The benefits of tannin-containing forages

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The Benefits of Tannin-Containing Forages
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Jennifer W. MacAdam, Dept. of Plants, Soils and Climate, Utah State University
Joe Brummer, Dept. of Soil and Crop Sciences, Colorado State University
Anowarul Islam, Dept. of Plant Sciences, University of Wyoming
Glenn Shewmaker, Dept. of Plant, Soil, and Entomological Sciences, University of Idaho
What Are Tannins?
Tannins, a group of chemical compounds pro-
duced by a number of broadleaf forage plants,
can bind proteins. Typically, grasses don’t con-
tain tannins, although sorghum (Sorghum bicol-
or) has a significant tannin content. Tannins are
often found in higher concentrations in broadleaf
plants adapted to warm climates. For example,
sericea lespedeza (Lespedeza cuneata) is a for-
age cultivated in the midwestern and southern
U.S. that can accumulate tannins to as much as
18% of herbage dry matter (Mueller-Harvey,
2006).
Because tannins bind salivary proteins, they
produce an astringent or puckery sensation in the
mouth when foods with a high tannin content
(such as unripe fruits) are eaten. Tannins are
effective in preserving (tanning) leather because
they bind to the collagen protein in animal skins,
preventing microbial breakdown. The French
word “tannin” is related to the German word
“tannenbaum” (meaning fir tree), and is derived
from an older Latin term for oak bark, which
was an early source of tannins for leather-
making.
Proteins are needed to carry out the metabolic
activities in living cells, so the content of protein
is high in plant cells. For this reason, tannins in
plants are segregated in vacuoles, which are wa-
ter-filled structures in the center of most plant
cells, or in special compartments called tannin
sacs (Fig. 1). This segregation keeps tannins
from interfering with plant metabolism.
Figure 1. Cell sap pressed from the tannin sacs of
three sainfoin leaflets before (left) and after (right)
staining for tannins.
What Western Forage Plants
Contain Tannins?
The amount of tannin, the location of tannins in
leaves, stems or flowers, and the chemical struc-
ture of tannins vary greatly among the plants
that accumulate these compounds. Alfalfa
(Medicago sativa) can produce tannins, but they
only occur in seed coats; white clover (Trifolium
repens) produces tannins, but they only occur in
flowers. In these cases, the amount of tannin
consumed by ruminants grazing these forages is
negligible. Two forage plants that grow well in
the western United States and contain significant
tannins are birdsfoot trefoil (Lotus corniculatus)
and sainfoin (Onobrychis viciifolia, Fig. 2). Both
of these forages express tannins in their leaves.
September2013AG/Forages/2013‐03pr

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Much of the re-
search on tannin-
containing forag-
es has been car-
ried out by ani-
mal and forage
scientists in New
Zealand, where
cattle and sheep
are raised pri-
marily on peren-
nial ryegrass
(Lolium perenne)
pastures. In an
effort to identify
forage species
capable of im-
proving ruminant
production on
pastures, many
legume and other forb species were studied and
the beneficial traits of tannin-containing forage
plant species were documented.
The tannin produced by birdsfoot trefoil has rou-
tinely been found to increase ruminant produc-
tivity, and there is evidence that the tannins pro-
duced by sainfoin and sulla (Hedysarum
coronarium) may also have positive effects on
ruminant productivity. However, the results of
studies on sainfoin and sulla are not consistently
positive (Waghorn, 2008).
What Do Forage Tannins Do?
Two general traits of tannins relevant to grazing
ruminants are the prevention of bloat (Lees,
1992) and the suppression of internal parasites
(Hoste et al., 2006). Pasture bloat occurs when a
substantial amount of fresh, high-protein forage,
such as alfalfa, is digested quickly, resulting in a
rapid increase in the protein content of the ru-
men. This causes the rate of microbial fermenta-
tion in the rumen to increase, and results in rapid
accumulation of carbon dioxide and methane
gases in the rumen. Microbial slime, plant cellu-
lar membranes and proteins all combine with
fermentation gases to create a stable foam that’s
perceived as a liquid at the valve leading from
the rumen into the esophagus, causing it to re-
main closed (Howarth et al., 1991). As the gases
trapped in the rumen continue to accumulate, the
rumen becomes distended, interfering with
breathing and blood flow. Left untreated, bloat
can result in death from suffocation or cardiac
arrest. Tannin-containing forages are non-bloat-
ing because tannins bind excess plant proteins,
precipitating them out of rumen fluid, and in the
process, preventing the creation of the stable
foam that’s characteristic of pasture bloat.
The suppression of internal parasites by tannins,
specifically the suppression of numerous nema-
tode species, has been documented for sainfoin
and birdsfoot trefoil, and for purified tannins
from woody plant species used as dietary sup-
plements (Younie et al., 2004). The effect of
tannins on nematodes depends on the tannin
concentration and chemical structure as well as
the species of nematode. The effectiveness of
tannins also differs by the stage of growth of the
nematode, and the location in the gastrointestinal
tract where the tannin is active.
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How Do Tannin-Containing For-
ages Alter Forage Utilization?
Compared with grasses, legumes have less fiber
and the fiber in legumes is digested more rapidly
than the fiber in grasses (Smith et al., 1972).
Therefore, legumes are digested more quickly
than grasses, which means that intake and
productivity can be higher on legume than on
grass pastures (Crampton et al., 1960). The
problem with a diet consisting of highly digesti-
ble legume forages is that their protein content is
much higher than the dietary requirements of
ruminants, and their energy (carbohydrate) con-
tent is relatively low. In the rumen, this problem
is solved when microbes use the carbohydrate
“backbone” of proteins as energy. However, the
ammonia this creates isn’t good for the ruminant
or for the environment.
In tannin-containing forages, excess plant pro-
teins that become bound to tannins leave the ru-
men without being digested. Unfortunately, the
tannin chemistry or concentration in most forag-
es results in irreversible binding of proteins. In
these cases the protein is never digested, and
Figure 2. Sainfoin

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both forage intake and digestibility are reduced
(Reed, 1995). As a result, forages such as big
trefoil (Lotus pedunculatus) can prevent bloat,
but also reduce ruminant productivity (Barry and
Duncan, 1984).
Like other tannins, those in birdsfoot trefoil (Fig.
3) bind excess plant proteins in the rumen, pre-
venting bloat. However, unlike most tannins,
they release these proteins in the abomasum in
response to low pH. This allows the protein to
be digested and absorbed in the small intestine
(Waghorn et al., 1987) and results in high
productivity in both sheep (Douglas et al., 1995)
and cattle (Wen et al., 2002). In Utah, season-
long average daily gains of 2.87 to 3.35 lbs. per
day have been achieved on birdsfoot trefoil pas-
tures (MacAdam et al., 2011).
Tannin Environmental Benefits
High-protein forages can result in high nitrogen
concentrations in both milk and urine, but when
birdsfoot trefoil is fed and excess proteins are
digested in the abomasum instead of being used
for energy in the rumen, the nitrogen concentra-
tion of milk and urine is reduced and more ni-
trogen is excreted as solid waste. This has been
shown in studies by Woodward and others
(2009) where urinary nitrogen was reduced as
birdsfoot trefoil was increased relative to peren-
nial ryegrass in dairy cow diets, and by
Misselbrook and others (2005), where ammonia
emissions from dairy manure were reduced
when cows were fed birdsfoot trefoil silage in-
stead of alfalfa silage.
The rate of nitrogen released into the soil from
the manure of sheep fed birdsfoot trefoil was
reduced compared with the manure of sheep fed
white clover (Crush and Keogh, 1998). Over
time, this would increase the rate of soil organic
matter accumulation in pastures planted with
birdsfoot trefoil. Birdsfoot trefoil tannins have
also been shown to reduce the enteric (digestive)
methane production of dairy cows compared
with cows fed perennial ryegrass (Woodward et
al., 2004).
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Conclusions
The inclusion of highly digestible legumes such
as birdsfoot trefoil in pasture plantings can in-
crease the productivity of grazing livestock. Be-
cause forage legumes produce their own nitro-
gen as long as they’re inoculated with the proper
Rhizobium bacterium at planting, they can meet
their own nitrogen fertilization needs as well as
those of associated pasture grasses. Since
birdsfoot trefoil and other tannin-containing for-
age legumes are non-bloating, they can be plant-
ed as 50% or more of mixtures with no risk of
bloat.
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Figure 3. Birdsfoot trefoil flowers and seedpods.

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