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return to Farmers
Independent Weekly
July
4, 2002
By J.C. Plaizier, Department of Animal Science |
Feeding
Forage to Prevent Rumen Acidosis in Cattle
Upset
stomach? Take two bales and call me in the morning.
Rumen
acidosis, or high forestomach acidity, can result when you
feed your cattle a lot of grain and other concentrates. The
acidity of the rumen can increase so much that it affects
microbes in the rumen, especially the microbes that break
down fibre. This will result in reduced digestibility of the
feed, lower feed intake and a reduction in milk fat. Other
effects of acidosis can include damage to the rumen wall,
bacterial liver infections and loose feces. It is believed
that acidosis can cause laminitis, which is one of the main
reasons for lameness.
The key
to preventing rumen acidosis in cattle is in the coarseness
of the forage.
The rumen
is acidic because the microbial breakdown of feed in the rumen
generates volatile fatty acids. When a high concentrate diet
is fed, these acids are produced rapidly after a meal. They
do not leave the rumen as quickly as they are produced and
as a result, the acids accumulate, acidity increases and rumen
pH decreases.
It is
not clear below which rumen pH acidosis occurs. Microbial
breakdown of fibre is reduced when rumen pH drops below 6.0.
Losses in milk fat and feed intake will also occur when rumen
pH drops below this level. It is believed that the rumen wall
is damaged and that laminitis will occur if rumen pH drops
below 5.5. Acute and clinical signs of acidosis will occur
if rumen pH drops below 5.0. Fortunately, this is not common
in dairy cows. However, acute acidosis can be seen in beef
cattle on high grain finishing diets.
Stimulating
the production of saliva is vital in helping the prevention
of ruminal acidosis. Saliva contains buffers such as sodium
bicarbonate. The more a cow chews the cud (ruminates), the
more saliva is produced. Rumination is stimulated by the fibre
content and the particle size (coarseness) of the forage.
A term used to describe the coarseness of the diet is the
effective fibre. So, coarser diets stimulate rumen buffering
and prevent rumen acidosis. However, excess coarseness will
reduce feed intake. It is, therefore, very important to measure
the coarseness of your forages and your diets. This can be
done with the Penn State Particle Size Separator, which is
a user-friendly tool consisting of a set of sieves
Recommendations
from the new Nutrient Requirements for Dairy Cattle from the
National Research Council are that the dry matter of dairy
cow diets should contain at least 25% neutral detergent fibre
(NDF) and that three quarters of the NDF should come from
forages. Most of the studies used to determine these requirements
have used corn-based diets. The microbial breakdown (degradability)
of barley in the rumen is much faster that that of corn. The
buffering diets containing barley grain and barley silage
is more of a challenge than the buffering of corn based diets.
A study conducted at Agriculture and Agri-Food Canada in Lethbridge
suggests that barley based diets for dairy cows should contain
at least 34% NDF.
Acidosis
often occurs after a diet change, especially a switch from
a forage based diet to a concentrate based diet, such as after
calving. This is because the rumen and the rumen microbes
need time to adapt to the new diet. Fresh dairy cows are especially
at risk for acidosis. This risk can be reduced by giving fresh
cows some long hay, about 1 to 2 kg per day, in addition to
their normal diet. This long hay will give additional buffering
of the diet, improving the fiber digestion and feed intake.
Instead of reducing the intake of the normal diet, feeding
additional hay will increase the intake of this diet.
Mineral salts such as sodium bicarbonate, calcium carbonate
and magnesium oxide are added to dairy cow diets for additional
buffering. These buffers can aid in the prevention of acidosis,
but the use of their use should not replace balancing diets
for fibre content and coarseness. It is recommended that these
buffers be fed at 0.6% to 0.8% of the dry matter.
In order
to avoid low rumen pH, make sure that diets contain enough
fibre and coarse particles (effective fibre) and realize that
these fibre requirements depend on the rumen degradability
of the grain. More work is required to determine the relationship
between fibre requirements and the grain source. The use of
buffers such as mineral salts is an additional aid in the
prevention of acidosis.
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