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. 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. The first cotton crop was planted in May 1999. It is anticipated that at least two more cotton crops will be produced with continuous water quantity and quality monitoring. Results obtained during the 1999-growing season, the first year following establishment, are discussed in this report. Differences in water quantity and quality between plots maintained under strip and conventional tillage are highlighted.

^* USDA-Agricultural Research Service, Southeast Watershed Research Laboratory, Tifton, GA; ^t Department of Crop and Soil Sciences, Coastal Plain Experiment Station, University of Georgia, Tifton, GA

^& corresponding author e-mail: tom.potter@ars.usda.gov

Site Description. The soil is a Tifton loamy sand with 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 restricts rooting depth and deep percolation of infiltrating precipitation and under certain circumstances induces lateral subsurface flow.

The parcel was divided into 7 plots. Topographic features and locations of the plots are shown in Figure 1. Plots 1 to 6, approximately 0.5 acres each, were surrounded by 2.0-ft. earthen berms. They permitted 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 installed. 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 (Figure 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 loops of 6-inch drain tile were installed so that they surrounded plots 1, 3 and 5 and 2, 4 and 6 (Figure 1). Flumes were installed at the 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). In November 1998, all were planted with a rye grass cover crop. Subsequent fertilizer and pesticide applications and crop management practices are outlined in Table 1. During the growing season, farm managers irrigated all plots with a traveling gun on an as needed basis.

Environmental Monitoring. Since planting and up to the present (January 2000), precipitation, temperature, soil water content (Troxler soil capacitance), 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 the plot 1 to 6. Water quality samples were also collected daily at the tile drain outlets whenever flow occurred and monthly from the monitoring wells. Samples were submitted for suspended sediment, pesticide residue and nutrient analysis. One day before planting, 1 hour after planting (and pre-emergence herbicide application), 1, 4 and 7 days later and weekly thereafter until July 25, 1999, composite soil samples, at three depth increments in the plow layer, 0-2 cm, 2-8 cm and 8-15 cm, were collected on plots 1 to 6. Samples were also collected one day prior to and 1 hour after defoliant application and during the first week of January 2000. Samples were frozen after collection with selected sub-samples submitted for physical and chemical characterization and pesticide residue analysis.

Rainfall Simulation Studies. During the first and second weeks of September, a series of six, (three in the 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 three defoliant active ingredients were evaluated utilizing simulated rainfall applied at 2 inches per hour for one hour. Source of the water was a deep well on the farm. Rainfall was simulated 1 hour after defoliant application. Runoff was collected continuously in 5-minute intervals and analyzed for total suspended sediment and defoliant residues. Defoliants were applied in two tank mixtures using a backpack sprayer. The composition of tank mix 1 was identical to that applied to plots 1 to 6. It was applied on two conventional and two strip-till plots at nominal rates indicated in Table 1. Tank mix 2 contained Harvade, Prep and Dropp 50W. It was applied to 1 conventional and 1 strip-till sub-plot at rates equivalent to 0.1 lbs/acre for Dropp 50W, 6 oz/acre for Harvade and 21 oz/acre for Prep.

Table 1. Crop and soil management

RESULTS AND DISCUSSION

Water Quantity. Precipitation during the growing season was well below average. Thus, even though supplemental irrigation was applied, total water which plots received was about 10 inches below normal. Superimposed on this were dramatic differences in the hydrology of the two different tillage systems. Soil water content, monitored 3 times per week since plot establishment in April, indicated that strip tillage lead to an increase of up to 50 % more soil water in the top 5 ft of the soil profile. Greatest differences in the soil water conditions were observed in the zone from 30 to 60 inches and during the early part of the growing season. In August when soil conditions were the driest, differences as great as 10 % by volume were observed between the conventional and strip-till plots.

Differences were also found in the surface runoff and lateral subsurface flow. Maximum runoff rates from natural precipitation was up to 5 times greater on conventional-till plots. As discussed below under Tillage Effects on Surface runoff and erosion, no such differences were observed in the rainfall simulation studies. This may be due to differences in the hydrologic behavior of the small plots (ca. 0.001 acres) used for rainfall simulation studies when compared to plots 1 to 6 (0.5 acres), variations in intensity of natural versus simulated rainfall and other factors.

Lateral subsurface flow was also greater on strip-till plots. Although a relatively small volume of flow was observed, rates from the strip till plots were up to 10 times greater. This was a reflection of greater soil water contents observed in these plots. A soil profile with high soil water content is expected to yield more subsurface flow and provide more plant available water.

Agrichemical Fate and Transport. Fate and transport studies focused on active ingredients in pre-emergence herbicides, Cotoran and Prowl and defoliants, DEF, Harvade and Dropp. Motivation to study these herbicides and defoliants was based on published studies which indicated that some of the active ingredients have the potential to contaminate ground and surface water via surface runoff or leaching. Thus, there is need to document how the chemicals behave in conventional and conservation tillage systems.

Analyses of each of the active ingredients in soil, water, and suspended sediment samples are in progress. To date (January 2000), fluometuron residue measurements in soil and tile drain samples and dissolved tribufos tests in runoff samples collected during rainfall simulator studies have been completed. Fluometuron is the active ingredient in Cotoran and tribufos in DEF.

Soil fluometuron concentration data are summarized in Table 2. For reference, the computed surface soil concentration based on the nominal Cotoran application rate (1 qt/acre) is included. There was remarkably close agreement between the computed soil concentration based on application rate and the measured (in soil) concentration for conventional-till plots (2.97 versus 2.94 ug/g). For strip-till plots, the measured concentration was significantly less than the computed application rate (2.04 vs. 2.94 ug/g). This can be attributed to interception of the herbicide spray by the crop residue remaining on the surface of the strip-till plots. Only the soil was sampled, thus fluometuron intercepted by and adsorbed on cover crop residue would not have been detected.

Table 2. Fluometuron concentration in soil plow layer (0-15 cm)

In spite of the fact that the initial concentration was higher in the conventional-till samples, one day later there was no significant difference when total fluometuron in strip and conventional-till samples was compared. This can be attributed to differences in the fluometuron evaporation rates due to differences in surface soil temperature. Measurements made on 5-May show that mid-afternoon temperatures in bare surface soil reached more than 43^oC while soil temperatures taken under cover crop residue on the strip-till plots time were about 7^oC lower (Table 3). It appears that the residue shaded the surface soil, keeping temperatures lower and reducing evaporation. Overall, nearly 33 % of fluometuron applied on the conventional-till plots was lost during the first 24 hours after application while less than 10 % was lost from strip-till plots. On the afternoon of 7-May, plots received a 0.7-inches of rain which cooled the soil and leached a portion of the remaining fluometuron below the soil surface. This appears to have limited further evaporative loss of the compound.

Subsequently, fluometuron concentration in the strip and conventional-till plots decreased at approximately the same rate. On day 46, the data show that about 20 % of the fluometuron applied remained in the plow-layer. The loss of the fluometuron between day 1 and day 46, is best explained by biodegradation of the compound and to lesser extent, off-site transport in runoff and leaching below the plow layer. As data become available, the relative significance of runoff and leaching will be evaluated. In turn this will permit evaluation of potential water quality impacts.

Table 3. Soil Temperature on at three depths in the plow-layer^&

^& measurements made on 5-May 1999 between 2:30 and 2:45 P.M.

Preliminary data on tile drain water samples summarized in Table 4, show that some of the fluometuron was leached through the soil and that more was leached on the strip-till plots. However, total mass lost by leaching was only a small fraction (0.00008) of the compound applied. It should be emphasized that the concentration levels of fluometuron detected in the tile drain samples were very low. Ecological and public health impacts are unlikely.

Table 4. Fluometuron concentration in tile drain samples.

^& "Fraction leached" equals mass of fluometuron in tile drainage divided by total mass in soil 1 hour after application.

Tribufos residue data obtained by analysis of runoff samples collected during rainfall simulation studies are compiled in Figure 2. Higher dissolved tribufos concentrations were observed in runoff from strip-till plots. Explanations include the possibility of higher "dislodgeable" residue concentrations on plant surfaces and crop residue on strip-till plots. This would contribute to higher residue levels in run-off. Regardless of tillage, tribufos concentrations in runoff were relatively high when compared to levels, which have been reported to kill or injure aquatic invertebrates, an important food source for many fish. Future studies will evaluate tribufos removal efficacy runoff as it flows through from grass and other buffer systems designed to trap dissolved pesticide and fertilizers residues and prevent them from entering streams.

Tillage Effects on Runoff and Erosion. Simulated rainfall studies were used to evaluate how conventional and strip-tillage 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 are summarized in Table 5. No differences were found in hydrological properties (rainfall intensity, initial water content, runoff, and infiltration) of the two tillage systems. Significant differences were found between total sediment and maximum sediment delivery rates for the two tillage treatments. Total sediment lost (476 vs. 257) and maximum sediment loss rate (0.14 vs 0.09) for the conventional-till treatment were 1.9 times (85%) and 1.4 times (45%) greater than that for the strip-till treatment. Similar results were found for runoff and sediment losses occurring during each simulated rainfall event for each tillage treatment. Overall, the data were encouraging in that just one growing season after establishing strip-tillage it was shown to significantly reduce sediment delivery.

Table 5. Selected properties measured from both tillage systems during rainfall simulation events (3 reps per treatment) ^&

^& Values in parentheses are coefficients of variation.

Crop-performance. Lint yields, based on a 35% turnout, averaged 854 lbs/acre for the conventional-till plots and 678 lbs/acre for the strip-tillage plots. The relatively low yields on the strip-till plots appeared to be related to an early season nitrogen deficiency in combination with herbicide injury from post-directed applications of MSMA.

Poultry litter (2 tons/acre), was the only source of pre-plant nitrogen applied to the plots. It was surface applied on April 12^th and on the conventional-till plots tilled into the soil when these plots were ripped and bedded April 21^st. This protected litter from loss in runoff generated during a 0.5-inch rain event on April 28^th. Visual observations at runoff flume outlets on strip-till plots indicated that some litter was washed off during the storm event. Thus, when the plots where strip-tilled planted on May 6^th less nitrogen in the form of poultry litter was tilled into the soil. This resulted in less nitrogen being available and slower rates of growth. By mid-June, nitrogen deficiency symptoms were visible in all strip-tillage plots. This was treated by side-dress nitrogen application on June 29^th.

Less vigorous early season cotton growth on strip-till plots resulted in a shorter crop, and appears to have contributed to herbicide injury when MSMA was post-directed in early June for weed control. The MSMA went "over-the-top" on some of the plants on strip-till plots and likely retarded growth.

Improvements in crop management, which will be implemented during the next growing season includes banding a starter fertilizer on strip-till plots and shortening the time after poultry litter application and strip-till planting.

SUMMARY AND CONCLUSIONS

During the first year following establishment, significant differences between conventional and strip-till tillage treatments in water quality and quantity and crop yield were observed.

1. Total sediment and maximum sediment delivery rates were substantially greater in runoff from conventional-till plots.

2. Strip-till plots typically had higher soil water content.

3. Tile drainage occurred more frequently with higher volumetric flow rate from strip-till plots.

4. Peak surface runoff due to natural rainfall was up to five times greater on conventional-till plots.

5. 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.

6. Fluometuron leached at higher rates on the strip-till plots but, fluometuron leaching accounted for only a small fraction of that applied.

7. Relatively high concentrations of dissolved tribufos were detected in runoff from all plots where it was applied. The highest concentrations were from strip-till plots.

8. Lower lint yields on the strip-till plots were associated with early season nitrogen deficiencies and herbicide injury from post-directed application of MSMA.

In summary, findings indicated that strip-tillage conserved soil water and reduced sediment transport and runoff when compared to conventional tillage. Some potentially negative observations associated with strip-tillage 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-tillage versus conventional-tillage. 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. Discussions with Glenn Harris of the University of Georgia benefitted the research. The able assistance of Herman Henry, Ricky Fletcher, Margie Whittle, Sally Belflower and Luz Marti in the performance of field and laboratory work is appreciated.

Figure 1. Topographic map of research plots.

Figure 2. Tribufos concentration in runoff