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Chapter 9 — Irrigated Systems, No. 3, Fall 1987

Conservation Tillage Works with Furrow Irrigation

Roger Veseth

Conservation tillage has not been used on firrow irrigated land because it was assumed that fields could not be irrigated satisfactorily with crop residue lefl on the soil surface. Quite to the contrary, recent research has shown that irrigation efficiency can actually improve with conservation tillage. Production costs can also be reduced without reducing yields.

As part of the STEEP conservation farming research effort, research on the use of conservation tillage to control soil erosion on furrow irrigated land has been spearheaded by David Carter of the USDA-Agricultural Research Service. Carter is a supervisory soil scientist at the Snake River Conservation Research Center at Kimberly, ID.

Need for Management Change

Carter's research has demonstrated that topsoil loss because of water erosion under furrow irrigation is a severe problem. Through extensive field studies in southcentral Idaho, he determined that about 75 percent of all fields had sustained sufficient furrow erosion on the upstream ends that subsoils are now exposed. Original topsoil thickness in the area averaged about 15 inches over the white, highly calcareous (high lime content) subsoils. The average portion of the fields exhibiting exposed subsoils was 33 percent. An additional 10 to 20 percent of the field areas had lost significant topsoil but not enough for tillage to expose the white subsoil.

The topsoil loss has had a major effect on crop yield. Carter conducted topsoil depth-yield relationships for a wide variety of crops grown in the region. He determined that, on an overall field basis, crop yield potential has decreased approximately 25 percent from topsoil loss caused by furrow erosion. The soils in the study areas are representative of more than a million acres of irrigated cropland.

Carter stresses that short of.hauling topsoil back on the eroded areas, the erosion damage has proven irreparable under current technology with fertilizers and soil amendments. Implementation of erosion and sediment loss control technologies to limit further yield potential losses is urgently needed. Carter's research indicates that conservation tillage greatly reduces fhrrow erosion and at the same time, reduces production costs without reducing yields.

Field Research

Rotation Selection

Most growers in the study area in southcentral Idaho produce alfalfa as part of their crop rotation. Commonly, alfalfa is grown for 3 years, then plowed out and rowcrops are grown for 2 or 3 years. A cereal crop then follows before replanting alfalfa.

Up to 10 tillage operations are used by some growers in this furrow irrigated area to take out alfalfa and prepare a seedbed for row crops. The operations are done to assure that the alfalfa is killed. The alfalfa plants and roots are buried and the soil is "worked down" or compacted sufficiently that it can be irrigated. This tillage process is expensive and time consuming and leaves the soil highly vulnerable to erosion.

The focus of Carter's research has been to eliminate all or most of the tillage operations when alfalfa is removed from the rotation. In a 1984-85 study, alfalfa was chemically killed in the fall of 1984. Several tillage options were then compared for planting winter wheat, spring wheat and corn.

Tillage Options

Winter WheatFour tillage options were used: (1) "conventional tillage" consisting of disking, moldboard plowing, disking, roller harrowing twice and seeding with a conventional, double-disk, irrigated-land drill; (2) direct seeding with the conventional drill; (3) light disking (about 2 inches deep) parallel to the fimows and seeding with the conventional drill; and (4) direct seeding with a Haybuster no-till drill. The plots were seeded parallel to the furrows with Stephens winter wheat between October 11 and 16.

Spring Wheat – Two tillage options included: (1) fall plowing, spring disking, roller harrowing twice and seeding with the conventional drill; and (2) direct seeding with the conventional drill. Seeding with Fieldwin spring wheat was completed April 8.

Corn – Two tillage options were compared: (1) fall plowing, spring disking, roller-harrowing twice and seeding with a conventional, furrow-irrigation, row crop planter; and (2) direct planting with the conventional, row crop planter. To assure planter penetration and proper seeding depth in the direct seeding, bull tongues (shanks with narrow chisel points) were attached to a toolbar directly ahead of the planter units. Pioneer F 15-3901 corn for silage was seeded by May 2.

In all tillage plots for wheat, furrows were made or cleaned with a conventional corrugating implement. Furrows in the conventional tillage corn were formed during the seeding operation. In direct-planted corn, furrows were cleaned with the conventional corrugating implement.

Irrigation Ease and Efficiency

No problems were encountered irrigating the direct drill or minimum tillage wheat and corn. Water infitration was reasonably even along the length of the furrow. Under conventional tillage, however, water could not be forced to the lower ends of the furrows on the first irrigation because of the high infiltration rate. One day of drying and a second water application were required to irrigate the entire furrow length.

Carter determined that 40 percent more water was required for the first irrigation, and excess water infiltration occurred on the upper one-third of the conventional tillage plots compared to the direct drill or minimum tillage plots. No measurable furrow erosion occurred on the direct drill or minimum tillage plots of either cereals or corn. Significant erosion did occur on conventionallyplanted corn. Furrow slopes ranged from 0.4 to 0.8 percent. Through numerous tillage-soil erosion studies under furrow irrigation, Carter and cooperating researchers have determined that minimum tillage reduces soil erosion by 60 to 90 percent compared to conventional tillage. Direct drill results in an 80 to 100 percent reduction.

Yields and Costs

Yields of winter wheat and spring wheat were about 15 percent above average for the area in 1985. No significant difference was found between yield, test weight or protein content of the different tillage treatments.

Corn silage yields were also above average for the area. Yields on the conventional tillage and direct drill treatments were nearly identical. Production costs for direct drill and minimum tillage wheat and corn ranged from 10 to 30 percent lower than under conventional tillage. Therefore, with no significant yield differences, net returns were that much greater with the conservation tillage systems.

Nitrogen Efficiency

Carter points out that growers can eliminate the need for nitrogen fertilizer by planting spring grain or corn after alfalfa, instead of the common rotation crop of dry beans or other low nitrogen (N)-requiring crop. This is also important to reduce N leaching into groundwater after alfalfa in the crop rotation. The N released from decomposing alfalfa roots and nodules provided adequate N to produce high yields with satisfactory protein contents. Supplemental N fertilizer, up to 100 pounds N/acre, did not increase yields or quality.

Winter wheat yields were increased by adding N fertilizer, but the increase was about equal to the cost of the N fertilizer. Since the alfalfa was killed when soil temperatures had cooled in the fall, decomposition and N release did not progress much until the soils warmed the following spring. Winter wheat needed N earlier at a higher rate than available from the decomposition process. Consequently, the application of some N fertilizer may be needed in the fall or early spring with winter wheat.


Carter concluded that growing cereals or corn after alfalfa under conservation tillage systems can reduce production costs, reduce soil erosion and improve N utilization efficiency while still maintaining yields equivalent to these under conventional tillage. He also concluded that furrow irrigation can be accomplished with less water, time and labor than under conventional tillage, and no specialized equipment is needed. Since this 1984-85 study, his research on the use of conservation tillage on furrow-irrigated land has confirmed these results. The data from his recent research are currently being summarized for publication.


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Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
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