensing and GIS techniques were employed, in this study, to predict potato tuber crop yield on three 30 ha center pivot irrigated fields in an agricultural scheme located in the Eastern Region of Saudi Arabia. Landsat-8 and Sentinel-2 satellite images were acquired during the potato growth stages and two vegetation indices (the normalized difference vegetation index (NDVI) and the soil adjusted vegetation index (SAVI)) were generated from the images. Vegetation index maps were developed and classified into zones based on vegetation health statements, where the stratified random sampling points were accordingly initiated. Potato yield samples were collected 2–3 days prior to the harvest time and were correlated to the adjacent NDVI and SAVI, where yield prediction algorithms were developed and used to generate prediction yield maps. Results of the study revealed that the difference between predicted yield values and actual ones (prediction error) ranged between 7.9 and 13.5% for Landsat-8 images and between 3.8 and 10.2% for Sentinel-2 images. The relationship between actual and predicted yield values produced R2 values ranging between 0.39 and 0.65 for Landsat-8 images and between 0.47 and 0.65 for Sentinel-2 images. Results of this study revealed a considerable variation in field productivity across the three fields, where high-yield areas produced an average yield of above 40 t ha-1; while, the low-yield areas produced, on the average, less than 21 t ha-1. Identifying such great variation in field productivity will assist farmers and decision makers in managing their practices.

Large-Scale On-Farm Implementation of Soil Moisture-Based Irrigation Management Strategies for Increasing Maize Water Productivity

Irrigated maize is produced on about 3.5 Mha in the U.S. Great Plains and western Corn Belt. Most irrigation water comes from groundwater. Persistent drought and increased competition for water resources threaten long-term viability of groundwater resources, which motivated our research to develop strategies to increase water productivity without noticeable reduction in maize yield. Results from previous research at the University of Nebraska-Lincoln (UNL) experiment stations in 2005 and 2006 found that it was possible to substantially reduce irrigation amounts and increase irrigation water use efficiency (IWUE) and crop water use efficiency (CWUE) (or crop water productivity) with little or no reduction in yield using an irrigation regime that applies less water during growth stages that are less sensitive to water stress. Our hypothesis was that a soil moisture-based irrigation management approach in research fields would give similar results in large production-scale, center-pivot irrigated fields in Nebraska. To test this hypothesis, IWUE, CWUE, and grain yields were compared in extensive on-farm research located at eight locations over two years (16 site-years), representing more than 600 ha of irrigated maize area. In each site-year, two contiguous center-pivot irrigated maize fields with similar topography, soil properties, and crop management practices received different irrigation regimes: one was managed by UNL researchers, and the other was managed by the farmer at each site. Irrigation management in farmer-managed fields relied on the farmers’ traditional visual observations and personal expertise, whereas irrigation timing in the UNL-managed fields was based on pre-determined soil water depletion thresholds measured using soil moisture sensors, as well as crop phenology predicted by a crop simulation model using a combination of real-time (in-season) and historical weather data. The soil moisture-based irrigation regime resulted in greater soil water depletion, which decreased irrigation requirements and enabled more timely irrigation management in the UNL-managed fields in both years (34% and 32% less irrigation application compared with farmer-managed fields in 2007 and 2008, respectively). The average actual crop evapotranspiration (ETC) for the UNL- and farmer-managed fields for all sites in 2007 was 487 and 504 mm, respectively. In 2008, the average UNL and average farmer-managed field had seasonal ETC of 511 and 548 mm, respectively. Thus, when the average of all sites is considered, the UNL-managed fields had 3% and 7% less ETC than the farmer-managed fields in 2007 and 2008, respectively, although the percentage was much higher for some of the farmer-managed fields. In both years, differences in grain yield between the UNL and farmer-managed fields were not statistically significant (p = 0.75). On-farm implementation of irrigation management strategies resulted in a 38% and 30% increase in IWUE in the UNL-managed fields in 2007 and 2008, respectively. On average, the CWUE value for the UNL-managed fields was 4% higher than those in the farmer-managed fields in both years. Reduction in irrigation water withdrawal in UNL-managed fields resulted in $32.00 to $74.10 ha-1 in 2007 and $44.46 to $66.50 ha-1 in 2008 in energy saving and additional net return to the farm income. The results from this study can have significant positive implications in future irrigation management of irrigated maize systems in regions with similar soil and crop management practices.

Political Economy of Rural Schools in the Heartland

The sense of serenity and beauty of a drive through the Heartland often hides the desperation that many communities are experiencing as they struggle to survive. Among the winter wheat fields of western Kansas and Nebraska spaced between the center-pivot irrigated fields of corn and alfalfa, towns and cities struggle to maintain their vitality and quality of life. These communities are slowly giving up the civic institutions that for so many generations provided a sense of worth and pride. Children are bused or drive up to an hour to get to schools and in some of the more remote areas, families take up apartments and second homes “in town” where “mom and the kids live” during the week so they can mitigate the daily commutes. New immigrants fill the void left by the out-migration of young adults and families. They assume jobs that either pay poorly or are considered difficult and monotonous. In many communities, in particular some of the most vibrant communities, English is no longer the dominant language. With over 60% of the school-age students living in homes where English is not the first language, towns in Kansas like Garden City, Dodge City, and Liberal are thriving with the growth in feedlots and meatpacking plants. The social and cultural fabric in many of these communities in the western half of the Heartland is rapidly changing.

First Report of Soilborne wheat mosaic virus on Wheat (Triticum aestivum) in the Columbia Basin of Oregon.

The Columbia Basin of Oregon consists of six counties (Gilliam, Hood River, Morrow, Sherman, Wasco, and Umatilla) and is the major wheat-producing region in the state. In 2005, these counties produced 300,277 ha of mostly fall-planted wheat. While primarily a dryland production area, wheat (approximately 8,094 ha) is also grown as a rotational crop under irrigation. Stunted and chlorotic winter wheat plants with leaves exhibiting a mosaic pattern consistent with that caused by Soilborne wheat mosaic virus (SBWMV) were observed in March 2005. These plants originated from four center-pivot irrigated fields in Umatilla County with each field approximately 50.6 ha. One-half of one field was planted with cv. Western Breeders 470 (WB470) and the other half with cv. Tubbs, while the three other fields were planted to Tubbs. In the split-planted field, symptoms were widespread in the WB470 half but only observed in low-lying areas planted with Tubbs. ELISA with a monoclonal antibody (Agdia Inc., Elkhart, IN) confirmed the presence of SBWMV, which is transmitted by the soilborne organism Polymyxa graminis. Electron microscopy confirmed rigid, rod-shaped particles that were 19 nm wide and of two size classes, 138 to 222 and 416 to 471 nm long. Presence of SBWMV was further verified by reverse transcription (RT)-PCR using SBWMV RNA-2 specific primers (sense 5'-AAAGAGTCTIGCGTRTARCAYTC-3' and antisense 5'-AACGGTGTTAGTAARYTRGGKGA-3'), which amplified the predicted 338-bp product from the coat protein gene (1). Additional positive samples were found in 2006 from fall-planted wheat cvs. WB 528 and MJ9 from two additional 50.6-ha fields in Umatilla County. In 2005, yield of WB470 in the split-planted field was reduced by approximately 15% compared with yields obtained in similar fields planted with WB470 not exhibiting symptoms. SBWMV has been reported previously in Oregon (2) but nearly 322 km to the west in an area that is not the major wheat-producing region in Oregon. Because of the apparent reduced susceptibility of Tubbs, which is an older cultivar, as compared with WB470, WB528, and MJ9, which are three new high-yielding cultivars, additional research is needed to identify the reaction to SBWMV of cultivars adapted for production in the Pacific Northwest, particularly if this disease becomes widely distributed in both irrigated and dryland production areas.

Sunflowers as a trap crop for reducing soybean losses to the stalk borer Dectes texanus (Coleoptera: Cerambycidae)

Larvae of Dectes texanus LeConte cause significant losses to soybean production in the American High Plains by girdling the stalks of mature plants at their base, causing them to lodge. The authors demonstrated that cultivated sunflowers can reduce rates of D. texanus infestation in adjacent soybean fields because adult females prefer sunflower over soybean for feeding and oviposition. Since females do not avoid ovipositing in plants already containing their own eggs or those of conspecific females, sunflower plants can accumulate multiple eggs, and subsequent larval combat typically results in the survival of only one. In west central Kansas, planting one half of a center pivot irrigated field to sunflower in 2004 significantly reduced infestation of soybean plants in the other half of the field within 200 m of the crop border. Beyond 200 m from the sunflowers, the rate of soybean infestation increased significantly. Planting sunflowers in the non-irrigated corners of a center pivot irrigated soybean field in 2005 reduced D. texanus infestation of soybeans by 65% compared with a control field without adjacent sunflowers. Surrounding a 0.33 ha soybean field with six rows of sunflowers in 2006 reduced soybean infestation to < 5% of plants, compared with 96% of sunflower plants. These results reveal that sunflowers can reduce D. texanus infestation in adjacent soybeans by acting as an 'ovipositional sink'. Further research is warranted to determine the optimum crop area proportions and spatial configurations that will maximize the efficacy of a sunflower trap crop to reduce soybean losses due to D. texanus-induced lodging.

MAPPING PHYMATOTRICHUM ROOT ROT OF COTTON USING AIRBORNE THREE-BAND DIGITAL IMAGERY

Phymatotrichum root rot, caused by the fungus Phymatotrichum omnivorum, is a serious and destructive diseasethat significantly reduces cotton yield and lowers lint quality. Cultural practices are commonly recommended for the controlof cotton root rot, and fungicides and fumigants that may suppress the disease have also been used. Because of the high costsof these chemicals, their use may be economically feasible only when the infested portions of the field are treated. Theobjective of this study was to evaluate airborne multispectral imagery for detecting and mapping root rot areas in cotton fieldsfor site-specific management of the disease. One center-pivot irrigated field and one rainfed field near Corpus Christi, Texas,were selected for this study. Airborne three-band digital imagery was taken from the two fields shortly before harvest in 2001when the infested areas with wilted and dead plants were almost fully pronounced for the season. The imagery wasgeoreferenced and then classified into healthy and root rot areas using unsupervised classification. Accuracy assessment onthe classification maps for the two fields indicated that airborne imagery effectively and accurately identified root rot areaswithin the fields. Ground samples taken from the fields showed that cotton yield and some lint quality indices were significantlylower in root rot areas than in healthy areas. Buffer zones around the root rot areas were generated to account for the spreadof cotton root rot on the classification maps. The mapping procedures and maps presented in this study will be useful forsite-specific management of the disease.

CHARACTERIZATION OF APPARENT SOIL ELECTRICAL CONDUCTIVITY VARIABILITY IN IRRIGATED SANDY AND NON-SALINE FIELDS IN COLORADO

Recent advances in apparent soil electrical conductivity (ECa) sensor technology have provided the opportunityto rapidly map soil spatial variability for site-specific management. However, characterizing (or identifying the causes of)the ECa variability has remained difficult, with ECa and soil property (such as soil water and clay content) relationshipsshowing a wide range of varying strength across fields. In this study, our objective was to characterize the main soil propertiesthat alter ECa and evaluate the temporal variability of ECa and soil property relationships using multi-year measurements(1998 to 2003) in three center-pivot irrigated fields in eastern Colorado. Results show that for the irrigated sandy andnon-saline fields, ECa maps may be viewed as surrogate maps for soil water content (.w), clay, cation exchange capacity,and/or organic matter content due to their strong correlations (with correlation coefficients between 0.66 and 0.96). Theexperimental linear functions of ECa versus soil properties changed over time when soil solution concentration (ECw)changed considerably. That finding was supported by theory (the dual-pathway ECa model) showing the relationship betweenECa and soil stable properties (such as clay content) to be governed by the status of the soil transient properties of ECw and.w at the time of the ECa mapping. The temporal effect of varying soil temperature on ECa could be significant but was notaccounted for due to lack of temperature data. Results collectively suggest that because of the lack of reliability of usingempirical ECa and soil property relationships for predictive purposes over time, on-site calibration of ECa versus soilproperties of interest are needed at each ECa mapping.

Maize yield potential: critical processes and simulation modeling in a high-yielding environment

Understanding processes of maize ( Zea mays L.) growth and production of grain in high-yielding, irrigated conditions offers hope to understand yield potential in many other environments. In this study we investigated such processes at the plant level, and attempted to simulate maize yields at the field level and county level in the high yielding region of the High Plains of Texas. In addition, we used the normalized difference vegetation index (NDVI) from satellite data of year 2000 to update leaf area index for yield simulation in three counties. In the field study, we measured maize leaf area index (LAI), the fraction of photosynthetically active radiation intercepted (FIPAR), and the harvest index (HI) in irrigated plots near Dumas, Texas. The light extinction coefficient ( k) for Beer's law was calculated with the FIPAR and the LAI. The radiation use efficiency (RUE) was determined with sequential measurements of the fraction of photosynthetically active radiation (PAR) intercepted and biomass harvests. The RUE was 3.98 g of above-ground biomass per MJ of intercepted PAR in 1999 and 3.41 in 2000 for three sampling dates prior to silking. These values are 106 and 93% of the expected RUE values at the measured vapor pressure deficits, using a previously published response function. The mean k value was −0.46 in 1999 and −0.47 in 2000, similar to the expected value of −0.43 reported in the literature for this row spacing. The mean HI measured in 2000 was 0.52, similar to values of 0.53 and 0.54 in the literature. Application of these parameters to maize simulation with the Agriculture Land Management Alternatives with Numerical Assessment Criteria (ALMANAC) model for 13 center pivot irrigated fields near Dumas in 1999 provided simulations within 1.0 Mg ha −1 with a mean error of 0.03 Mg ha −1 and a mean square error of 0.10. For five years of grain yields reported for each of four counties in this region of Texas, ALMANAC simulations were within 5% of the mean measured yields. Introduction of PAR interception, based on the satellite-derived normalized difference vegetation index (NDVI), into ALMANAC resulted in slight increases in accuracy of yield prediction for two counties and a slight decrease in accuracy in one county for year 2000. Consistency in values of RUE, k, and HI in this study as compared with values reported in the literature will aid modelers simulating maize growth and grain yields in similar high-yielding, irrigated conditions.

Effect of Chemical Defoliation, Irrigation Water, and Distance From the Pivot on Late Blight Tuber Rot in Center-Pivot Irrigated Potatoes in the Columbia Basin.

Chemical desiccation of potato foliage 2 to 3 weeks prior to harvesting tubers is commonly recommended to reduce late blight tuber rot. Observations in commercial fields indicate that this practice may not be needed to manage tuber rot in the semiarid environment of the Columbia Basin. Potatoes in this region are mostly grown under center pivot irrigation, and a relatively high volume of irrigation water is often delivered near the pivot in center-pivot irrigated fields. This study evaluated the effect of chemical desiccation of potato foliage on late blight tuber rot and quantified the incidence of late blight tuber rot in relation to distance from the pivot and the amount of irrigation water applied in center-pivot irrigated fields. Potato foliage was chemically desiccated or not desiccated within 41 to 76 m of the pivot of center-pivot irrigated potato fields 2 to 3 weeks before harvest in replicated plots in five field trials during 3 years. Incidence of late blight tuber rot did not differ significantly between the defoliated and nondefoliated plots in all five trials. Incidence of tuber rot significantly increased as amount of irrigation water applied increased, and was significantly greater within 30 m of the center pivot than at greater distances in two trials in 1998 and two trials in 1999. Tuber rot did not develop in a fifth trial in 2000 where a water gradient did not occur among the plots. The largest percentage of tubers infected with late blight was observed within 30 days of harvest at a storage temperature of 9°C. Chemical desiccation of potato foliage in the semiarid environment of the Columbia Basin was not beneficial in reducing late blight tuber rot, but growing potatoes within 30 m of the pivot will likely increase the number of infected tubers going into storage.

Approaches to Management Zone Definition for Use of Nitrification Inhibitors

Site-specific use of nitrification inhibitors has been proposed as one means of reducing the cost of nitrification inhibitor use on fields that vary in their potential for leaching or denitrification. This study evaluated one approach for site-specific use of the nitrification inhibitor nitrapyrin (2-chloro-6-[trichloromethyl] pyridine) based on slope and surface texture. Treatments of uniform and variable N fertilizer and nitrapyrin were applied to field length strips in a center-pivot irrigated field planted to maize (Zea mays L.) from 1995 to 1998. Nitrapyrin application was controlled spatially according to an arbitrary leaching potential derived from slope and surface soil texture. Growing seasons were grouped into dry, wet, or average precipitation years. Alternative approaches for nitrapyrin management zones based on fuzzy cluster analysis of grain yield or soil apparent electrical conductivity (EC,) were compared with the slope-texture management zone approach. There were small effects of nitrapyrin application method (none, uniform or variable) on grain yield. Uniform nitrapyrin application increased yield in a wet year. Fuzzy cluster analysis of grain yield or soil EC a identified unique patterns of yield or soil EC a . Yield clusters corresponded to patterns of water availability or excess. There were small effects of N or nitrapyrin application method on soil residual NO 3 -N. Yield Cluster 3 soils had higher residual NO 3 -N levels all 4 yr. In a wet year (1996) NO 3 -N levels were lowest in yield Cluster 2 soils-in the other 3 yr of the study, there was no difference in NO,-N levels between yield Clusters 1 and 2. Soil EC a clusters corresponded closely to soil series. There were significant correlations between soil EC a and yield clusters. Fuzzy cluster analysis has the potential to define management zones for use of nitrification inhibitors from relatively easily obtained spatial yield or soil EC a , rather than expensive grid sampling of soil chemical and physical properties.