G.A. Peterson², D.G. Westfall², F. B. Peairs³, L.R. Ahuja⁷, L.Sherrod⁴, D. Poss⁵, W. Gangloff⁵
  K. Larson⁶, D.L. Thompson⁶,  M.D. Koch⁵, and C. B. Walker⁵
 

INTRODUCTION
         This dryland agroecosystems project is a joint effort of the Colorado State University Department of Soil and Crop Sciences, Colorado Agricultural Experiment Station, and the USDA-ARS Great Plains System Unit in Fort Collins, CO.  This interdisciplinary project was initiated in 1985 and expanded in 1998 to evaluate the bio-control of plant pests in dryland systems by the Department of Bioagricultural Sciences and Pest Management.  The geographic region targeted by this project is the Central Great Plains region of the US where 44,000,000 acres are devoted to dryland crop production.
         As we closed out the 20th century it became evident that the traditional stubble-mulch tillage winter wheat-fallow cropping was neither economically nor environmentally sustainable. Tillage and wheat-fallow cropping practices had resulted in increased erosion by wind and water, and declines in soil organic matter that altered soils properties that in turn decreased precipitation capture and precipitation use efficiency.  In short the system was not environmentally sustainable.  It was evident that a different approach to managing our fragile dryland agroecosystem in this semi-arid environment was needed for the 21st century.

Efficient use of the precipitation is critical if cultivated agriculture is to remain viable in the region.  Dryland agriculture in this region is highly dependent on precipitation, both snow and rainfall.  The water budget in Figure 1 shows how water received as rain or snow can be easily lost before it has an opportunity to be used by a crop.  Water use by weeds and evaporation are the two most negative pathways of water loss that must be avoided if improvements in precipitation use efficiency are to be accomplished.  Each unit of precipitation is critical to production in this highly water limited growing environment.  For example, at Akron, CO each additional inch (25 mm) of precipitation above the initial yield threshold will result in 4.5 bu/A of wheat (12 kg/ha/mm); returns are highly related to water conservation.  Consequently, capture and efficient use of our limited precipitation is vital to the success of dryland farming.

Figure 2.  Water budget for dryland systems.

OBJECTIVES
         The general objective of the project is to identify dryland crop and soil management systems that will maximize water use efficiency of the total annual precipitation and economic return while ensuring environmental sustainability.

 Specific objectives are to:
 1.  Determine if dryland cropping sequences with fewer and/or  shorter summer fallow periods are
            economically and environmentally sustainable.
 2.  Quantify the relationships among climate (precipitation and evaporative demand), soil type and cropping
            sequences that involve fewer and/or shorter fallow periods.
  3.  Quantify the effects of long-term use of no-till management systems on soil structural stability, micro-organisms
            and faunal populations, and the organic C, N, and P content and cycling of the soil as impacted by various
            crop sequences.
 4.  Identify crop management systems that will minimize soil erosion by promoting crop residue maintenance on the
            soil surface.
 5.  Develop a data base across climatic zones that will allow economic assessment of  entire management systems.
 6.  Evaluate the effect of different cropping systems on beneficial and pest insect populations with the ultimate goal of
            determining the bio-control potential of crop insect pests, particularly Russian Wheat Aphid.

EXPERIMENTAL PROCEDURES
         From 1986 - 1997 we studied interactions of climate, soils and cropping systems at three sites, located near Sterling, Stratton, and Walsh, in Eastern Colorado, that represent a gradient in potential evapotranspiration (PET)
(Fig. 2).

Elevation, precipitation and evaporative demand for each site are shown in Table 1. All sites have long-term precipitation averages of approximately 16-18 inches (400-450 mm), but increase in PET from north to south. Open pan evaporation is used as an index of PET for the cropping season.

Table 1.  Elevation, long-term average annual precipitation, and evaporation characteristics for each site.

Site	Elevation	Annual Precipitation 1	Growing Season Open Pan Evaporation 1	Deficit (Precip. - Evap.)
	--Ft. (m) --	---In. (mm) ---	---In. (mm) ---	---In. (mm) ---
Briggsdale	4850 (1478)	13.7 (350)	61 (1550)	-48 (-1220)
Sterling	4400 (1341)	17.4 (440)	63 (1600)	-45 (-1140)
Akron	4540 (1384)	16.0 (405)	63 (1600)	-47 (-1185)
Stratton	4380 (1335)	16.3 (415)	68 (1725)	-52 (-1290)
Lamar	5250 (1600)	14.7 (375)	76 (1925)	-62 (-1555)
Walsh	3720 (1134)	15.5 (395)	78 (1975)	-61 (-1555)

¹Annual precipitation = 1961-1990 mean ²Growing season = March - October
 
 

The cropping system at these sites during the previous 50 years had been primarily stubble-mulch tillage of dryland wheat-fallow with some inclusion of grain sorghum at Walsh and corn at Sterling. At the these three original sites we placed cropping system treatments over the soil sequence (Figs. 2 & 3) to study the interaction of cropping systems and soils.  In 1998 we initiated three new research sites at Briggsdale, Akron and Lamar, CO to evaluate Objective 6 above.  At the three new sites we have only one soil type and the individual plots are larger, ranging in size from 1 to 6 A.  Larger plots were needed to properly evaluate the interaction of cropping systems on insect population dynamics.  Systems being studied at each site are listed in Tables 2a & 2b.  All crops in each cropping system are present in the rotation each year.

Figure 4.  Wheat-corn-fallow system at Sterling CO (note landscape  aspect of experimental design.)

         Prior to 1998, wheat fallow (WF) was the "standard system" with which we compared the new cropping systems.  In 1998, the WF system was discontinued because it had become obvious that WF was not economically viable under no-till management.  We adopted WC(S)F as the "standard system" in 1998 and converted the WF plots to more continuous cropping systems and included soybeans.  Prior to this time we had grown other summer cash crops such as millet and sunflowers.  Millet is a well adopted crop for the region but we discontinued it because of the low price and are now investigating the potential of soybeans as a summer cash crop.  All experiments are managed with no-till techniques which allows us to maintain as much crop residue on the soil surface as possible.

Table 2a.  Cropping systems for each of the original sites in 1999.
  Site                                                                         Rotations
Sterling                                                      1)  Wheat-Corn-Fallow (WCF)
                                                                 2)  Wheat-Corn-Soybean (WCSy)
                                                                 3)  Wheat-Wheat-Corn-Soybean (WWCSy)
                                                                 4)  Opportunity Cropping*
                                                                 5)  Perennial Grass

Stratton                                                     1)  Wheat-Corn-Fallow (WCF)
                                                                 2)  Wheat-Corn-Soybean (WCSy)
                                                                 3)  Wheat-Wheat-Corn-Soybean (WWCSy)
                                                                 4)  Opportunity Cropping*
                                                                 5)  Perennial Grass

Walsh                                                       1)  Wheat-Sorghum-Fallow (WSF)
                                                                 2)   Wheat-Corn-Soybean (WCSy)
                                                                 3)  Wheat-Wheat-Sorghum-Soybean (WWSSy)
                                                                 4)  Continuous Row Crop (Alternate corn & sorghum)
                                                                 5)  Opportunity Cropping*
                                                                 6)  Perennial Grass
____________________________________________________________________________________
*Opportunity cropping is designed to be continuous cropping without fallow, but not monoculture.

Table 2b.  Cropping systems for the new sites in 1999.
    Site                                                                         Rotation

Briggsdale                                             1) Wheat-Fallow (WF)
                                                            2) Wheat-Hay Millet-Fallow (WHF)
                                                            3) Wheat-Wheat-Corn-Soybean-Sunflower-Pea (WWCSySnPea)
                                                            4) Opportunity

Akron                                                   1)  Wheat-Fallow (WF)
                                                            2)  Wheat-Corn-Fallow (WCF)
                                                            3)  Wheat-Corn-Proso-Fallow (WCPF)
                                                            4)  Wheat-Corn-Proso (WCP)

Lamar                                                   1)  Wheat-Fallow (WF)
                                                            2)   Wheat-Sorghum-Fallow (WSF)

SUMMARY OF PROJECT RESULTS
         Grain yields, stover yields, crop residue amounts, soil water measurements, and crop nutrient content have been published annually in Colorado State University Agricultural Technical Bulletins.  Technical Bulletins can be accessed at www.colostate.edu/Depts/AES/ Pubs.  Select TB 98-1 or TB 99-1 from the publications displayed.

Figure 5.  Grain yields averaged over soil position and 14 years of production for each location.

Complete data for each crop are available in previously published bulletins.
         We included yields in Figure 4 from all years, even those where yield losses occurred due to hail, early and late freezes, insect pests, winter kill of wheat, and herbicidal carryover.  Fluctuations in corn and sorghum yields are of most interest because they represent the highest input crops. Corn yields have averaged 65 bu/A (Ranging from 14 to 107 bu/A) at Sterling and 76 bu/A (Ranging from 37 to 112 bu/A) at Stratton.  These averages include the disastrous yields recorded in 1994, which were caused by drought, and the low yields caused by early frost in 1995.  Grain sorghum was produced at Stratton for 4 years and yielded an average of  44 bu/A (ranging from 20 to 63 bu/A), but corn has averaged 76 bu/A for the past 9 years, making it a better choice for this environment.  At Walsh grain sorghum yields have averaged 48 bu/A (ranging from 27 to 75 bu/A), including the results from the very dry 1995 season and severe hail in 1996.  Dryland corn yields at Walsh, using Bt varieties, have averaged 57 bu/A from1997-1999.
         The 3- and 4-year systems like wheat-corn(sorghum)-fallow and wheat-corn-millet-fallow or wheat-sorghum-sorghum-fallow have increased average annualized grain production by 74% compared to the 2-year wheat-fallow system (Figure 5). Yields are annualized to account for the nonproductive fallow year in rotation comparisons.  Economic analysis has shown there was a 25-40% increase in net annual income for the three-year rotation in northeastern Colorado. However, in higher ET region of southeastern Colorado the three year wheat-sorghum-fallow rotation, using stubble mulch tillage in the fallow prior to wheat planting, netted about the same amount of return as reduced till wheat-fallow.  New herbicide programs with fewer residual materials have shown promise and are less expensive.

         Our data have shown that cropping intensification is certainly possible in the west central Great Plains.  More intensive rotations like wheat-corn(sorghum)-fallow and wheat-corn(sorghum)-millet-fallow have more than doubled grain water use efficiency in all three study environments when compared over years.  Water conserved in the no-till systems has been converted into increased grain production. Our opportunity cropping systems have maximized production at all sites relative to all other rotations, but have not been the most profitable.  The 3-year rotations have been most profitable.
         Producers in northeastern Colorado have been adopting the more intensive cropping systems at an increasing rate since 1990.  Corn is one of the principal crops used in more intensive systems, and we use its acreage as an "index" of adoption rate by producers (Table 3).  Area planted to dryland corn in northeastern CO increased from about 20,000 acres per year in years previous to 1990 to 220,000 acres in 1999.  Total dryland corn acreage in Colorado increased from 23,700 historically to 290,000 in 1999.   Corn acreage is expanding into areas once thought to be too dry for corn production as exemplified in Lincoln county where corn acreage increased from 1500 in 1996 to 18,000 in 1999. Adoption of the new systems also is reflected in sunflower and proso millet acreage increases.  For example, sunflower acreage increased from 63,000 in 1991 to 270,000 in 1999.
Table 3.  Dryland Corn Acreage in Eight Northeastern Colorado Counties and total for state from 1971 to 1998.

Year	Eight NE Counties	Total for State
	Acres
1971-1988	21,200	23,700
1989	27,000	28,000
1990	26,000	26,000
1991	32,500	33,000
1992	48,500	50,000
1993	79,000	90,000
1994	92,500	100,000
1995	95,500	100,000
1996	104,000	110,000
1997	138,500	150,000
1998	191,000	240,000
1999	220,000	290,000

 *Data from Colorado Agricultural Statistics (Adams, Kit Carson, Logan, Morgan, Phillips, Sedgewick, Washington, Yuma)
        Intensive dryland cropping systems are adapted to this semi-arid environment, the key to success of these systems is improved water use efficiency.  Water use efficiency can only be maximized by adoption of no-till cropping management.  With the use of herbicides for weed control producers must plan a year ahead to ensure that the herbicides used on one crop do not have a detrimental effect on the following crop.  Since glyphosate is the main herbicide used in weed control in these systems, and as its price continues to decrease, the intensive cropping systems will become even more profitable than they are now.  We, as well as many growers, have concluded that no-till WF definitely is not economically viable.  There is a big question if stubble-mulch tillage WF is a long term economically viable enterprise, especially on rented/leased farmland.  Therefore, we believe that the adoption of intensive cropping systems that include summer crops is the economically sustainable future of the Central Great Plains; a system that also adds environmental stability to this agricultural environment.

The key to the future of Central Great Plains agriculture is the expansion no-till systems with more crop diversity, meaning more markets for farmers.  Our most recent system changes even include the use of Round Up Ready soybean (Fig. 7).  We are testing continuous cropping systems where we plant winter wheat directly into the soybean stubble a few days after harvest.

Figure 8.  Soybean planted no-till into corn stalk stubble in a wheat-corn-soybean continuous crop system.

__________________________
¹Funding is provided by:  Colorado Agricultural Experiment Station; USDA-ARS Great Plains System Unit, and various industry supporters.
²Professors,  Department of Soil and Crop Sciences, Colorado State University, Fort Collins, CO 80523
³Professor, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523
⁴USDA-ARS Technician - Great Plains Systems Research Unit
⁵Research Associate - Department of Soil and Crop Sciences, or Department of Bioagricultural Sciences and Pest Management, Colorado State University
⁶Researchers  -   Plainsman Research Center at Walsh, Colorado
⁷USDA-ARS Research Leader - Great Plains Systems Research Unit