In dryland agriculture, crop production is entirely dependent upon precipitation as the source of water. To maximize the usefulness of this water, producers need to save as much of it as possible and use the management practices that provide for its most efficient use. Unfortunately, many producers still do not recognize the importance of soil water conservation strategies that provide them with the additional capability to increase the amount of soil water available for crop production. Advances in minimum- and no-till equipment technology has provided additional opportunities to conserve soil water and to manage seed-zone soil moisture to obtain optimum plant populations for crops grown in minimum- or no-till crop production systems.

The focal point of soil moisture management and water conservation is crop residue (stubble) management. Residues simultaneously reduce water evaporation rate, retain precipitation (especially snow) in place and improve infiltration into the soil. Effective water conservation, and hence soil moisture management, is realized when residues are retained at the soil surface.

The purpose of this paper is to discuss the role of crop residue management in conserving soil water in conventional-, minimum- and no-till cropping systems during seasonal segments of the noncrop period.

It is important to conserve the precipitation received in August, September or October following harvest of cereal grain crops. The opportunity to conserve water in the soil profile before soil freeze-up diminishes when full-season crops like sunflower are grown compared to small grains. Crops like winter wheat or spring barley, which are harvested in late July or early August, widens the time between harvest and fall soil freeze-up and increase the opportunity to conserve precipitation received during that time. Conversely, full season crops, like corn, sunflower or safflower, use water into September, and thereby limit the time between harvest and soil freeze-up that precipitation can add to the stored soil water supply.

Aside from the effect of the length of growing season of the previous crop on the amount of water potentially available for storage before winter, there are two other management decisions that affect potential water storage that must be made at time of harvest and following harvest, particularly with cereal crops. First, the cutting height of the swathing or combine platform must be adjusted for stubble height most effective for snow trapping. (Stubble height as related to over-winter soil water storage will be discussed in the following section.) The combine should also be equipped with a straw and chaff spreader that spreads the crop residue over the platform width from which it originated. Second, the choice of methods of control of weeds and volunteer grain must be made and the decision implemented. A dense population of volunteer spring grain can remove 1.2-inches of stored soil water from mid-August to soil freeze-up time.

Weed control in the fall should be accomplished without destroying the upright position of the stubble of the previous crop so as not to destroy the snow trapping potential. Some wide V-blade or flat-sweep type implements can be used to undercut the stubble and control weeds and volunteer grain and still leave the stubble in an upright position to trap snow. Herbicides, can also be used for the control of weeds and volunteer grain, thus leaving the stubble upright.

1/ Soil Scientists USDA-ARS, Northern Great Plains Research Laboratory, P.O. Box 459, Mandan, ND 58554.

Researchers in the northern Great Plains of the United States and Canada have documented the additional soil water conservation aspects of standing stubble compared to use of fall tillage systems that flatten or bury the stubble. Over the 45 location-year studies shown in Table 1, the average overwinter soil water gain was 1.54-inches greater for upright stubble (10- to 17-inches tall) than when the stubble was flattened or incorporated by tillage.

TABLE 1. Research comparisons of overwinter soil water storage for standing stubble versus stubble flattened or incorporated by tillage in the Northern Great Plains.

Over a 12-year period at Mandan, North Dakota, Bauer and Black (1990) made comparisons of no-till stubble heights categorized as short (2-inches), medium (8 to 10-inches) and tall (13 to 15-inches) in relation to overwinter soil water conservation. The overwinter change in soil water content from fall to spring to a depth of 5 feet is shown in Table 2. The average gain in soil water storage overwinter (12-years) for short, medium and tall stubble was 2.5, 3.1 and 4.1 inches, respectively. For these same years nearby black fallow land gained an average of only 1.3 inches.

TABLE 2. Overwinter change in soil water content to 5 feet as affected by stubble height management (Bauer and Black, 1990).

*Short (2-inches)

Medium (8 to 10-inches)

Tall (13 to 15-inches)

In this no-till stubble height management study by Bauer and Black (1990), stubble heights were established by adjusting the combine platform height at time of harvest for the medium and tall stubble heights with remainder of straw passing through the combine being chopped and spread on the soil surface over the width of the platform. The short stubble height was established by mowing the stubble at a 2-inch height returning most of the stubble to a flat position on to the soil surface. Because of the relatively large amount of surface residue present with the 2-inch stubble height, soil water gains were 1.2 inches greater than nearby black fallow and the medium and tall stubble heights gained 1.8 and 2.8 inches more than black fallow.

Bauer and Black (1991) have shown that the grain yield per inch of water used in evapotranspiration after the initial yield point is reached averaged 5.1 bushels per acre per inch of water for wheat. Therefore, the additional soil water gained by short, medium and tall stubble heights should provide wheat yield gains of 6.1, 9.2, and 14.3 bushels per acre over the yield gain from the additional water gain on black fallow.

Assuming the stubble was managed in an upright position overwinter to trap snow, the producer should make an estimate of the quantity of residue present in the spring at the beginning of summer fallow. If residue production from the previous crop was less than 2500 pounds per acre, a producer should consider using only a chemical fallow program. If residue production was in the 2500 to 4000 pounds per acre range, then a producer should consider a combination of minimum tillage and herbicide sprayings.

During the past 7-years, crop-fallow strips have been maintained on section 20 of the Area IV SCD-USDA, ARS Research Farm with crop residue levels being managed by using minimum-till and/or no-till concepts to provide30 to 60% cover after 21 months of fallow. As shown in Table 3, residue cover was maintained above 900 pounds per acre (30% cover) after summerfallowing in all years except 1989 following the 1988 drought year when residue production was only 1080 pounds per acre at the start of the 21-month fallow period. These data illustrate that a target residue level of 30% cover was achieved in 7 out of 8 years using minimum- and no-till practices.

TABLE 3. Tillage and/or herbicide spray operations used during summerfallow to achieve 60% cover based on crop residue present in the spring of the summer fallow period.

1/ Used Haybuster undercutter with 32-inch V-blades.

2/ Used Roundup alone or in combination with 2,4D. (We often used Landmaster BW late in the fallow season.)

Spring wheat grain and straw yields on summer fallow have averaged 34.4 bushels per acre and 3090 pounds per acre, respectively, using minimum- and no-till practices on the Area IV SCD Research Farm. Most importantly, the additional soil water conserved by controlling fall weeds, leaving the stubble upright to trap snow, and practicing minimum- and no-till summerfallow has increased spring wheat grain and straw production by 56% over the Morton County average of 22.0 bushels per acre and, 1980 pounds per acre, respectively. Since minimum- and no-till fallow practices enhance soil water conservation, grain and straw yields are improved and the additional crop residue produced will sustain a higher level of soil water conservation and protection from erosion than conventional-till systems. When soil water conservation is improved, crop yields and crop residue production are enhanced, thus making the system somewhat self-perpetuating as adequate levels of crop residue are sustained for soil and water conservation purposes.

In spring grain-summer fallow cropping system, the second winter period is of major concern from the standpoint of maintaining sufficient residue cover to protect the soil from wind erosion. In this respect, minimum- and no-till fallow methods have the potential to reduce relative wind erodibility about 10-fold (Black and Power, 1965).

The amount of precipitation stored in the soil during the second winter of fallow averages about 10% of the total quantity stored during 21-months (Black, 1969). During an 11-y@ar period, Black (1969) showed soil water storage ranged from a net loss of 0.8 inches to a gain of 1.2 inches following a severe drought year. In 4 of 11 years, there was a gain of 0.6inches or more of soil water but likewise in 4 out of 11-years there was either no gain, or a loss, of stored soil water. These results are readily understood if one considers that the soil water stored is near the maximum storage capacity during this period, therefore little or no additional water can be stored. In addition, soils freez3 during this period and fallowed soils frozen with a high water content ii the upper two feet virtually prevent infiltration of snowmelt or rain while in the frozen state.

Our experience over an 8-year period with minimum- and no-till cropping systems on the Area IV SCD Research Farm has demonstrated the advantages of using only the undercutter or a sprayer alone, or in combination, for residue management in annual cropping or crop-fallow systems. Surface residue management is the key to water conservation and seed-zone soil moisture management for improved crop yields. The importance of plant population to grain yield was shown by Black and Bauer (1990) in winter wheat survival studies.

Desirable seed zone soil moisture management requires the proper use of available crop residue (position and orientation) and/or the use of the undercutter operated at a shallow depth (no deeper than 2 to 2.5 inches).Producers need to manage crop residue and tillage depths to keep soil moisture within reach of the capabilities of the seeding equipment used so that the seed can be placed into moist soil at the optimum planting depth. With about 50% cover or more, soil moisture usually will be present at about the 2-inch depth sufficient for seed germination. If residue cover is low, below 30%, soil moisture may disappear to the 4 to 5-inch depths. When residue levels are less than 50%, an undercutter operated at a shallow depth in early spring will preserve seed zone soil moisture at the desired shallow depth.

As an example, the minimum-till system we developed for sunflower production is a crop residue-soil moisture management system. We have ongoing studies of both a spring wheat-winter wheat-sunflower rotation and a spring barley-winter wheat-sunflower rotation. The winter wheat stubble (12-15 inch height) is left standing over winter to trap snow. In mid-April we apply Sonolan G-10 granules with a granular applicator mounted on the front of the undercutter while simultaneously undercutting the stubble at a 2.0 to 2.5-inch depth. The undercutter operated at a 2-inch depth in mid-May constitutes the second tillage incorporation of the herbicide. The sunflower is planted following the second undercutter operation with a no-till, offset-double disk, unit row planter. In each of the past 8 years, we have obtained sunflower plant populations of 18500 to 21000 plants per acre from a seeding rate of 22000 seeds per acre. Soil moisture was adequate for seed germination at the 2-inch depth even when no April-May precipitation occurred in 1988 and 1989.

Reference Cited

1. Bauer, Armand and A. L. Black. 1990. Effects of annual vegetative barriers on water storage and agronomic characteristics of spring wheat. North Dakota Agric. Exp. Stn. Res. Rpt. No. 112. 16p.
2. Bauer, Armand and A. L. Black. 1991. Grain yield production efficiency per unit of evapotranspiration. North Dakota Agric. Exp. Stn., North Dakota Farm Res. 48:15-20.
3. Bauer, Armand and Henry L. Kucera. 1978. Effects of tillage on some soil physiochemical properties and on annually cropped spring wheat yields. North Dakota State Univ., Agric. Exp. Stn. Bull. No. 506.
4. Black, A. L. 1969. Summer fallow conserves soil water? Montana Farm-Stockman 53:40-41 (April 7).
5. Black, A. L. and Armand Bauer. 1990. Stubble height effect on winter wheat in the Northern Great Plains: 11 Plant population and yield relations. Agron. J. 82:200-205.
6. Black, A. L. and J. F. Power. 1956. Effect of chemical and mechanical fallow methods on moisture storage, wheat yields, and soil erodibility. Soil Sci. Soc. Amer. Proc. 29:465-468.
7. Black, A. L. and F. H. Siddoway. 1977. Winter wheat recropping on dryland as affected by stubble height and nitrogen fertilization. Soil Sci. Soc. Amer. J. 41-1186-1190.
8. Smika, D. E. and C. J. Whitfield. 1966. effect of standing wheat stubble on storage of winter precipitation J. Soil and Water Conserv. 21:138-141.
9. Staple, W. J. 1964. Dryland agriculture and water conservation. IN Research on Water, ASA Spec. Pub. No.4, So Sci. Soc. Amer., Madison,WI.