Conservation tillage has significant potential as a management tool for cotton production on sandy soils that are drought-prone and susceptible to erosion. Planting directly into a residue cover (no-till) or in narrow rows tilled into a residue cover (strip-till) has been shown to reduce erosion and conserve water by enhancing infiltration and increasing soil water holding capacity. This can reduce irrigation requirements and runoff which transports sediment, nutrients, pesticides and other agrichemical residues into surface waters.

While potential benefits of conservation tillage are widely recognized, actual benefits in terms of water conservation and quality vary, depending on numerous factors including soil characteristics, topography, pest pressure, agrichemical use and weather. There is a continuing need for systematic research to provide growers with the best available information on benefits of different tillage systems so that they can make informed choices which will enhance profitability and sustainability while minimizing adverse environmental impacts. To meet this need, a collaborative research effort was established between USDA-ARS -Southeast Watershed Research Laboratory and University of Georgia (UGA) scientists to systematically evaluate impacts of strip tillage on water quantity and off-site water quality. In 1999, a 4.6-acre parcel on the UGA Gibbs Farm located in Tift County, GA was selected for the study. The site was divided into six half-acre plots with a seventh 1-acre plot set aside for companion rainfall simulation studies (Fig. 1). Results obtained during the 2000-growing season are discussed in this report. Differences in water quantity and quality between plots maintained under strip and conventional tillage are highlighted. Additional details of the study and results from the 1999-growing season can be found in the 1999 Georgia Cotton Research and Extension Report.

Site Description. The soil is a Tifton loamy sand with a 3 to 4 % slope. Past agronomic practices resulted in substantial soil erosion. General soil properties delineated in a high-intensity soil survey included sandy surface soil to a depth of 10 to 20 inches underlain by dense sandy clay loam and sandy clay whose plinthite concentrations increase with depth. Because of its relatively low permeability the subsoil is believed to restrict rooting depth and deep percolation of infiltrating precipitation and to induce lateral subsurface flow.

Plots 1 to 6, approximately 0.5 acre each, were surrounded by 2.0-ft. earthen berms. The berms facilitated installation of metal runoff flumes equipped with automatic water sample collection and flow monitoring devices. On the down-slope side of each of these plots, 2-in (i.d.) PVC groundwater monitoring wells and soil water monitoring access tubes were used to monitor and sample the groundwater. Six-inch (i.d.) tile drain was installed across the slope between the lower boundary of plot 7 and the upper berm of plots 1 and 2 (Fig. 1). The drain was designed to intercept lateral subsurface flow originating on plot 7 and redirect it away from other plots lower on the slope. To capture lateral subsurface flow originating on the remaining plots two separate loops of 6-inch drain tile were installed so that they surrounded plots 1, 3 and 5 and 2, 4 and 6 (Fig. 1). Flumes were installed at the tile drain outlets to measure flow and provide a point for manual water sample collection.

Management. Tillage treatments were assigned as follows: plots 1, 3 and 5, conventional-till; plots 2, 4 and 6, strip-till; plot 7, half strip and half conventional. All were planted with a rye grass cover crop in the fall. Crop management practices for 2000 are outlined in Table 1. A solid set irrigation system was established in the spring of 2000 and used to supply additional water needs.

Table 1. Crop and soil management

Environmental Monitoring. Since planting and up to the present (January 2001), precipitation, temperature, soil water content, the volume of surface runoff, lateral subsurface flow (tile drain) and water table elevation have been monitored. During each storm event composite water quality samples were collected from plots 1 to 6. Water quality samples were collected daily at the tile drain outlets whenever flow occurred and monthly from the monitoring wells. Samples were analyzed for suspended sediment and pesticide residues. Soil on plots 1 to 6 was sampled intensively in both 1999 and 2000. In crop year 2000, this included composite soil samples on each plot, at four depth increments in the plow layer, 0-1 cm, 0-2 cm, 2-8 cm and 8-15 cm on the schedule shown in Table 2. As indicated sampling was most intensive immediately after planting and after defoliant application. This was done so that we could compare pre-emergence herbicide (fluometuron) soil dissipation results to data collected in 1999 and expand our dissipation research to pendimethalin, tribufos and thidiazuron. Soil samples were frozen after collection and are currently being analyzed for the four active ingredients, fluometuron, pendimethalin, tribufos and thidiazuron, and two fluometuron degradates, desmethylfluometuron and trifluormethylaniline. Selected sub-samples will also be tested for organic matter content and other physical and chemical properties.

Table 2. Soil sample collection schedule for crop-year-2000.

Rainfall Simulation Studies. Each fall, and also in the spring of 2000, a series of six, (three conventional-till and three strip-till area) 2X3-m subplots were established on plot 7 using aluminum frames. Impacts of tillage on runoff volume and rate, sediment delivery and transport of agrichemicals were evaluated utilizing simulated rainfall applied at 2 inches per hour for one hour. The source of the water was a deep well on the farm. Runoff was collected continuously in 5-minute intervals and analyzed for total suspended sediment and chemical residues.

RESULTS AND DISCUSSION

Water Quantity. Precipitation during the 2000 season was 36 inches, well below the 48-inch average. Even though some supplemental irrigation was applied (4.16 inches), total water which the plots received was about 8 inches below normal. In the first two years of the study, significant differences in runoff from plots 1-6 were observed. Surface runoff was again greatest from the conventional-till plots, while subsurface runoff was the greatest from the strip-till plots.

Tillage Effects on Runoff and Erosion. Simulated rainfall studies were used to evaluate how conventional and strip-till systems partition rainfall into infiltration and runoff with subsequent sediment generation. Runoff and sediment were determined gravimetrically, and infiltration was calculated by difference (rainfall–runoff). Data for the three rainfall simulation studies are summarized in Table 3. Infiltration and runoff characteristics for the different tillages have changed over the first two years of the study. In the fall of 1999, no differences were observed in the runoff volumes from the two tillage systems. In the spring and fall of 2000, we observed approximately twice the runoff from the conventional-till plots than from the strip-till plots. Thus, approximately twice the amount of water infiltrated the soil profile under strip-till. During natural rainfall events this would ultimately increase plant available water and decrease irrigation requirements.

Table 3. Summary data for the three simulation experiments, rainfall intensity was 50 mm/hr for 1 hour.

⁺conventional tillage
⁺⁺strip tillage

Soil loss from the strip-till plots has been consistently lower than from the conventional-till plots. The greatest differences were observed during the fall of 2000 simulation when the soil loss from the conventional-till plots was over 4 times that of the strip-till plots. The lowest differences were observed during the spring of 2000 simulation when we observed only a 15% increase. The strip-till system appears to have the potential to substantially decrease sediment losses.

Agrichemical Fate and Transport. In the 1999 Report, preliminary results were provided for soil dissipation of the pre-emergence herbicide, fluometuron (Cotoran) and its concentration in tile drain samples. Data were also provided on the concentration of the defoliant tribufos (DEF) in runoff samples collected during the September-1999 rainfall simulation. Updates on the defoliant runoff work and fluometuron studies are described below. Analyses of samples collected in the current crop-year-2000 are in various stages of completion. They will be completed in time for review in next year’s report.

Defoliant Runoff. All analyses of samples collected during the September-1999 runoff-rainfall simulation study were completed in Spring-2000. This includes the dissolved and sediment bound concentrations of thidiazuron, tribufos, and dimethipin in runoff samples and the mass of these chemicals on spray targets (filter papers) clipped to plants prior to defoliant application. Tribufos is the active ingredient in DEF, thidiazuron in Dropp and dimethipin in Harvade. A summary of results for each chemical for each tillage are presented in Table 4. The application rate shown is based on the results of the spray-target analyses. It was computed by dividing the mass of the chemicals detected on the filters (5 per plot) divided by the area of filters. The volume weighted concentrations were determined by summing the mass of each chemical detected dissolved in water and bound to sediment and dividing by the total volume of runoff. The fraction lost was computed by dividing the mass of each chemical detected in runoff (dissolved and sediment bound) by the mass of the chemical applied to each plot.

These data have provided a much clearer picture of the behavior of the three chemicals. The value "fraction lost in runoff" was particularly useful since it allowed direct comparison of results while taking into account the higher than anticipated variation in application rates. In the case of tribufos, differences in application rates between the two tillages varied by a factor of 5. We have no explanation for this except that one or more of the spray-nozzles on the back-pack sprayer used to apply the chemicals may been have become partially plugged between the time we sprayed the strip and conventional-till plots.

Taken together, the results did not reveal any tillage related differences with the possible exception of dimethpin. The fraction lost in the strip-till treatment was 0.06 whereas it was 0.02 from the conventional till treatment. However, it is unknown whether the difference was significant since only one plot from each tillage treatment was treated with dimethipin.

While tillage did not appear to affect chemical behavior, chemical properties did. This indicated by comparing the averages (across tillage treatments) of the fraction of each chemical lost. The dimethipin average was 0.04. It was 0.16 for tribufos and 0.15 for thidiazuron. The fraction of dimethpin lost was significantly lower (P=0.001) when compared to the other two chemicals. Possible explanations for the dimethipin behavior include more rapid absorption by the plants. If this were the case, less would be available for wash-off from foliar surfaces. Another possibility was that much more of the dimethpin was leached below the soil surface prior to initiation of runoff thus becoming unavailable for runoff. Dimethipin is 100 times more soluble in water than thidiazuron and 1000 times more soluble than tribufos. A prior study with dimethipin on Tifton soil showed that it can leach rapidly (Potter et al, 2000).

A detailed report of all data collected in the study is in preparation. It will be published later this year. Overall, results showed that differences in the amount of defoliants lost in runoff may be strongly influenced by properties of the active ingredients. Much more of tribufos and thidiazuron (>3 X) was detected in the runoff than the dimethipin. The study also showed that the mass lost in runoff of the later two compounds can be as high as 15 % of that applied under a worst-case scenario where an intense storm occurs soon after application. Adverse environmental impacts on rivers, streams and other surface waters receiving runoff could result. This finding lead to development of a study in which we are examining how effective grass-filters in removing defoliants from runoff. Preliminary findings for the establishment year, 2000, are also provided in this report (Potter et al, 2001).

Herbicide Dissipation and Leaching. Completion of the analysis of soil samples collected in crop-year 1999, tile drain samples collected in 1999 and 2000 and samples from a laboratory-based soil incubation study, confirmed results reported previously. This included observations that soil degradation is relatively rapid (t_1/2 = 10 to 15 days) and although some leaching of the fluometuron was observed, it is relatively minor process in terms of the mass of the chemical applied to the plots. The concentration of the fluometuron detected at the bottom of plow layer was much lower than in the surface soil (see Fig. 2). In addition, the compound was detected only at trace levels in tile drain samples and was not detected in samples pulled from the monitoring wells. Results support the conclusion that the low leaching rate was directly attributable to rapid soil degradation. Continued monitoring of fluometuron in soil and water at the site and addition of the fluometuron soil degradates, desmethylfluometuron and trifluormethylaniline, to the list of chemicals tested should help to conform this result.

Crop-performance. Yields for the 2000 growing season are shown in Table 5. Lint yields averaged 512 lbs/ac. for the conventional-till plots and 524 lbs/ac. for the strip-till plots. Yields from plot 6 have been significantly lower than the other plots for both 1999 and 2000. If this plot is removed from the average for the strip till plots, the average is then approximately 600 lbs/ac. The low overall yields obtained in 2000 appeared to be a reflection of the unusually dry and hot spring. Problems with installation of the irrigation equipment prevented us from being able to adequately irrigate the cotton crop to meet water needs.

Table 4. Summary statistics: active ingredient application rates, volume-weighted concentrations, and fraction of the chemicals applied lost in runoff.

Notes: cv = coefficient of variation

 

Table 5. 2000 Cotton yields.

SUMMARY AND CONCLUSIONS

During the first two years of the study significant differences in water quantity, water quality, and crop yield were observed between conventional- and strip-till treatments. The impact on water quality was restricted to sediment content. To this point no apparent differences in agrichemical behavior have been observed with the possible exception of slightly greater fluometuron leaching on the strip plots.

1. Greater runoff is observed from conventional-till plots.
2. Total sediment and maximum sediment delivery rates in runoff are greater from conventional-till plots.
3. Greater infiltration rates are observed on the strip-till plots.
4. Tile drainage occurred more frequently with higher volumetric flow rates from strip-till plots.
5. Peak surface runoff due to natural rainfall was up to five times greater on conventional-till plots.
6. More fluometuron, the active ingredient in the pre-emergence herbicide, Cotoran, reached the soil surface during application on conventional-till plots; however, higher rates of evaporative loss rapidly reduced levels to those found on strip-till plots.
7. Fluometuron leached at higher rates on the strip-till plots but, fluometuron leaching accounted for only a small fraction of that applied.
8. Up to 15 % of the defoliants, tribufos and thidiazuron applied was detected in runoff from small plots when simulated rainfall was applied 1 hour after defoliant application. In the same study only 4 % of another defoliant, dimethipin, was carried from the plots in runoff.
9. No differences in the rate of loss in the defoliants was observed between the two tillage treatments.
10. While overall yields from the plots have been relatively low, yields from the strip-till plots have been equal to or greater than those observed from the conventional-till plots.

In summary, findings indicated that strip-till conserved soil water and reduced sediment transport and runoff when compared to conventional till. Some potentially negative observations associated with strip-till included enhanced herbicide leaching and higher loss rates of dissolved defoliant residues in runoff. As subsequent crops are produced on the plots and environmental monitoring continued, more definitive data will be available to evaluate positive and negative aspects of strip-till versus conventional-till. These results should be of interest to growers and water managers who are concerned with optimizing water use to lower production costs and at the same time protect water quality.

ACKNOWLEDGMENTS

This work was supported in part by a grant from the Georgia Cotton Commission and research support funds provided by the U.S. Department of Agriculture-Agricultural Research Service. The able assistance of Herman Henry, Ricky Fletcher, Dudley Cook, Margie Whittle, Sally Belflower, and Luz Marti in the performance of field and laboratory work is greatly appreciated.