Nutrient Boost for Alfalfa Silage
Molecular biologist Michael
Sullivan places small-scale
experimental "silos" in a
water bath for fermentation.
Behind him, technician
Ursula Hymes-Fecht will
analyze the contents for
It's a familiar scene in the countrya tractor chugging
its way across a field, mowing down swath after swath of green alfalfa.
Many farmers store and ferment these alfalfa clippings in silos. By
doing so, they turn the forage into silage, the cow's equivalent of
As a forage crop, alfalfa has many benefits. It fixes
nitrogen in the soilmeaning there's no need to add nitrogen fertilizerand
it's a good scavenger of excess soil nitrate left by overfertilized
row crops. And because it's high in protein, it's great for livestock,
such as dairy cattle. Unfortunately, when alfalfa is processed into
silage, up to 85 percent of its protein breaks down into nonprotein
nitrogen (NPN) through a process known as proteolysis. Cows use NPN
much less efficiently than protein.
Now, researchers at ARS's
U.S. Dairy Forage Research Center in Madison, Wisconsin, and ARS's Plant
Science Research Unit in St. Paul, Minnesota, have found an environmentally
friendly way to reduce protein degradation in ensiled crops such as
alfalfa. ARS has filed a patent application on the discovery, which
could save farmers more than $100 million per year.
A row of wilted, chopped,
green alfalfa is collected into
a wagon before being taken
to the silo.
"Right now, no practical techniques are available
to farmers who want to reduce protein breakdown in alfalfa silage,"
says plant physiologist Ronald D. Hatfield, of the Madison center. Research
has shown that applying formic acid or using heat treatments can reduce
protein degradation by 12 to 28 percent, but these methods are either
too caustic or too expensive for farmers to use profitably. Formic acid,
for example, must be handled with care and can be hard on some equipment.
But Hatfield and two other scientists at the centeragricultural
engineer Richard E. Muck and molecular biologist Michael L. Sullivanalong
with Deborah A. Samac, a plant pathologist in the St. Paul unit, have
discovered a way to reduce protein loss by using ingredients extracted
from potato skins and red clover. Research leading up to their invention
began more than 10 years ago.
Clues in the Clover
In the early 1990s, Muck and Hatfield were helping Beth Jones, a graduate
student at the University of Wisconsin, research red clover and alfalfa
silage. They found that although red clover and alfalfa have similar
protein levels, the protein in red clover does not degrade during ensiling
nearly as dramatically as the protein in alfalfa. In fact, red clover
silage preserves 65 to 80 percent of its protein as true protein.
In hopes of identifying
possible ways to improve
alfalfa silage, plant
Hatfield harvests red clover
from the greenhouse to study
why it undergoes limited
protein degradation in the
The researchers wanted to know why red clover, which outwardly seems
so similar to alfalfa, made such excellent silage. "We looked to
see whether there were different types of proteins in the two plants
or differences in their protease activity," says Muck. (Proteases
are the enzymes responsible for breaking down proteins.) They didn't
find anything at first.
But later, a clue emerged. Alfalfa clippings would remain green for
a while after being cut, but red clover clippings would turn brown right
away. Further studies revealed that red clover contains large amounts
of polyphenol oxidase (PPO), the same enzyme that turns cut surfaces
brown in apples, bananas, potatoes, and many other fruits and vegetables.
Alfalfa has insignificant amounts of PPO.
For PPO to cause the browning reaction, it needs something to act ona
substrateas well as exposure to oxygen. The substrates of choice
for PPO are O-diphenols. They include compounds such as caffeic
acid and related compounds, or conjugates, such as chlorogenic acid.
In addition to containing high levels of PPO, red clover contains high
amounts of caffeic acid and its conjugates. Alfalfa doesn't.
While a front-end loader
works to fill this bunker silo,
agricultural engineer Richard
Muck takes forage samples
that will be analyzed later for
nutritive value and moisture
Hatfield explains how red clover safeguards its protein. "When
the clover is chopped up, its cells release PPO," he says. "Once
the PPO is exposed to oxygen, it reacts with the plant's caffeic acid
and forms a very reactive molecule known as an o-quinone. Quinones
bind themselves to the proteases and keep them from degrading red clover's
Since making these discoveries, Sullivan has been able to extract the
PPO gene from red clover, and Samac has inserted it into an alfalfa
plant. They recently conducted an experiment in which they chopped some
transgenic alfalfa plants into 2-centimeter pieces, treated them with
a bacterial inoculant, applied caffeic acid to about half of them, and
let them sit for 2 weeks. Bacterial inoculants are the principal silage
additives in the United States; they ensure fast and efficient fermentation
in the silo.
The alfalfa plants treated with caffeic acid had 15 percent less protein
degradation than untreated plants. The scientists believe they can preserve
even more alfalfa protein if they improve their processing technique
and grind the plant into smaller pieces.
A Potato Mash Alternative
Caffeic acid is present in high concentrations in a variety of fruits
and vegetables, most notably potato skinsa common agricultural
waste product. The scientists are currently working with different potato-processing
plants to determine how easy it would be to extract large amounts of
caffeic acid from leftover skins.
They are also looking at ways to insert the PPO gene into a bacterial
inoculant. Such inoculants would excrete the protective PPO enzyme and
enhance fermentation of the silage. Farmers could apply the inoculant
and the potato-derived caffeic acid to their alfalfa crop. In this way,
they could achieve results similar to the ones reached with transgenic
alfalfa without having to grow a transgenic plant.
This technology should work on other ensiled crops as well, including
corn and rye grass.By Amy
Spillman, formerly with ARS.
This work is part of Food Animal Production (#101) and Rangeland,
Pasture, and Forages (#205), two ARS National Programs described on
the World Wide Web at www.nps.ars.usda.gov.
Ronald D. Hatfield, Richard
E. Muck, and Michael L. Sullivan
are with the USDA-ARS U.S. Dairy
Forage Research Center, 1925 Linden Dr., West, Madison, WI 53706-1108;
phone (608) 264-5358 [Hatfield], (608) 264-5245 [Muck], (608) 264-5397
[Sullivan], fax (608) 264-5147.
Deborah A. Samac is in the
USDA-ARS Plant Science
Research Unit, 317 Christensen Laboratory, 1515 Gortner Ave., St.
Paul, MN 55108; (612) 625-1243, fax (651) 649-5058.
"Nutrient Boost for Alfalfa Silage" was published
in the December
2003 issue of Agricultural Research magazine.