facilitates getting the second crop planted quickly and limits potential moisture losses from the germination zone in the seedbed, allowing greater flexibility in cropping sequence or rotation (Sandretto and Bull, 1996).

The 1988-95 Cropping Practices Surveys (CPS) provide detailed data on residue levels and tillage systems for individual field crops in major producing States (for more discussion, see "Cropping Practices Survey" in the appendix). The advantages of the CPS for analysis of CRM is that it allows the linking of CRM practices to other relevant details about the farm production system, such as the type of tillage equipment used and the number of trips made over a field. These annual surveys indicate a decline in the use of the moldboard plow and other conventional tillage systems and an increase in the use of all types of conservation tillage for most of the major field crops. Less than 10 percent of the surveyed area in major producing States used a moldboard plow in 1995, down from 20 percent in 1988.

Corn. Tillage systems used for corn production in the 10 major producing States indicate a trend toward the use of conservation tillage systems (table 4.2.2). No-till systems were used on 17 percent of the acreage in 1995, up from only 5 percent in 1989, and exceeded 20 percent in several Corn Belt States. Ridge-till systems increased to 3 percent of the total acreage, but this expansion was mainly confined to Nebraska and Minnesota. A moldboard plow was used on 8 percent of 1995 corn acres, down from 20 percent in 1988.

Soybeans. Soybean production also indicated a trend toward greater use of conservation tillage systems. The 14 major soybean producing States were divided into northern and southern areas. The northern area showed a steady increase in no-till system use from 3 percent of the acreage in 1988 to 30 percent in 1995. At the same time, mulch-till increased from 14 to 24 percent and use of the moldboard plow dropped from 28 to 8 percent. The small share of soybean acreage with ridge-till was located mainly in Nebraska and Minnesota, where some soybeans are grown in rotation with ridge-till corn. The southern area increased no-till system use from 7 percent of the acreage in 1988 to 25 percent in 1995.

Cotton. Nearly all cotton was produced using conventional tillage methods in the six major cotton States. However, use of the moldboard plow decreased to less than one-half of the 1988 level. Arizona, California, and parts of Texas have State

"plow-down" laws requiring that the cotton plant be disposed of to eliminate the over-winter food source for bollworms and boll weevils. Some producers have misinterpreted these laws to mean that the previous crop must be plowed under with a moldboard plow. California producers mainly use multiple passes with a heavy disk. In some areas of Texas, the moldboard plow is also used to bring up clay subsoil in order to cover the soil surface with clods to help control wind erosion. The large number of tillage trips across the field (averaging 6.1) leaves very little residue, even without use of the moldboard plow. Research is being conducted in a number of cotton producing States on the use of strip-till and no-till systems and the "stale seedbed" system, which uses cover crops or weeds to provide vegetative cover on the field from harvest to the next planting season.

Winter Wheat. Except for 1994 and 1995, a steady decline in moldboard plow use occurred in winter wheat production since 1988 (table 4.2.2). Meanwhile, no-till and conventional tillage without the plow showed a corresponding increase. The heavy rains and flooding in some States during 1993 affected planting of the 1994 crop. Siltation from flooding and the impact from heavy rains may have contributed to increased use of the moldboard plow in 1994 and 1995 (Bull and Sandretto, 1996).

Spring and Durum Wheat. Variations in the type of tillage system used in the production of spring and durum wheat may be partly due to weather-soil relationships in the areas producing these crops. Much of the wheat produced in the Great Plains and the Western States is grown after a fallow period. Implement passes made during the fallow year are included in determining residue levels, hours per acre, and trips over the field. Normal fallow procedure in these regions starts with chisel plowing and other noninversion tillage operations in the fall instead of a pass with a moldboard plow. For these regions, therefore, more trips over the field occur under conventional tillage without the moldboard plow than for tillage with the moldboard plow.

Factors Affecting CRM Adoption

The trend toward adoption of conservation tillage and a corresponding decline in clean tillage has been stimulated by the prospect of higher economic returns with conservation tillage and by public policies and programs promoting conservation tillage for its conservation benefits. The major limitations to adoption of soil-conserving tillage systems for some farmers include additional management skill requirements, expectations of lower crop yields and/or

economic returns in specific geographic areas or situations, negative attitudes or perceptions, and institutional constraints.

Prospects for Higher Economic Returns

Higher economic returns with CRM result primarily from some combination of increased or stable crop yields and an overall reduction in input costs, with both heavily dependent on characteristics of the resource base and appropriate management (Clark and others, 1994).

Yield Response. Yield response with soil-conserving tillage systems varies with location, site-specific soil characteristics, climate, cropping patterns, and level of management skills. In general, long-term field trials on well-drained to moderately well-drained soils or on sloping land show slightly higher no-till yields, particularly with crop rotations, compared with conventional tillage (Hudson and Bradley, 1995; CTIC, 1996). Experienced no-till farmers claim greater yields from increased infiltration and improved soil properties such as reduced erosion and soil compaction, increased soil organic matter and earthworm activity, and improved soil structure (tilth) in 4-7 years from when the system becomes established (CTIC, 1996). A mulch-till system may be more appropriate where soil varies greatly within a field, where pre-plant incorporated herbicides are used for weed control, or where equipment or management limitations preclude the use of no-till or ridge-till (CTIC, 1996).

The benefits from improved moisture retention in the root zone that derive from reduced water runoff, increased infiltration, and suppressed evaporation from the soil surface-usually increase crop yields, especially under dry conditions. In some areas of the northern Great Plains, these benefits permit a change in the cropping pattern to reduce the frequency of moisture-conserving fallow periods (Clark and others,

1994).

Increased crop residue on the soil surface tends to keep soils cooler, wetter, and less aerated (Mengel and others, 1992). These characteristics under cool, wet planting conditions, especially in some Northern States, have been blamed for delayed plantings, uneven stands, and lower corn yields (Griffith and others, 1988). However, with hot, dry weather later in the growing season, the effects of increased organic matter, improved moisture retention and permeability, and reduced nutrient losses from erosion all benefit crop yields. No-till is particularly well suited for double-cropping because farmers can plant