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Posted March 9, 2006: You might not think
much about the mice in your fields, but these pint-sized seed
predators are proving to be powerful “little hammers”
in the war against weeds, according to new research from Iowa
State University (ISU).
“In sustainable farming systems where you don’t
rely heavily on the ‘big hammer’ (ie. herbicides),
seed predation from mice and insects matters a great deal,”
said Matt Liebman, an ISU agronomist. “While an individual
weed management technique or ecological process is usually
inadequate for suppressing weed populations, a combination
of ‘little hammers’ can get the job done.”
For the last four years, Liebman and fellow ISU researchers
have been keeping a detailed account of what goes into and
what comes out of the bank of weed seeds in the soil. They
measure the number of seeds in the soil at the start of every
growing season, count the number of emerged seedlings, and
determine the number of new weed seeds produced and shed back
onto the soil.
“Using simple input-output accounting methods, we
figure that 70 to 90 percent of the weed seeds that should
be in the soil aren’t there,” Liebman noted.
Where are they going? Right down the gullet of a surprising
weed management star—the prairie deer mouse, Peromyscus
maniculatus. Just over 6 inches in length (including
its 3-inch tail), this weed seed connoisseur can adapt to
many North American habitats but prefers open spaces in crop
fields. It tag teams with its cousin, the white-footed mouse,
who prefers gobbling weed seeds along field edges. Since neither
species hibernates, these tiny dynamos remain active throughout
the winter months and eat weed seeds year round.
“These guys are working hard around the clock every
day,” said Brent Danielson, a biologist from ISU’s
Department of Ecology, Evolution and Organismal Biology. “We
found that they’re out there in significant numbers
and can do a lot of weed killing.”
Scientists track velvetleaf, giant foxtail
The seed predation research is part of a larger project that
began in November 2002 on the ISU Marsden Farm in eastern
Boone County, Iowa. Liebman and fellow ISU agronomist Bob
Hartzler set up 36 plots, each 60 feet by 275 feet, to study
the effects of different crop management systems on velvetleaf
and giant foxtail. The study includes three rotations:
- A conventional two-year system (corn-soybean).
- A three-year system suitable for producers with a need
or market for small grains (corn-soybean-triticale underseeded
with red clover).
- A four-year system suitable for producers with livestock
on forage (corn-soybean-triticale underseeded with alfalfa-alfalfa
for hay).
Each phase of each rotation occurs every year. During the
past three cropping seasons, measurements have been taken
to determine weed seed longevity in the soil, weed seedling
emergence and survival, weed seed production, and weed seed
loss to insect and animal predators.
Predation losses were determined by lightly gluing giant
foxtail and velvetleaf weed seeds onto squares of sandpaper.
The squares were placed throughout the fields for 48-hour
periods between May and November in 2003 and 2004. Some of
the cards were placed in cages designed to keep out the mice
and other vertebrates. After each card was collected, researchers
counted the number of seeds left on the card.
Averaged over 27 sampling periods in 2003 and 2004, about
a third of the velvetleaf seeds and half of the giant foxtail
seeds were lost to predators within two-day periods. In 2003,
the loss of velvetleaf seeds to predators was greater in the
four-year rotation than in the two-year rotation.
“When the rodents are allowed to forage for weed seeds,
they can consume more than 40 percent of the seed in a single
night,” Danielson noted.
Overall, seed predation patterns in the different crops
were complementary. For example, in triticale the largest
number of weed seeds was consumed in the spring, whereas weed
seed consumption in corn and soybean was greatest in late
summer and early fall.
While ISU researchers don’t have data on how different
field sizes impact seed predation, they know mice are abundant
in the borders and in the middle of the fields. Mice are much
less likely to be found in mowed areas, Danielson noted. Seasonal
moisture levels may also impact seed predation. Mice can move
under layers of snow and may be more active in the winter
than previously suspected, although there’s no research
yet to confirm this, Danielson said.
Predation slows a fast-growing problem
One concern about more diversified cropping systems is the
possible influx of annual weeds in the three-and four-year
rotations when red clover and alfalfa are grown with the small
grain crop. Paula Westerman, a research associate in the ISU
Agronomy Department, used data collected in the study to model
long-term effects of management and seed predation on weed
seedbanks.
In these fast-growing weed populations, seed predators have
tremendous impact, Westerman said. “A 25 percent loss
of weed seeds due to predators may be enough to reduce increases
in giant foxtail populations,” she said. “In other
words, predation can reduce the risk of weeds [overall].”
Are mice and other seed predators picky about their food
options? It depends. Research from the University of Maine
has shown that when ground beetles (Harpalus rufipes)
are offered a choice of weed seeds, they prefer barnyard grass
seeds to wild mustard seeds. “Most animals and insects
do have a preference when they have a choice of food,”
Danielson said. “With seeds, this preference could be
tied to various chemicals in the seed coat, or how easy or
hard it is to chew through the seed coat.”
While mice eat velvetleaf seeds and foxtail seeds, ISU researchers
theorize that foxtail is their favorite, since it’s
easier for the mice to chew through the seed coat. Picky eaters
aren’t a limiting factor with seed predation, however,
Liebman said. “Research has been conducted around the
globe, and I’ve never run into a study where none of
the weed seeds were taken.”
In addition, research has shown that mice (and ground beetles)
can harvest more weed seeds than they can eat and they store
their surpluses below the ground. “The seeds are buried
below the plow line, so they’re down in the ground eight
to 10 inches,” Danielson said. “Seeds buried at
this depth can’t emerge.”
Do mice pose a threat to crop seeds? “We haven’t
experienced lots of mice-related losses in terms of crop establishment
or yields,” Liebman said. “Even if a mouse knows
a crop seed has been planted, it’s hard to reach the
seed due to the planting depth. There’s really no downside
to seed predation that we’ve observed.”
Evaluating the economics of the rotations
ISU Extension field economist Craig Chase has calculated
costs and returns for the three cropping systems from the
weed research averaged over three seasons. Herbicide use in
the four-year rotation was 78 percent lower than in the two-year
rotation, which used herbicides (but not Roundup®) at
conventional rates. Reductions in herbicide use in the three-
and four-year rotations were achieved by the use of banded
sprays, rotary hoeing and inter-row cultivation in corn and
soybean years, and the elimination of herbicides in years
when triticale, red clover, and alfalfa were grown.
The four-year rotation used 73 percent less synthetic nitrogen
fertilizer than the two-year rotation. The longer rotation
relies on nitrogen fixation by alfalfa and application of
a low rate of cattle manure (seven tons per acre) prior to
corn. “Despite the management differences, our corn
and soybean yields have been essentially the same in all systems,”
Liebman said.
Production costs in the three- and four-year rotations were
lower by 23 percent and 28 percent, respectively, than the
two-year rotation. Labor requirements were 59 percent and
81 percent higher in the three-year and four-year rotations.
Averaged over the three-year period of 2003-2005, returns
to land, labor and management were greatest in the four-year
system at $192 per acre, least in the three-year system at
$160 per acre, and $173 per acre in the two-year system.
How to make mice work for you
The bottom line? The key to enhancing seed predation is
to keep weed seeds on the soil surface as long as possible
by delaying tillage. “The longer seeds remain on the
surface, the more likely they’ll be destroyed,”
Liebman said. ISU scientists are now conducting research to
find out, in more detail, how the timing of tillage affects
weed seeds.
In addition, it's probably advantageous to provide a longer
opportunity for seed removal during the season by having a
diversity of crops that comes from a longer rotation, Liebman
said. In corn and soybean crops, the demand for weed seeds
by predators is high only during the middle and later parts
of the summer, but the triticale and alfalfa provide a better
season-long habitat for predators.
Finding more answers
The ISU weed seed research builds on work funded by the
Leopold Center for Sustainable Agriculture, and scientists
have received a three-year $499,500 grant from USDA’s
Cooperative State Research, Education and Extension Service
to support a national research initiative on the biology of
weedy and invasive species that cause environmental damage
and losses adding up to more than $100 billion per year.
Upcoming research will include:
- Special cages to regulate access to seeds between November
and April so the scientists can measure rates of weed seed
predation that occurs after the harvest of one crop and
before planting another.
- An examination of DNA in the feces of mice live-trapped
during the winter to determine what and where the animals
have been eating.
- The impact of corn stalks and other residue on seed predation
during the winter. “We also hope to learn more about
how different crop types, tillage methods and rotations
may affect the abundance of mice in the winter and how these
factors impact their diet,” Danielson said
- The impact of soil quality/biological activity on seed
predation.
ISU researchers are also applying for a grant so they can
study whether soils in organic farming systems are more likely
to have higher seed predation rates. “We’re really
interested in the effects of different crop habitats and tillage
practices on seed predation,” Liebman said. “We
know huge numbers of mice are already in the fields eating
weed seeds every day, so we want to help farmers benefit as
much as possible from this free weed-control service.”
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