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SOIL MANAGEMENT

Post-Contract Grassland Management and Winter Wheat Production on Former CRP Fields in the Southern Great Plains

^a USDA-ARS, BARC-East, Bldg. 306, Room 102, 10300 Baltimore Ave., Beltsville, MD 20705 USA
^b Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078 USA
^c Dep. of Plant and Soil Sciences, Oklahoma State Univ., Altus, OK 73521 USA
^d Usda-Nrcs, Beaver, OK 73932 USA
^e Usda-Nrcs, Altus, OK 73521 USA

thdao{at}lpsi.barc.usda.gov

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Summary and conclusions REFERENCES

 
Integrated management guidelines for postcontract land use Conservation Reserve Program lands in semiarid regions are generally lacking. We determined the relative efficacy of four systems of transitional conservation practices for producing `Old World' bluestem (OWB) (Bothriochlora ischaemum L.) and dryland wheat (Triticum aestivum L.) and cotton (Gossypium hirsutum L.) on former CRP fields. The sites were located on Dalhart fine sandy loam (Aridic Paleustalf) and La Casa–Aspermont clay loam (Typic Paleustoll) near Forgan and Duke, OK, respectively. Removing old growth increased cumulative OWB yields between 1994 and 1997. Applications of 67 kg N and 16.5 kg P ha^-1 increased yields by 0, 70, and 180% at Forgan and 290, 70, and 280% at Duke in 1995 to 1997, respectively. Removing the old dry matter and regrowth vigor also enhanced chemical suppression and killing of the grass, the performance of conservation tillage, and achieving a uniform crop stand. Early OWB suppression conserved stored water that was vital to cool-season crop production in the year the contract expired. First-year wheat yields averaged 970, 490, and 1002 kg ha^-1 at Forgan and 1590, 600, and 830 kg ha^-1 at Duke under unfavorable weather conditions (i.e., drought, late freeze) of 1995 through 1997, respectively. No-till generally produced higher yields, averaging 10 and 35% greater than conservation systems at Forgan and Duke, respectively. In variable semiarid environment, the chance of success for agronomic production decreased in the order of grass production, NT wheat, tilled wheat, and dryland cotton on former CRP lands.

Abbreviations: CRP, Conservation Reserve Program • OWB, Old World bluestem • NT, no-till • ST, sweep-tillage • DT, disc-tillage

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Summary and conclusions REFERENCES

 
CONGRESS mandated the set-aside of 14.7 million ha of erosion-prone croplands for 10 yr to curtail erosion of the soil resource base and protect water quality across the nation. The Conservation Reserve Program (CRP) was established in Title XII of the Food Security Act of 1985. Soon after its inception, the very nature of its merit, implementation strategies, benefits, and deficiencies, and its future was extensively debated (Grazinglands Forum, 1988). The program was reauthorized in 1996, adding environmental benefits to the requirements for new enrollment or renewal of expired contracts. Landowners would have to requalify under more stringent environmental benefit criteria or choose alternate land use for their CRP fields.

In response to CRP landowners' concerns, studies of grazing or haying CRP grasslands were conducted throughout the Great Plains, from Montana (Printz, 1993) to Wyoming (Schuman et al., 1995), Nebraska (Schacht, 1998), Kansas (Ohlenbusch and Langemeier, 1995), New Mexico (Donart et al., 1996), and Texas (Colette et al., 1996) to evaluate the potential value of CRP grasslands for livestock production. These studies suggested that returning CRP lands to grazing lands was an economically competitive choice with dryland cropping options. Management considerations for grazing of CRP lands included removal of the dead forage, weed and brush control, and improving forage quality and stand longevity (Gillen, 1996, 1998; Schacht, 1998). Although maintaining the grass cover and undertaking a transition to forage–livestock production on CRP lands appeared to be the most sensible option, the trend toward conversion of these former croplands to row crop production was also apparent (Novak et al., 1991; Sudduth et al., 1993; Krall and Schuman, 1996). Cropping studies were initiated across the Great Plains between 1994 and 1996, from Montana (Tanaka, 1995) to South Dakota (Stymiest, 1995), Nebraska (Holshouser et al., 1995), Kansas (Schlegel et al., 1996), Colorado (Bowman and Anderson, 1996), Texas (Unger and Jones, 1996), and Oklahoma (Dao et al., 1996). Fertility management and chemical control of CRP grass and weeds (Anderson, 1996; Medlin et al., 1998) were the primary focus of these state research and demonstration projects. Although many management insights were drawn from past experience with the conversion of native pastures to croplands, converting temporary CRP grasslands and destroying a warm-season, bunch-type grass, as was the case in Great Plains states, proved more challenging in subhumid and semiarid climates.

Oklahoma has 485000 ha enrolled in the CRP. Forty percent of this land area is in the panhandle and another 48% is in counties along the Oklahoma–Texas border. Before CRP, much of this land was cropped annually to wheat. Cotton production is also important in SW Oklahoma. Sediments, airborne dust, and particulate-associated nutrient discharges have been significant problems in the production of both crops. Old World bluestem (OWB) (Bothriochlora ischaemum L.) and native grasses were extensively used for permanent soil cover in the panhandles of Oklahoma and Texas. Benefits of the CRP have included erosion control, enhanced wildlife habitat, reduction in surplus commodities, and farm income support (Dicks, 1994). Most often the program has been credited with substantial reduction in wind and water erosion of marginal croplands (Margheim, 1994; Lindstrom and Schumaker, 1995; Gilley et al., 1997). In Oklahoma, the majority of CRP contracts were scheduled to expire in the 3 yr beginning in October 1996. A similar trend was occurring across the Great Plains. Contract holder concerns have revolved around the best courses of action. The questions often were whether it was most beneficial to use the lands for livestock production and what were the optimal grass management and grazing procedures. If these temporary grasslands were to revert to crop production, what were the best management practices to kill the sod, plant crops, and preserve the improvements in soil properties accrued under the program? Research and education agencies of USDA have endorsed a 1993 resolution by the National Association of Conservation Districts. Resolution ARC-11 anticipated that no-tillage and other methods of residue management would be production tools of choice for controlling the continual erosion of the soil resource across the nation. The organization recommended no-tillage and residue management practices in recropping CRP lands. In dry and semiarid regions, residue management methods have been shown to conserve soil water and enhance crop production (Unger and Wiese, 1979; Deibert et al., 1986; Wilhelm et al., 1989; Dao, 1993).

The lack of integrated management guidelines on returning highly erodible CRP lands to agronomic production and the eminent expiration of the popular conservation program led to coordinated research and educational efforts among USDA research and action agencies across the USA. The objective of this multiagency collaborative study was to determine the relative efficacy of four postcontract transitional practices for grass management and conversion to dryland winter wheat and cotton production while providing adequate erosion protection on former CRP fields. Active involvement of local, state, and federal action agency field offices in conducting the field research also was a high-priority objective to facilitate technology transfer efforts.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Summary and conclusions REFERENCES

 
Field Sites
One experimental site was in Beaver County near the town of Forgan in northwestern Oklahoma. Annual precipitation averaged 450 mm and mean minimum and maximum temperatures were 5 and 21°C, respectively. The major soil at the site was Dalhart fine sandy loam (fine-loamy, mixed, mesic Aridic Haplustalf) on a 1 to 3% slope. The field was planted to `WW spar' OWB in 1989. The southwestern Oklahoma site was in Jackson County near the town of Duke. Annual precipitation was approximately 750 mm. Annual minimum and maximum temperatures averaged 9 and 23°C, respectively. The major soil was La Casa–Aspermont clay loam (fine, mixed, thermic Typic Paleustoll) on a 1 to 3% slope. The field was under CRP contract since 1987 and planted to `Plains' OWB. Selected properties of soils at the study sites are presented in Table 1 . Automated weather stations were used to collect on-site climatological data. Daily minimum and maximum air temperature and precipitation from 1995 to 1997 are shown in Fig. 1 .

View larger version (46K): [in this window] [in a new window]   Fig. 1 Temperature and cumulative precipitation at the Forgan and Duke experimental sites during 1995–1997

At Forgan, replicated plots measuring 50 by 100 m were established each year in these areas to evaluate the following post-CRP management options: (i) minimum grass management (no fertilizer added following controlled burning or swathing), (ii) optimal grass management (fertilizer added following controlled burning or swathing), (iii) conventional-tillage (i.e., sweep-tillage) conversion to wheat, and (iv) no-till (NT) conversion to wheat. At Duke, field plots were established to evaluate the same four management systems and the row-till dryland cotton system. Two plot sizes were used at this location. A set of large field plots measuring 80 by 300 m was established for research and demonstration purposes, while the other three replications had plots that measured 15 by 30 m.

Detailed field practices and timing of management operations were as follows.

Grass Treatments
For the minimum OWB management treatment, the grass was not fertilized and left to grow without further management input on half of the OWB plots. On the other half of the OWB plots, we made a uniformly broadcast application of commercial fertilizers in March. At Forgan, 89 kg N ha^-1 (46–0–0) were applied in 1995, and 67 kg N and 16.5 kg P ha^-1 (34–19–0) in 1996 and 1997. At Duke, we applied 67 kg N ha^-1 (46–0–0) in 1995. In 1996 and 1997, we used a mixed fertilizer (34–19–0) to apply a total of 67 kg N and 16.5 kg P ha^-1. Forage production was determined in five random samples from 1 m² areas. Peak standing dry matter accumulation was determined in mid-July to August of each year. Similar estimates of untreated CRP forage production were made in undisturbed areas of the fields. Total Kjeldahl N of the harvested forage was determined to calculate crude protein concentrations. The first-year OWB plots were reestablished after swathing and bailing to remove the previous year's growth in March 1995, 1996, and 1997. Forage crude protein and yields were determined as previously described.

Wheat Cropping Treatments
At Forgan, sweep tillage was performed to kill the OWB with a 90-cm wide V-blade in July 1994. The sweep cut and lifted approximately a 10- to 15-cm layer of sod and allowed the soil to dry. No other tillage was subsequently performed during the summer because precipitation was negligible and grass regrowth was not observed. In 1995, sweep tillage was performed as needed in July, August, and again in September before planting wheat because of significant regrowth of the sod and weeds. In 1996, sweep tillage was again done to undercut the growing sod of a newly prepared plot area. The NT plots were sprayed with glyphosate¹ [isopropylamine salt of N-(phosphonomethyl) glycine] at the rate of 1.12 kg a.i. ha^-1 in August 1994 and in June 1995 and 1996, and a second time in September of all years before planting wheat. A no-till drill was used to seed all ST and NT plots to `Tomahawk' wheat at the rate of 78 kg ha^-1 in mid-October. Starter fertilizer (18–20–0) was placed in the seed row at the rate of 112 kg ha^-1. All wheat plots were topdressed with 34 kg N ha^-1 (46–0–0) in March. Spring weeds were controlled with an application of chlorsulfuron [2-chloro-N-((4-methoxy-6-methyl-1,3,5-triazin-2-yl) aminocarbonyl)-benzene sulfonamide] at 28 g a.i. ha^-1. Chlorpyrifos was applied by air in March to control greenbugs (Schizaphis graminum) as needed. Grain yields were determined using a plot harvester (0.6 by 5 m area) and a commercial combine in mid-June of each year.

At Duke, disk tillage (DT) was performed to kill and partially incorporate any regrowth in June, following controlled burning. The OWB regrowth in NT plots was sprayed with a mixture of glyphosate, ammonium sulfate, and surfactant at the rate of 1.12 kg ai ha^-1. Another herbicide application was made before wheat planting. `Pioneer 2180' wheat was seeded at the rate of 78 kg ha^-1 in all DT and NT plots using a NT drill set for 20-cm row spacing. A starter fertilizer (45 kg ha^-1 of 18–46–0) was applied in the seed row. Additional fertilizer was surface-broadcasted to provide a total of 100 kg N and 16.5 kg P ha^-1. In March, wheat was topdressed with 34 kg N ha^-1 (46–0–0). Spring weed and insect control was performed as needed. Forage and grain yields were determined as detailed above.

The first-year ST, DT, and NT plots were reestablished after the initial grain harvest for each of the 3 yr. Conventional-tilled plots were sweep-tilled (Forgan) or disked (Duke) in July and September. Weeds and OWB regrowth in NT plots were sprayed with 1.12 kg ha^-1 of glyphosate in July and September and plots were planted back to wheat. This schedule resulted in replicated plots with a 1-, 2-, and 3-yr crop history following the initial breakout of the OWB sod.

Cotton Cropping Treatments
At Duke, cotton plots were also established by disking twice to destroy the OWB sod and incorporate any remaining grass residues following controlled burning in 1994. In October, the plots were fertilized and planted to winter wheat as a cover crop. All cultural practices were similar to the DT wheat treatment, except that a mixture of glyphosate, ammonium sulfate, and surfactant was applied at the rate of 1.12 kg a.i. ha^-1 in late February to terminate the wheat cover crop. In March, strip-tillage was performed to prepare clean-tilled 45-cm wide strips for planting cotton. A preplant incorporation of trifluralin (2,6-dinitro-N,N-dipropyl-4-[trifluoromethyl] benzenamine) was made at the rate of 1.12 kg a.i. ha^-1 to control weeds. Cotton (var. `All-Tex Express') was planted at a population of 30000 plants ha^-1 in June of 1995. An early cultivation was performed after emergence and the operation was repeated to control blowing soil conditions and/or keep weeds under control. Scouting was regularly done during the boll-forming stage and insecticides were applied by air as needed. In mid-October, whole plots were stripped to determine cotton yield.

In June 1995, new cotton plots were similarly established in another area of the CRP field and the first-year cotton plots were disked to prepare for seeding a wheat cover crop in October. Pioneer 2180 wheat was planted at the rate of 78 kg ha^-1. The wheat cover was terminated in late February 1996. However, cotton was not planted that spring because no precipitation occurred at the site from October 1995 to June 1996 during the drought that affected the Southern Great Plains and beyond (Fig. 1).

Plant Analysis and Soil Water Measurements
Dry matter of OWB and wheat forage and grain samples were determined following drying at 65°C. Forage and grain yields were reported on an oven dry wt. basis. Subsamples were ground in a Wiley mill to pass a screen with 1-mm openings. Triplicate 0.25-g samples were weighed into 100-mL tubes and acid-digested to determine total Kjeldahl N (Nelson and Sommers, 1980). These values were used to calculate forage crude protein concentrations.

Soil water contents were determined at 0.3-m increments to a depth of 1.2 m using the neutron attenuation technique (Model 3331A, Troxler Electronics, Research Triangle, NC). Duplicate access tubes, made from EMT tubing of 38-mm i.d. were installed in each experimental plot. Measurements were made when tillage treatments were imposed and at planting and crop harvest.

At each location, land management treatments (4) and replications (4) were established based on a randomized complete block design. Significant differences in treatment means were detected following analysis of variance and a multiple range test at the 0.05 level of probability.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Summary and conclusions REFERENCES

 
Old World Bluestem Management
Old World bluestem plots were not fertilized in the summer of 1994 at either location. Forage production averaged 3.2 Mg ha^-1, at Forgan after the removal of the old dry matter (Fig. 2) .

View larger version (28K): [in this window] [in a new window]   Fig. 2 Field-scale effects of fertilization, soils, and climate on Old World bluestem forage production in two CRP fields during 1994, 1995, 1996, and 1997

The differential responses to fertilizer applications showed the impact of soil–climate interaction on the productivity of the grass stands. Over the 3-yr period, average forage crude protein concentrations were consistently higher at Duke than Forgan, respectively (Table 2). Optimal use of OWB still depended upon a delicate balance between forage yield and peak nutritive value of the forage. The dependence between time of sampling and crude protein concentrations of OWB is shown in Fig. 3 for the Forgan stand during the 1996 summer. Crude protein concentrations of the new growth declined with advancing plant maturity and growth.

View larger version (20K): [in this window] [in a new window]   Fig. 3 Forage crude protein concentration and yields of fertilized Old World bluestem at Forgan, OK, during the 1996 summer

Conversion to Winter Wheat Production
In Oct. 1994, stored soil water at wheat planting showed no difference between grass and cropped plots at Forgan (Fig. 4) . Negotiation delays in CRP contract release resulted in late application of herbicides and tillage to prepare seedbeds for planting wheat. The OWB growth went unsuppressed until mid-July. The grass sod depleted stored soil water and that which would be stored during the spring months. In an attempt to minimize tillage intensity, a single sweep-tillage to the 0.1- to 0.15-m depth was performed to undercut and kill the sod. Tillage also created a soil water deficit under the semiarid climate of Forgan. High evaporation potential and hot temperatures dried soil and the sod mulch, as the undercut layer remained dry for several days following tillage. The water depletion extended deep in the root zone of Dalhart fsl (Fig. 4).

View larger version (23K): [in this window] [in a new window]   Fig. 4 Soil water content at (A) wheat planting and (B) preharvest at the Forgan site in 1994–1995

View larger version (23K): [in this window] [in a new window]   Fig. 5 Soil water content at (A) wheat planting and (B) preharvest at the Duke site in 1994–1995

In this study, the success of sweep-, disk-tillage, or chemical suppression of the sod for no-till wheat production was achieved with the removal of old grass growth by controlled burning or swathing and baling. Although it was highly desirable to conserve as much of the fixed C in the surface mulch, the issue has sparked a debate on whether to remove or not remove the old growth (Schertz, 1995). The moldboard plow was suggested to be an excellent tool to bury and incorporate this large amount of C into the soil. With liberal application of N fertilizers, Medlin and coworkers (1998) attained high crop yields. However, clean-till practices brought back images of the dusty days following the Soil Bank program of the 1950s. Susceptibility of loose plowed soil to wind and water erosion has always been high (Gilley et al., 1997). Recent research also showed that tillage stimulated the mineralization of organic C and resulted in rapid loss of incorporated residues (Reicosky and Lindstrom, 1993; Dao, 1997). They found that decomposition of crop residues released between 55 to 70% of the C as CO₂ to the atmosphere. Loss of soil C has often been associated with decline in soil productivity components that included such fundamental properties as aggregate stability, macroporosity, water-holding capacity, nutrient availability, and microbial biodiversity and activity. Traditional land management approaches such as mechanical tillage and mixing or burial of the organic mulch would undo the many benefits to these field soils that were accrued under the CRP (Dao et al., 2000).

Conversion to Cotton Production
Dryland cotton 1995 yields averaged 100 kg ha^-1 because dry weather conditions existed during the boll-setting stage. The low cotton yields was typical of poor growing conditions that existed on producers' fields in the region for the current and previous two growing seasons. A 1996 crop was not planted at the study site and was considered a production failure due to the drought conditions that prevailed during the optimal time window for cotton planting in the region. Precipitation during June was scant and came in small events for a monthly total of 71 mm (Fig. 1). Significant precipitation came late in July and much of the local cotton land area produced luxurious vegetative growth and low boll counts due to late planting and prolonged cool temperatures in August.

	Summary and conclusions

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Summary and conclusions REFERENCES

 
The CRP has curtailed the degradation of marginal croplands and reduced soil erosion across Oklahoma and the Great Plains. The estimated 610000 ha of CRP lands planted to OWB in the Oklahoma and Texas panhandles required immediate management action at the end of the contracts to convert these grasslands into more productive lands. Best management strategies to prepare CRP lands for grazing or haying included removal of the old litter in early spring of the year the CRP contract expired. Commercial fertilizers should be applied to improve nutrient status of these field soils. Integrated systems of conservation practices are available to successfully convert these CRP lands to annual crop production. The timing of suppression of a warm-season grass cover was critical for conserving stored water and ensuring the success of producing winter wheat in the year a CRP contract expires. The amount of dry matter removed and regrowth was crucial to how well conservation tillage performed, the growing cover killed, and a good plant stand established. No-till practices most efficiently conserved stored water and maintained a smooth and firm seedbed for at least the first-year and the second-year crops (Dao, 1996). If tillage was necessary, intensive tillage was required to break up sod-bound clods and produce a smooth seedbed. According to local soil and cultural practices, a combination of deep sweep and secondary tillage operations or repeated disking and harrowing provided the desired seeding conditions. However, additional storage of soil water would be needed either through additional rainfall, fallow, or irrigation. A cool-season crop such as wheat would minimize the risk of production better than a summer crop that would not be very competitive with uncontrolled grass growth (Dao et al., 1996). A simple suppression of OWB was needed and not an immediate complete kill of the sod. Active spring growth and plant stature would allow wheat to compete effectively with suppressed OWB during the grain production phase. Grass control should be completed during the second year following wheat harvest. Climatic variability served as a constant reminder of the precarious environment and the high risk of agricultural production in the Great Plains. Maintaining a productive grass stand on CRP lands appeared to be the least-risk option. The study showed that the chance of success decreased in the order grass production > no-till wheat > tilled wheat > dryland cotton.

	ACKNOWLEDGMENTS

 
The authors acknowledge the technical assistance of R.D. Meyer, L.S. Pellack, M. Heath, and R. Thacker during the conduct of the research. Sincere appreciation is extended to R.B. Masters of Duke, OK, and A. Hodges of Forgan, OK, for providing access to the CRP fields and their support during much of the research. The discussions with Bill Berg, USDA-ARS, Woodward, OK (retired), and the suggestions of many producers during prestudy meetings are sincerely appreciated. The authors sincerely acknowledged the partial financial support provided by the Southern Region SARE/ACE program under grant LS94-58.

	NOTES

TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Summary and conclusions REFERENCES

 
¹ The mention of trade names or manufacturer names is made for information only and does not imply an endorsement, recommendation, or exclusion by the USDA–Agricultural Research Service. Mention of a pesticide does not constitute a recommendation for use nor does it imply registration under FIFRA as amended.

Received for publication January 27, 2000.

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TOP ABSTRACT INTRODUCTION Materials and methods NOTES Results and discussion Summary and conclusions REFERENCES

 

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