1) Feeding diets high in unsaturated fats like those found in distillers grains and corn can cause milk fat depression by altering the rumen environment. 2) Effective fiber from long forage particles is needed to stimulate chewing cud and saliva production which buffers rumen pH. Small fiber particles in distillers grains do not provide this. 3) Low rumen pH shifts rumen bacteria populations, lowering fiber digestion and the acetate to propionate ratio, which can inhibit fat synthesis and cause milk fat depression. Proper forage particle size is important to prevent this.

1. With possible changes of up to
three percentage units, fat is the
milk component most easily modified
by the diet. Nutrition is the factor
you control with the greatest impact
on milkfat. Low milkfat results when
specific diets cause a reduction in
milkfat concentration and yield.
Due to the price of distillers grains
(DDG) per unit of nutrient supplied,
least-cost formulations usually
call for its maximum inclusion. To
achieve adequate nutrient balance,
however, the maximum inclusion
level suggested for DDG has been
20 percent of diet dry matter. There
seems to be agreement between pro-
ducers and their nutritionists that
the high fat concentration in DDG
can cause low milkfat.
Part of this problem has been that,
if one goes by the ration “on paper,”
it is easy to replace some forage fiber
with fiber from DDG. Regrettably,
and due to its small particle size,
the fiber in DDG is not “effective.” It
has a limited capacity to stimulate
cud chewing and rumination. These
factors can lead to rumen acidosis.
Scratch factor, saliva are key
During rumination, the rumen
contents are thoroughly mixed, and
the larger, less digested particles
are regurgitated, chewed, and re-
swallowed. This cycle is repeated
constantly as the cow ingests new,
longer feed particles. Feed particles
escape this cycle once they become
smaller and dense enough to sink to
the bottom of the rumen. From here,
particles are transported through
the digestive tract.
In addition to mechanically break-
ing the forage into smaller particles,
cud chewing stimulates the produc-
tion of saliva. High bicarbonate lev-
els present in saliva neutralize the
continuous rumen acid conditions
produced by microbial fermentation.
However, the stimulus that initiates
rumination and, in turn, saliva pro-
duction results from the coarse fiber
present in the rumen.
This feed fraction forms the float-
ing rumen mat which scratches
the rumen walls, contributing to
the physical stimulus needed for
the rumen to contract and start
each rumination cycle. The rumen
mat also slows particles down long
enough for them to be degraded by
rumen microbes. A reduction in
rumen contractions reduces cud
chewing and saliva production. Con-
sequently, the capacity of the rumen
to deal with acid loads also drops.
Watch particle size
What is really important in diets
then is not the fiber concentration
alone but also the particle size of the
forage. Research has shown that,
with rumen pH greater than 6.0,
milkfat percent in Holsteins runs at
or above 3.5. Rumen bacteria ferment
dietary carbohydrates and proteins to
produce volatile fatty acids as excre-
tion products of their metabolism.
Higher volatile fatty acid concen-
trations in the rumen, as a result
of this fermentation, drop the pH
which shifts microbial populations.
These changes may cause a reduc-
tion in the digestion of fiber and the
acetate to propionate ratio.
Work by Hippen, here at South
Dakota State, reported a reduction
in total volatile fatty acid (VFA) pro-
duction in diets with 20 percent DDG.
The authors believe this was due to
possible reductions in effective fiber
and the presence of unsaturated fatty
acids. This lower acetate concentra-
tion also resulted in a linear reduc-
tion in the acetate/propionate ratio.
The results of this experiment
indicated that the rumen envi-
ronment had been altered; a typi-
cal example of milkfat depression
(MFD). In addition, low rumen pH
can result in elevated formation of
conjugated linoleic acid intermedi-
ates that depress synthesis of fat in
the mammary gland.
Therefore, what is important is not
the fiber concentration alone, but the
particle size of the forage fraction.
Fatty acid amount, profile
Most lipids that occur natu-
rally in feedstuffs are unsaturated
and later saturated in the rumen.
Higher dietary unsaturated fatty
acid concentrations in the rumen
disturb normal fermentation due
to their negative effects on some,
although not all, rumen microbes.
Microorganisms that resist start to
dominate which causes a reduction
in fiber digestibility, lower acetate
production (milkfat precursor), and
reduction in rumen pH.
Under these altered conditions,
alternate pathways for fatty acid
saturation generate specific inter-
mediates which have been dem-
onstrated to inhibit fat synthesis
in the mammary gland. The more
unsaturated fats that are fed and
available for hydrogenation in the
rumen, the more intermediates will
be produced. This overabundance of
dietary unsaturated fatty acids has
been termed rumen unsaturated
fatty acid load (RUFAL). This is
commonly associated with oleic, lin-
oleic, and linolenic acids.
Based on initial research, with
RUFAL in the diet at 2.2 percent
(500 grams per day in a cow eat-
ing 50 pounds of dry matter daily),
intake drops and milkfat depression
occurs. It is not just the absolute
amount that is important but also
the profile of these fatty acids.
Linoleic acid, for example, con-
stitutes over 50 percent of the total
fatty acids in corn fat. It is hydroge-
nated in the rumen in reactions that
generate intermediate fatty acids.
A 2011 study from the University
of Wisconsin by Armentano and
co-workers evaluated the effects of
feeding free vegetable oils with dif-
ferent fatty acid profiles. There was
a negative effect on milkfat yield
and concentration associated pri-
marily with unprotected oils rich in
linoleic acid.
It is not clear from this study if
the MFD seen in the diets with saf-
flower (high oleic) and linseed (high
linolenic) oils was due to these acids
or the linoleic acid also present in
the diets. However, if oleic and lino-
lenic depressed milkfat yield, they
appear to be equal in potency and
less potent than linoleic acid.
Fat levels variable
According to the NRC (2001), tra-
ditional full-fat distillers grains have
10 percent fat. However, the New
York State Dairy One Lab reported
an average of 12.6 percent fat in
4,819 DDG samples analyzed dur-
ing 2011. Samples ranged from 9.4
to 15.57 percent fat. Research has
shown that diets without distillers
grains have variable concentrations
of unsaturated fats. Table 1 shows
diets without distillers grains that
contain 371 to 713 grams of RUFAL,
and 221 to 440 grams of linoleic acid.
As the amount of distillers rose in
the diet, RUFAL and linoleic acid
also surged. Needless to say, this will
be aggravated in diets where other
corn-derived feedstuffs contribute its
share of linoleic acid (Table 1).
Current high corn grain prices
have dictated the use of high forage
diets, including elevated amounts of
corn silage. Today, it is not unusual
to see diets in the field with 70
pounds of corn silage as fed. The
corn grain present in that silage will
have around 4 percent fat, of which
more than 50 percent is linoleic acid.
Diets with corn silage as the pre-
dominant forage source seem to
be more prone to result in milk-
fat depression. This is particularly
true when supplementary unsatu-
rated fats are added to the diet.
This milkfat drop is explained by
the compounded effect of the fat in
the corn-based feedstuffs plus that
of the fat supplement. Research has
shown that adding fat to diets when
corn silage was the predominant
forage resulted in low milkfat. This
was corrected by substituting corn
silage with alfalfa silage.
Dilution effect possible
On occasion, diets can boost milk
production with no changes in fat
yield which results in reduced milkfat
concentration. This does not constitute
milkfat depression because fat synthe-
sis was not affected, the concentration
drop results from a dilution effect.
When looking at trials that have
resulted in low milkfat concentration,
they have one thing in common: diets
had less than 50 percent forage or
under 22 percent of neutral detergent
fiber coming from forage.
One example is the trial by Hip-
pen et al. (2010) mentioned previously
(Table 2). In their experiment where
cows developed milkfat depression, the
diet had 45 percent forage, with only
19.3 percent as forage fiber. When they
measured ration particle size with the
three-sieve Penn State particle separa-
tor, the feed amounts retained in the
first two screens were similar. In the
diet with DDG, however, there was
approximately 3 percent less and 3.8
percent more in the third screen and
bottom pan, respectively.
Additional feed particles showing
up in the bottom pan represented a
higher content of easily fermentable
nutrients which has been associ-
ated with reduced fat concentration
and yield.
Similar results between TMR par-
ticle size and milkfat percent were
reported by Nydam et al. (2008).
They also observed that, when all
TMR variables and herd manage-
ment factors were analyzed, the
factors with the greatest effect on
milkfat were the DM concentration
of the TMR and the percentage of
particles remaining on the bottom
pan of the two-sieve Penn State
particle separator. These two fac-
tors were responsible for 21 percent
of the variation observed in milkfat
percent in their experiment.
Maximize DDG without MFD
Some practical recommendations
can be drawn from the results of
these experiments in order to max-
imize distillers grains inclusion
without significant negative effects
on milkfat yield or concentration.
To produce MFD, two conditions
are necessary. One is the presence
of unsaturated fatty acids in the
rumen (linoleic acid in particular).
Second, an altered rumen environ-
ment that causes their incomplete
biohydrogenation is required.
When formulating diets:
1. Although fiber concentration is
important, it needs to be accompa-
nied by a particle size assessment of
the ration in cows fed TMR.
2. Adding buffers to the diet (e.g.
bicarbonate) can ameliorate the
effects of low rumen pH, their effect
is transitory and is not a substitute
for adequate particle-sized rations.
3. Linoleic acid in diets may be
more important in causing low milk-
fat than the rest of the unsaturated
fatty acids. Rather than the total
rumen unsaturated fatty acid load,
it is suggested to check the amounts
of linoleic acid the diet supplies.
4. The amounts and fat composition
of other feedstuffs included in the
diet will dictate the level of distillers
grains that can be safely included.
5. Even with adequate particle
size, avoid the inclusion of more than
10 percent wet DDG products when
fermented corn feedstuffs constitute
over 50 percent of diet dry matter.
Distillers grains don’t have to cause low milkfat
Feeding
by Fernando Diaz-Royon and Alvaro D. Garcia, D.V.M.
The authors are with the dairy science depart-
ment at South Dakota State University.
Circle No. 24 on Reader Response Card
Reprinted by permission from the June 2012 issue of Hoard’s Dairyman.
Copyright 2012 by W. D. Hoard & Sons Company, Fort Atkinson, Wisconsin.

2. 417June 2012
HOARD’SDAIRYMAN
Table 2. Productive response and milkfat depression with varied DDG levels
Inclusion (DM) Production
DDG, % Forage, % Milk, lbs. Fat, lbs. Fat, %
Abdelquader et al. 2009 0-30 55 -- -- Decrease
Anderson et al. 2006 0-10-20 50 Increase Increase --
Leonardi et al. 2005 0-5-10-15 45 Increase -- Decrease
Hippen et al. 2010 0-20 45 Decrease Decrease Decrease
-- = no effect
Table 1. Rumen unsaturated fatty acid load and productive response of cows fed different DDG concentrations
Inclusion (DM) RUFAL Linoleic acid
DDG Forage Grams/day Grams/day
% CS (%) ALF (%) 0% DDG
inclusion
Max. DDG
inclusion
0% DDG
inclusion
Max. DDG
inclusion
Abdelqader et al. (2009) 0 - 30 30 25 554.5 868.4 303 537
Leonardi et al. (2005) 0 - 5 - 10 - 15 27 18 712.9 949.6 439.3 595
Ranathunga et al. (2010) 0 - 7 - 14 - 21 27 22 524.8 723.6 283.4 421
Schingoethe et al. (1999) 0 - 31 31 18 464.1 736.8 293.9 484.6
Sasikala et al. (2008) 0 - 18.5 27.5 27.5 370.6 596 221.1 368.7
Abbreviations: DM = dry matter; RUFAL = rumen unsaturated fatty acid load; CS = corn silage; Alf = alfalfa; DDG = distillers grains