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