Excerpt from "Steel in the Field"
Details of the Thompsons' fine-tuned weed management system

Chapter 14, Steel in the Field, a SARE publication

Dick & Sharon Thompson
Boone, Iowa

  • 300 acres corn, soybeans, oats, hay, cover crops
  • ridge-till for row crops slightly rolling fields in central Iowa

Weed management highlights

Strategies: early weeds managed to suppress late-germinating weeds... no preplant tillage... increased plant population... 36" rows... fine-tuned cover-crops

Tools: customized ridge-till planter... high-residue rotary hoe... high-residue cultivator... guidance mirror

Dick and Sharon Thompson and their son, Rex, didn't just write the book on sustainable weed control -- they update it with improvements every year. A high-residue cultivator and high-residue rotary hoe are prominent among other sustainable tools and practices in the scientific on-farm testing they have conducted since 1985.

No part of their cropping system exists long without being scrutinized for improvement. They still use cover crops, but have long ago moved away from fall-broadcast mixtures of winter rye and hairy vetch. Vetch posed too many management problems come spring.

For cover crops ahead of soybeans, their experiments found the most weed-suppressing benefit from rye alone, drilled at 20 pounds per acre only on the ridge top--two rows, 6 inches apart, each 3 inches from the crop row. A ridge-till planter can remove spring growth of 8 to 12 inches. Taller rye can be knocked down with a stalk chopper.

Corn's need for early nitrogen and moisture means rye is not a good cover crop choice for corn planted on ridges, the Thompsons found. Rather, they overseed oats at 2 bushels per acre with a high-clearance tractor at leaf-yellowing of soybeans--usually in late August. Freezing weather kills the oats, but stalks remain on the surface to protect the soil from spring erosion.

When they plant corn into a flat field following hay, however, they spread rye through Gandy seed boxes during fall plowing of the hay field as they incorporate strawy manure. The greater weed-suppressive effect of rye is needed for this transition, and standard spring soil preparation allows a way to control the rye that is not available in their no-herbicide, ridge-till-only system. Field cultivators kill most rye at the 6- to 8-inch stage, with scratchers dragging behind to bring the residue to the surface. After waiting about a week for the disturbed weed seeds to germinate, a second field cultivation takes out almost all the rest of the rye. At final weed cultivation with his maximum-residue, single-sweep cultivator, Thompson attaches ridging wings just above sweeps to divert soil into the row to create the elevated ridges that will be used the next several seasons.

"Crop rotation is the key," says Dick Thompson, who tries to maximize soil-building and weed-fighting benefits from the farm's mixed enterprises. Components include hogs, beef cattle and livestock manure, with aerobic digestion of municipal biosolids. The five-year crop sequence is corn-soybeans-corn-oats-hay (legume/grass mix).

Weed populations take a beating from this varied sequence of soil environments, preventing annuals and perennials from strengthening their populations. Existing weed seeds sprout between rows or between plantings--the places and times when light tillage or mowing can control them. Multiple cuttings in hay years knocks back species that thrive in undisturbed soil. The winter cover crops of rye and oats are selected and managed to mesh precisely with the intended crops to follow.

For row crops, Thompson uses ridge tillage, a system that plants rows in the middle of raised soil areas--ridges--that dry out and warm up faster in spring. By planting into the same row area each year, the system controls implement traffic. Ridge-till also can cut labor and fuel costs compared with conventional tillage because there is no pre-plant tillage and less soil is disturbed.

Ridge-till's permanent rows and necessary ridge-restoring cultivation provide the bridge from broadcast spraying to herbicide banding for some farmers. The next reduction can be to lower material rates within the bands. For Thompson and others, ridge-till's weed seed movement into the row middles at planting and faster, closer cultivation have allowed him to virtually eliminate chemical weed controls in most years. In any tillage regime, straighter rows can translate into easier, more efficient mechanical weed control with a lower risk of crop damage.

Thompson advises all farmers to refine management through their own on-farm testing. He offers these cumulative findings for evaluation:

Manage early weeds to control later weeds. Thompson claims this is the "best-kept secret in agriculture." He observes that the first weeds prevent more extensive germination of later-developing weeds. This may result from compounds released from roots or other factors such as shading and moisture competition. He regards early weeds as a natural cover crop. Despite their potential for good, early weeds need to be controlled in the row at planting with timely rotary hoeing or at first cultivation while they are small enough to be easily managed and before quick-maturing species go to seed.

Thompson's row-cleaning, ridge-till planter moves weed seeds and cover-crop residue into row middles as a mulch that protects soil and stifles weed development.

Select a planter that fluffs loose soil over firmly planted crop seed. This leaves crops in a good environment for germination but puts weed seeds in a poor environment for getting started. Be aware that packer wheels working on the surface or trailing scratchers improve weed germination rates, he warns.

Plant crops thicker for quicker in-row shading and to allow for some reduction in plant population from mechanical tillage. Twelve soybean seeds per foot and corn seed at 6-inch spacings work best, Dick Thompson finds.

Rotary hoe before and after crop emergence. He keeps a close eye on planted corn, waiting for seeds to sprout before using his M&W Gear high-residue rotary hoe--but he faithfully hoes soybeans three days after planting, weather permitting. The different approaches result in the same end: he hoes both crops just before emergence thanks to the longer time corn usually waits in cooler soil.

"You have to get off the tractor to check the hoe's penetration, weed kill and crop response," says Thompson. He looks for crop seed disruption or seedling damage. "Then I know how fast to drive and how deep to go." These preemergence passes are shallow (about an inch) and fast. "Fifteen miles per hour is about as fast as I can hang on," he admits. Extreme weed pressure justifies an immediate second trip, which Thompson does by half-lapping the previous round.

He waits about a week before hoeing a second time. At this point, corn is at about the two leaf stage (the third leaf is just beginning to emerge from the whorl) and soybeans have true leaves. These early "broadcast" tillage passes are important to suppress weeds when they are easiest to kill, and especially to control in-row weeds.

His M&W Gear high-residue rotary hoe handles residue well. Thompson believes that hoes need at least 20 inches between front and rear shafts to handle heavy residue. Rotary hoeing returned an average of $16.20 per acre more than even banded herbicides did in a three-year weed-control comparison, Thompson reports.

Outfit your cultivator for young crops--and have it ready to run! His standard practices at first cultivation include

  • Disk hillers with the leading edge angled into the row and cupped forward to make a cut as wide as possible that still moves soil away from the row. The hillers are set 5 inches apart--but even "tighter on the row that I watch," explains Thompson, to narrow his margin of error.
  • A "Culti-Vision" rearview mirror. This hooded, adjustable guidance aid is mounted on a bracket attached to the tractor frame just behind the front axle. It is aligned so the driver can see exactly where the hillers are running next to the crop. The mirror improves the driver's control enough to run about 3.5 mph at a stage when crop plants are too small to activate the sensing wands of an electronic guidance system. By also using a wide-angle, rear-view mirror inside the cab, Thompson can scan all four rows without turning around.

"Guys who use herbicides can afford to wait for four-inch corn that will activate guidance system wands. But those of us who are all-mechanical have to be out there earlier to stay ahead," he says. One species he particularly targets for early control is lambsquarter. Once the tap-rooted competitor is 6 inches tall, it becomes difficult to kill with a cultivator. Broadleaf weeds that have less of a tap root are easier to control when they are taller.

Use of 26-inch one-piece sweeps in 36-inch row spacings. "I need to be able to increase the pitch to get some soil-turning action," Thompson finds. "The flat, narrow surface of point-and-share sweeps basically undercuts weeds. I want to be harder on my weeds than that."

Metal tent shields with 18-inch rear extensions dragging in the soil. The shield fronts stay down if rows are clean--to keep out fresh soil with its newly exposed weed seeds--but are raised slightly to allow loose soil to flow in if enough small, in-row weeds are threatening. The rear extensions add extra stability and crop protection. At second cultivation, he replaces the tents with open-top, flat panel shields to avoid bending crops.

Stopping, dismounting and stooping low to inspect crops and weeds. Where he plans a hay crop and doesn't need ridges the coming year, Thompson does a second cultivation with wide sweeps before crops are a foot tall. He changes the angle of the disk hillers to throw soil toward the crop row, and moves them farther away from the row to avoid throwing soil around the plants. In soybeans, this prevents damage to lower pods and keeps mounded soil from interfering with harvest. In corn it limits root pruning that can hurt yield. His tests show that ridging in fall after corn harvest and removal of stalks can increase weed numbers in the following year's soybeans.

In wet seasons when crops are much more than a foot tall before he can make a second pass, Thompson uses a 14-inch sweep without ridging wings to avoid cutting crop roots.

Where he wants to build a ridge, Thompson begins the second cultivation with 14-inch sweeps after crops are a foot high. At that point the plants can tolerate contact by flowing soil that smothers in-row weeds. He moves the disk hillers to the row middles and turns them to push soil into the row. A "butterfly" ridging wing mounted on the shank behind each sweep diverts soil from the middles to the row ridge, smothering in-row weeds. Two sets of open-top row shields ride several inches off the soil surface next to the row to protect crop stalks and leaves.

Because bigger plants block his view of the hillers via the Culti-Vision mirror, Thompson removes it and uses his pivot-type automatic guidance system as crops mature. The electronic-hydraulic unit guides the implement to keep it in alignment with the row. Guidance greatly eases driver stress and allows Thompson to travel about 6 mph.

Night cultivation and tilling has yet to show consistent benefits in USDA-supervised tests on Thompson's farm. In other trials, the practice has reduced post-tillage germination of small-seeded broadleaf weeds. As part of his on-farm research in '96 with researchers from the National Soil Tilth Laboratory in Ames, he used night-vision goggles for nocturnal ridge-till planting. The goal is to deprive light-activated weed seeds the illumination necessary to trigger germination.

Improved implements and mechanical techniques hold the best opportunities for making weed management more sustainable, says Thompson. "If you use herbicides and still don't control weeds, you're building herbicide resistance. If you do control your target weed, you get weed-species succession and end up with weeds that are harder to control." He can trace a troublesome weed progression from horseweed to foxtail to velvetleaf during his own farm's "chemical era."

In '95, the Thompsons and other farmers who successfully practice integrated weed control with negligible herbicides formally described their systems to gatherings of weed management professionals. From those sessions, Dick Thompson sees a new appreciation for the importance of "alternative" practices.

"Weed scientists are finally taking field ecology seriously, talking about 'managing' weeds rather than 'controlling' them, and saying herbicides should be the last resort. I think they're on the right track."


This chapter in SARE's Steel in the Field is viewable online at www.sare.org/steel/item14.htm
This and other books from the Sustainable Agricultural Network are available at:
www.sare.org/htdocs/pubs.