Abstract: Research was conducted in 1997 and 1998 to evaluate narrow row spacing and glufosinate in glufosinate-resistant corn. Glufosinate-resistant corn was planted in 51- and 102-cm row spacings at the same plant populations. Herbicide treatments included glufosinate alone and in different herbicide combinations. Atrazine plus glufosinate enhanced Palmer amaranth control compared to glufosinate alone. Control of johnsongrass, ivyleaf morningglory, entireleaf morningglory, Texas panicum, smellmellon, browntop panicum, and toothed spurge with glufosinate was greater than 82%. Common sunflower control with glufosinate was greater than 95%. Atrazine followed by glufosinate applications provided at least 94% control of all species and was the most consistent herbicide system used. Row spacing had little effect on weed control. Crop injury to glufosinate-resistant corn was minimal with glufosinate and atrazine plus glufosinate combinations. Nomenclature: Atrazine; glufosinate; browntop panicum Panicum fasciculatum Sw. #3 PANFA; entireleaf morningglory Ipomoea hederacea var. intergriuscula Gray # IPOHG; ivyleaf morningglory Ipomoea hederacea (L.) Jacq. # IPOHE; johnsongrass Sorghum halepense (L.) Pers. # SORHA; Palmer amaranth Amaranthus palmeri S.Wats. # AMAPA; smellmellon Cucumis melo L. var. dudaim Naud. # CUMMD; common sunflower Heliathus annuus L. # HELAN; Texas panicum Panicum texanum Buckl. # PANTE; toothed spurge Euphorbia dentata Michx. # EPHDE; corn (Zea mays L.). Additional index words: Narrow-row corn, transgenic corn, atrazine, pendimethalin, primisulfuron, CGA 152005 (proposed name, prosulfuron). Abbreviations: DAT, days after POST treatment; fb, followed by; LPOST, late postemergence; POST, postemergence; PRE, preemergence.

New N management and conservation tillage systems are needed to improve agricultural sustainability on the Blackland Prairie of Texas. In 1994, an experiment was established to determine plant response to N fertilizer rate and timing within three different tillage systems. A split plot experiment with four replications was established on a Houston Black clay (fine, smectitic, thermic Udic Haplusterts) soil. The main plots were chisel tillage system without beds (conventional for the area), chisel tillage system with raised wide beds, and no‐tillage system with raised wide beds. The subplots were seven fertility treatments: four fertility rates (0, 56, 112, and 168 kg N ha−1 applied at planting) and three timing treatments (N applied in the fall, at planting, and split between at planting and 30 d later). The crop rotation was wheat (Triticum aestivum L.), corn (Zea mays L.), and sorghum [Sorghum bicolor (L.) Moench]. The experimental treatments were imposed on corn each year for 4 yr. Plant samples were collected for grain yield, biomass production, and N uptake. Grain yield ranged from 150 to 8435 kg ha−1. In wet years, grain yields and N uptake increased with N fertilizer up to 168 kg N ha−1, and fall application reduced yields 30% when compared with fertilizer application at planting. The highest yields were observed with the no‐tillage. Results from this study indicate that application of fertilizer in the fall may result in lost yield potential and that conservation tillage systems may be the most reliable in the Texas Blackland Prairie.

The objective of this study was to examine the impact of tillage intensity and crop residue on carbon (C) and nitrogen (N) cycling in a Vertisol. Soil samples (0-10- and 10-20-cm depth) were collected from a Houston Black soil (fine, montmorillonitic, thermic Udic Pellusterts) with three different levels of tillage intensity, varying from no tillage to complete residue burial. The experiment was a split plot design with five replications. The main plots were three crop species [corn (Zea mays L.), grain sorghum (Sorghum bicolor [L.] Moench), and soybean (Glycine max [L.] Merr.)] and the subplots were three levels of tillage intensity (chisel tillage, reduced tillage, and no tillage). Total N, organic C, inorganic N, and C:N ratio were measured on soil samples as well as the potential C mineralization, N mineralization, C turnover, and C:N mineralization ratio during a 7- and 30-d incubation. Chisel tillage reduced total N, C mineralization, and C turnover at the 0-10-cm depth compared to the other conservation tillage systems. Following corn production, soil organic C increased and C mineralization and C turnover were decreased at the 10-20-cm depth compared to the other crop species. This data indicates that, in the short term, tillage systems may control soil organic C at the soil surface, while changes in plant rooting may control soil organic C storage at deeper soil depths in Texas Vertisols.

Comprehensive models for agrichemical transport necessarily include runoff predictions to partition rainfall between infiltration and runoff, as this ability is fundamental to predictions of chemical runoff and leaching. We compared GLEAMS, Opus, and PRZM-2 model runoff predictions with runoff measured in a precisely controlled field site used for chemical runoff studies. In 1992 and 1993, two 14.5 m iA42.9 m corn (Zea mays, L.) field plots with 3% slope on Tifton loamy sand (fine-loamy, siliceous, thermic Plinthic Kandiudult) received six severe, artificial rainfall events over the growing season with each event consisting of a 25 mm h¨C1 rainfall for 2 h. Runoff was monitored continuously using a collector and flume. Model performance criteria included sensitivity analysis, graphical comparison and statistical analysis including mean, ratio of means, root mean square error (RMSE), and a paired difference t-test. Observed runoff averaged 20% of added rainfall. Lowest values occurred with freshly plowed soil or full canopy cover, while 24 to 34% runoff occurred when nearly bare soils had crusted over. Using an initial moisture condition-II curve number (CN) of 85, GLEAMS and Opus predicted runoff within 10%, overall, and produced a pattern of high and low runoff that closely followed observed. PRZM-2 overpredicted runoff by 90%, overall, and predicted its highest runoff when observed runoff was lowest. Paired difference t-tests indicated a significant difference between measured and predicted runoff for PRZM-2 (p<0.001 at ¦A= 0.05), but none for GLEAMS (p = 0.761) or Opus (p = 0.194). Mean, ratio of means, and RMSE showed that GLEAMS and Opus performed better than PRZM-2. All three models were very sensitive to CN values which were empirical and subjective, but less sensitive to measurable soil physical properties. With careful parameterization, GLEAMS and Opus could be used to simulate runoff from similar row-crop and soil conditions.

Because of erosion problems, an effort has been undertaken to evaluate the effect of tillage intensity on carbon (C) and nitrogen (N) cycling on a vertisol. Soil samples at 0-10, 10-20, and 20-30 cm depth were collected from a split plot experiment with five different levels of tillage intensity on Houston Black soil (fine, montmorillonitic, thermic Udic Pellusterts). The experiment was a split plot design with 5 replications. The main plots were chisel tillage, reduced tillage, row tillage, strip tillage, and no tillage. The subplots were soil fertility levels with either high or low fertilizer application rate. Total N, total phosphorus (P), organic C, inorganic N, and C:N ratio were measured on soil samples as well as the potential C mineralization, N mineralization, C turnover, and C:N mineralization ratio during a 30 d incubation. Total P and organic C in soil were increased, with 0.9 and 0.8 kg P ha(-1) and 20.6 and 20.0 kg C ha(-1), for high and low soil fertility, respectively. Fertilizer application had no effect on either total N at the 0-10 cm depth, or on soil nutrient status below 10 cm. Potential soil N mineralization was decreased at the 0-10 cm depth and increased at the 20-30 cm depth by the high fertilizer treatment. Chisel tillage decreased total N and P in the 0-10 cm depth, with 1.4 and 1.6 kg N ha(-1) and 0.8 and 0.9 kg P ha(-1). However, chisel tillage increased total N and p at the 10-20 cm depth, with 1.3 and 1.2 kg N ha(-1), and 0.72 and 0.66 kg P ha(-1) for chisel tillage and no tillage, respectively. Tillage intensity increased C mineralization and C turnover, but reduced N mineralization at the 0-10 cm depth. The results indicate that intensively tilled soil had a greater capacity for C mineralization and for reductions in soil organic C levels compared to less intensively tilled systems.

Red imported fire ants, Solenopsis invicta Buren, have been documented as damaging to planted field crop seed. The seeds are damaged before completion of germination. Five types of field crop seeds [wheat, Triticum aestivum L.; corn, Zea mays L.; grain sorghum, Sorghum bicolor (L.) Moench; cotton, Gossypium hirsutum L.; and soybean, Glycine max L.] were exposed to red imported fire ants under laboratory conditions to measure damage caused by feeding. The seeds were at an air-dry moisture condition, as typically sown farmers. Corn, sorghum, and cotton seeds were tested with and without commercial insecticide treatments used on bagged retail-market seeds. Wheat was tested only as untreated seeds and soybean were tested as both inoculated and noninoculated seeds. Our results show that red imported fire ants feed on dry wheat, corn, and sorghum seeds and to a lesser degree on cotton and soybean seeds. Seed treated with an insecticide sustained less damage; however, if dry Soil conditions allowed enough time for damage to exceeded 20-30%, as observed for sorghum, then there would still be a substantial risk to seed germination. Untreated wheat seed was the most heavily damaged and yet farmers in the red imported fire ant-infested region of the United States routinely plant untreated wheat seed. Red imported fire ant damage to the seeds of wheat, corn, sorghum, cotton, and soybean may vary from insignificant to severe, depending on exposure time to red imported fire ants, seed type, and whether or not a seed treatment was used.

Video image analysis, drop pins, and dot-screen methods were used to measure wheat residue cover in 1-m(2) rainfall simulator boxes under undisturbed field conditions. The data set consisted of 53 sites on a set of field plots which represented residue cover on chisel-till and no-till systems two months after wheat harvest. If the mean from the three methods is taken as the true cover, then there was a trend to produce 5% cover above average cover values with the pin method and 1% cover and 4% cover below average values with the dot and video methods, respectively. For no-till conditions of cover in the 80 to 100% cover range, the differences in values achieved with the three methods may be of little consequence, because the soil is adequately protected. For the low-residue cover chisel-till conditions, the differences were as much as 50% of the mean cover and could produce misleading information on the effect of residue cover on runoff, erosion, water quality, and other products of rainfall simulator studies.

Herbicide concentration and mass load of runoff depends, to a large extent, on soil management. This study was conducted to determine how tillage impacts herbicide losses in runoff from a vertisol soil on the Blackland Prairie of Texas. Atrazine [6-chloro-N-ethyl-N'(1-methylethyl)-1,3,5-triazine-2,4-diamine was applied at a rate of 2 kg a.i. ha(-1) to a Houston Black clay soil (fine, montmorillonitic, thermic Udic Pellustert) in 1993 at the Blackland Research Center in Temple, TX. For 4 yr, the test area was under continuous management using a wide-hed system with a wheat (Triticum aestivum L.), corn (Zea mays L.), and grain sorghum [Sorghum bicolor (L.)) Moench] rotation. Tillage treatments consisted of no-tillage or chisel-tillage. All experiments were repeated four times. A rainfall simulator with an intensity of 12.5 cm h(-1) was used to apply rainfall 24 h after the atrazine application. Sediment and runoff samples were collected during five time periods (from runoff initiation to 5, 5 to 10, 10 to 20, 20 to 30, and 30 to 40 min). No differences in atrazine concentrations vc ere found among treatments in either the runoff water or sediment from any of the five time periods; however, crop residues prevented surface seal development and erosion resulting in reduced runoff and sediment losses. No-tillage treatments significantly reduced runoff and sediment yield, rather than the atrazine concentration of the runoff, resulting in a 42% decrease of the atrazine load in the runoff and a 77% decrease in atrazine associated with the sediment. As a percentage of the total amount applied, runoff accounted for <2% of the atrazine. Sediment-transported atrazine was much less important and represented <0.03% of the total amount applied.

Sustainable production systems and conservation tillage practices are needed to control water erosion on vertisols. Five levels of tillage intensity were tested for 3 yr for effects on growth and yield of corn (Zea mays L.) and grain sorghum [Sorghum bicolor (L.) Moench.] on a Houston Black clay soil (fine montmorillonitic, thermic Udic Pellusterts). Tillage intensity treatments included: chisel plow with secondary tillage; disk only; no-till with residue rakes at planting; no-till with midseason cultivation; and no-till. Corn plant populations were greater in tilled treatments than in no-till treatments in 2 of the 3 yr. Corn above-ground biomass production was generally reduced in no-till treatments early in the growing season, but by silking differences among treatments were not significantly different. Corn yielded 840 lb/acre more on average with tillage than with no-till. Plant population differences accounted for much of the difference in corn grain yields, with low plant populations restricting yield in some years. Grain sorghum populations were not consistently affected by tillage intensity, and biomass production was less sensitive to tillage intensity than corn. Grain sorghum yields were as large or larger in no-till treatments than in tilled treatments, except in one instance where population was reduced.

Raised wide beds have been proposed as a conservation tillage practice for reducing erosion losses in vertisols, but few measurements of nitrogen (N) and phosphorus (P) losses have been reported. The objective of this study was to examine the impact of tillage systems and fertilizer N application methods on sediment and nutrient losses associated with interrill runoff. Simulated rainfall events (125 mm h(-1) until 30 min of runoff had occurred) were applied to raised wide beds (0.15 m high and 1.5 m wide with 0.5-m-wide furrows) on a Houston Black clay (fine, montmorillonitic, thermic Udic Pellusterts) which had been managed with either a no-till or a chisel-till tillage system. Three simulated methods of applying fertilizer N (surface band, coulter-nozzle, or spoke wheel simulated field practices) were compared in a split plot experimental design with four replications. Total N and P losses, as well as fertilizer N losses, in both sediment and solution from interrill runoff were determined from 1 m(2) plots. While no P was applied in fertilizer, greater P losses were observed with the chisel-till compared to no-till. While N losses in runoff were relatively low, fertilizer N application with surface banding or the coulter-nozzle applicator in no-till had greater total N and fertilizer N losses. Under relatively wet soil water conditions, respective losses of total inorganic N and fertilizer N in solution were greater from no-till with 4.0 and 2.0 kg N ha(-1) lost, as compared to 0.2 and 0.1 kg N ha(-1) lost from chisel-till per rainfall event. Losses of N in sediment were greatest in chisel-till, with 2.1 and 0.03 kg N ha(-1) lost from chisel-till, as compared to 0.6 and 0.01 kg N ha(-1) lost from no-till, of total N and fertilizer N per rainfall event, respectively The greatest N losses during the runoff event was observed with the surface banded and coulter-nozzle fertilizer application methods in no-till due to increased fertilizer N losses.

Management effects on water infiltration in vertisols is not well understood. Rainfall simulators and ponded and tension infiltrometers were used to characterize water infiltration rates as affected by traffic, crop residue, and tillage. Management was characterized by controlled traffic, with wide beds with either no-till or annual chisel-tillage on a Houston Black Clay soil (fine montmorillonitic, thermic Udic Pellusterts), a self-mulching vertisol. Traffic greatly reduced water infiltration rates compared to nontracked areas. Soil disturbance resulting from tillage in nontraffic areas was nor a significant variable in determining water infiltration rates. Rather, tillage effects on surface residue cover were more important in determining water infiltration rates. Surface residue was effective in controlling erosion from the wide bed management practices used in these studies. Erosion losses were greater than 4 t ha(-1) for the wet runs from both the no-till and tilled beds without adequate residue cover. Surface residue reduced erosion losses to less than one-tenth that of an unprotected surface.

Nine devices were used to visually measure percent residue cover on 21 field sites in 7 states. Three or more observers measured 1000 points with each device at each sire, with a total of 20 observers participating in the study. The residues were from crops of wheat, corn, cotton, sorghum, soybeans, and sunflowers. Cover ranged from an average of 11 to 84% for the 21 field sites. The measuring devices were four lines with various arrangements of bead markers, a 15.2 m steel measuring tape, and four wheels with markers located on or near the wheel perimeter. The time required to collect and record data for 10 consecutive replications of 100 points each varied between 16 and 31 min, with a mean of 20.7 min. General time savings were 3 to 6 min by using the measurement wheels. The "even-spaced residue wheel" equipped with a datalogger required the shortest time, while the steel tape rook the longest time. The observers preferred using the "dual-beaded" line and the wheel devices over other line devices. Not one observer preferred the steel tape or "random-spaced" single-bead line. Even though the fields visually appeared to be uniformly covered within measurement sites, data from each observer using the same measurement device revealed that there was variation of cover ranging from 7 to 20 percentage points. The variation among the observers was also greater than expected. The observers were mostly trained, USDA-Natural Resources Conservation Service employees, yet observer variations were nearly nine percentage points when using the same device on the same site. Superposition of such high levels of field site and observer variabilities exceeded arbitrary limits of acceptable performance, of +/- 3 percentage points, around the true value. All sources of variation increased with increasing field cover above 11%, but other residue characteristics such as diameter, length, and height were not highly correlated with measured values. Therefore, the measurement procedures used were equally effective across the wide array of field conditions. The nine measurement devices represented the variety of devices currently being used experimentally in research studies and standard types being used in the field. High levels of variability among observers obstructed any detailed performance comparisons among the nine devices. Precision of measurement appeared to group the four types of line devices and four wheel devices with generally less than four percentage points between groups of devices. Because cover was naturally distributed and not precisely controlled for any of the field sites, accuracy for individual devices could not be measured.

Heavy clay soils, such as vertisols of the Texas Blackland Prairie (Houston Black clay [fine, montmorillonitic, thermic Udic Pellusterts]), are difficult to manage for crop production. Such soils typically require high draft for tillage; have limited opportunities for mobility, tillage, and planting; and produce variable crop performance when associated with highly variable weather patterns. This study compared the soil and plant nutrient distributions and yield performance of an experimental no-till farming system with a farming system using modified conventional chisel-till soil management. Raised wide beds and controlled traffic were used for both systems. Wheat (Triticum aestibum L.), corn (Zea mays L.), and grain sorghum [Sorghum bicolor (L.) Moench] no-till yields were similar to those on chisel-tilled plots. Plant nutrients became stratified near the soil surface, with greater concentrations of C, N, and P occurring in the near-surface layers after a single 3-yr crop rotation cycle with no-till than with the chisel-till system. Grain yield responses to fertilizer rates were not affected by nutrient distribution. Nitrogen and P concentrations in plants tended to be high in the no-till system. Economic return for applied N was the same across tillage practices for each crop. The data demonstrated that the vertisol clay soils of the Texas Blackland can be farmed with no-till without comprising crop yield potential.

Reduced tillage and no-tillage cropping systems have periods of time lasting several months or years between tillage operations. If erosion is to be predicted for these systems, then there is a need to characterize the effects of such extended time since tillage on rill erodibility. Most erosion studies have been conducted on freshly tilled or disturbed soils which may respond differently to erosive forces than undisturbed soils. The objective of this study was to evaluate the effects of extended time since tillage on critical shear stress and rill erodibility for a Heiden vertisol under corn production. Rainfall simulation studies were conducted in March and August, before corn planting and after corn harvest, over a period of two years with time since tillage ranging from 7 to 624 days. Tests were replicated four times across a slope of 4 to 6%. The resulting data expands erosion information on one site included in the 1987 national tour under the Water Erosion Prediction Project (WEPP) studies. As time since tillage increased, the flow width in rills increased, and the hydraulic radius and hydraulic shear converged to more stable values. Three soil strength measurements, cumulative rainfall, or cumulative rainfall energy were not consistently correlated to rill erosion parameters and did not seem to be potentially useful for future predictions. Different approaches may be needed to relate repetitive wetting and drying cycles to measured soil properties and erodibility. Critical shear stress ranged from 0.3 to 2.47 Pa which indicated that rill erosion started at lower flow velocities or shallower flow depths than characterized by the 1987 WEPP data. Rill erodibility was found to be highly variable (0.73 - 16.31 x 10(3) s/m) on freshly tilled treatments, but variability decreased significantly for 89 days or more time since tillage, stabilizing in the range of 2 - 5 x 10(3) s/m. Erosion should be more accurately predictable on sites with extended time since tillage, for the soil in this study.

An experimental directed sprayer has been improved for use in conservation-tillage and no-tillage field conditions and variable row widths. The directed sprayer consists of a rear-mounted toolbar with individually flotational spraying units on lightweight runners which move over field surfaces without the collection of loose residues. Improvements allow convenient adjustments which combined lateral, vertical, angular, and fore-and-aft repositionings to correctly aim the hydraulic nozzles for desired pesticide targeting. Such new designs are needed to deliver pesticides to targets at minimum application rates and with minimum environmental impacts.

One goal in conservation farming systems is to optimize fertilizer use while reducing the loss of soil by water erosion. This requires evaluation of fertilizer application methods that minimize physical disturbance of protective crop residues and the soil surface and allow efficient uptake of added nutrients by the crop. We conducted this study to determine which of several surface and subsurface fertilizer application techniques would be the most beneficial to plant responses. A 3-yr randomized block study was conducted on a Houston Black clay (fine, montmorillonitic, thermic Udic Pellusterts) at Temple, TX. Nitrogen (N) and phosphorus (P) uptake and grain yield were measured for the sorghum [Sorghum bicolor (L.) Moench] test crop. All methods performed equally as indicated by crop yield responses and plant N and P contents. Based on these results and in conjunction with prior machine field performance testing, we have selected the coulter-nozzle applicator for fertilizer application in our conservation tillage research program.

A still video system was used to take RGB color images for system calibration, procedural developments, and initial analyses of percentage of soil covered by crop residues. A state-of-the-art PC image processor was used with customized software. A simplistic discrimination technique was used to classify individual image pixels as either residue cover or soil background. It was determined that analysis of only 12% of the image pixels would provide stable results. Shadows were not a significant problem. Color of the background affected the results, even with residues simulated with wooden dowels of known dimensions. Relative size of the residue pieces affected the performance, with better performance achieved with thick residues, such as corn, as contrasted with wheat residues. Analyses of actual residue images were generally too variable to be acceptable without further development on the discrimination of colors and soil backgrounds.

ABSTRACT AN experimental pneumatic downpressure system was designed to be interactive with the depth control system on a no-tillage planter. The interactive system uses position control monitoring and modulation of air flow to and from automotive pneumatic shock absorbers on individual furrow opener units. Automatic depth control is achieved by vertical force adjustments which occur for changing resistance to residue and soil cutting by a triple-disc furrow opener. The system response to dynamic pertubations was affected by the system pressure and somewhat by travel velocity.

ABSTRACT AN experimental hydraulic downpressure system was designed and tested on a no-tillage grain drill. Regulated downpressure to individual row units provided centralized downpressure adjustment and unit flotation for improved uniformity of planting depth. The system performed adequately in several years of field use. Passage of the drill over simulated soil undulations produced hydraulic pressure surges which were significantly affected by transmittal distance, field speed, and system pressure. An air damping system reduced the magnitude of the pressure surges for more uniform planting depth. The system could be used to convert existing planters and drills for use in no-tillage conditions.

Field experiments were conducted to evaluate placement techniques for preemergence applications of pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] in grain sorghum [Sorghum bicolor(L.) Moench.]. The first technique consisted of row shields mounted behind the planter units. Shields maintained an untreated strip over the crop drill and allowed successful crop establishment with pendimethalin at 1.1 kg ai/ha, despite a simulated, intense rainfall of 3.8 cm within 24 h after planting. A second technique, which consisted of a special nozzle arrangement, was evaluated in no-till grain sorghum. The nozzle arrangement allowed a broadcast herbicide application but maintained an untreated strip over the crop drill. No stand reductions occurred using this technique at pendimethalin rates of 1.1 and 2.2 kg/ha. In a growth chamber experiment, preemergence applications of pendimethalin severely injured grain sorghum when the soil was wet at the time of emergence, but injury was reduced under hot, dry conditions.