A terrace design for gentle slopes
A new type of terrace is lining the southern Iowa landscape. Grass-front farmable back-slope terraces are a growing trend on gently sloping cropland (1% to 6% slopes). These types of terraces are getting more popular because they are less expensive to construct than broad-base terraces and easier to farm around than narrow-base or grass back-slope terraces.
The first grass-front farmable back-slope terraces in Iowa were constructed just over a decade ago in Keokuk and Jefferson counties. Also called reverse grass back-slope terraces, these terraces allow farming right up to the edge of a gentle slope of vegetation that helps reduce the rate of runoff and reduce soil erosion.
State technician Scott Shifflett with the Iowa Department of Agriculture and Land Stewardship’s Division of Soil Conservation, or IDALS-DSC, says about 75% of the terraces they construct in Keokuk County are grass-front farmable back-slope terraces.
These terraces are considered storage terraces, much like a traditional grassed back-slope terrace, where water is collected and stored until it can infiltrate into the ground or is released through an outlet.
Doug Morningstar, area engineer with USDA’s Natural Resources Conservation Service in Williamsburg, says they originally designed grass-front farmable back-slope terraces as a modified broad-base terrace. Broad-base terraces, which are farmed on all sides, are terraces built on flatter ground.
Here are some of the other benefits of grass-front farmable back-slope terraces:
• Intake is against the front slope. The location of the intake makes navigating a tractor much easier and causes less damage to equipment. “The intakes are much easier to avoid while picking corn,” says Bill Bolinger, soil conservation technician for NRCS in Creston. “We have been offsetting the intake upwards 5 feet to 7 feet of the channel. There will be slight deviation in row pattern on both sides of the terrace instead of one larger deviation on the bottom side.”
• Less ground out of production. Compared to narrow-base terraces, where a farmer may need to swerve as much as 20 feet with a planter, grass-front farmable back-slope terraces allow for a much straighter pass.
• Reduced damage from heavy rain events. Farmers with grass-front farmable back-slope terraces are reporting less damage following heavy rains than farmers with other types of terraces. For example, when narrow-base terraces overtop, they will generally blow out clear to the bottom — a repair that requires a bulldozer to fix.
“If grass-front farmable back-slope terraces overtop, they will most often have sheet erosion where there is a lot of small areas along the backside that need filled in and repaired,” says Phoebe Wiley, soil conservation technician with NRCS in Fairfield. “In a lot of cases, this can be fixed much easier by the farmer, using a tractor with a loader bucket or a small scraper.”
• Less maintenance. The farmable backside of these terraces reduces the grassed slope, which has a tendency to grow up in trees and weeds if not maintained properly. “A farmable slope is also less attractive to varmints that like to dig holes in terraces,” says Wiley.
• Less expensive than broad-base terraces. On gentle slopes, where grass-front farmable back-slope terraces are appropriate, there is a cost savings compared to the cubic yards of soil needed to build a broad-base terrace. On the average, these types of terraces cost about 50 cents less per linear foot to construct than broad-base terraces.
• Contractors like them. Dirt movers prefer grass-front farmable back-slope terraces over broad-base terraces, due to reduced yardage. “This helps to reduce the problem of damaging existing shallow lines in the borrow area,” says Bolinger.
Establishing vegetation an issue
A potential problem with all terraces is keeping proper vegetation established. With grass-front farmable back-slope terraces, killing the vegetation on the
grass-front slope with the sprayer is a common problem.
“We encourage contractors to seed right behind the dozer,” says Wiley, “where the tracks make for a good, firm seedbed. Once the seeding is established, we struggle with commercial sprayer applications, in particular, spraying the front side killing the seeding.”
Wiley says leaving the front bare is an issue because the front toe will show signs of erosion after heavy rains. “Most of that soil sloughs off and falls around the intake, potentially affecting the entire function of the terrace,” she says.
If a farmer receives funding for terraces through one of the federal, state or local financial assistance programs, a maintenance agreement holds the farmer accountable to reseed a bare slope as soon as possible.
Financial assistance for terraces
NRCS offers financial and technical assistance for terraces through the Environ-mental Quality Incentives Program, or EQIP. In fiscal year 2011, NRCS offered farmers a payment of $1.85 per linear foot to construct grass-front farmable
IDALS-DSC and local soil and water conservation districts may also have programs available that help finance the cost of building terraces. For more information about terraces, visit your local NRCS office.
Johnson is a public relations specialist for NRCS in Iowa.
This article published in the October, 2011 edition of WALLACES FARMER.