QUESTION; What does per
cent cow days mean? Can we use
this number to manage our herd?
ANSWER: Percent cow days can
be found several places in reports
from PA DHIA. The first place we
see this number is on Herd Sum
mary I, which is blue in color.
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It is described as “Test Period
Daily Average," and the fourth
column is “% Days In Milk.”
We also find the same number
on the backside of Herd Summary
n, where it is also labeled as “Test
Period Daily Averages.” The
fourth column is “% Days In
Milk.”
Now, let us look at what this
number means.
At any given time, your herd has
cows that are milking and some
that are resting and dry.
Total cows in milking herd,
times (multiplied by) the days in
the test period, equals the “total
cows days” in the test period.
The actual total days that cows
were milking is the “milking days”
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in the test period.
Milking days, divided by total
days, times (multiplied by) 100,
gives the “percent cow days in
milk.”
(Editor’s Note: Days In Milk is
frequently abbreviated in dairy
industry text as DIM.)
Let’s look at an example:
GIVEN:
1. 100 cows in a herd.
2. Days from test day to test
day is 30.
3. There are 2,562 total Milk
Days
TOTAL DAYS - # COWS IN
HERD X TEST DAY
INTERVAL
% COW DAYS IN MILK - MILK
DAYS + TOTAL DAYS X 100.
. . THEREFORE:
1. 100 X 30 - 3,000;
2. (2,562 - 3,000) = .854
.854 X 100 ° 85.4 % COW
DAYS IN MILK
If this same herd is reported to
be at “91 % Days In Milk," then:
3,000 X 91 % - 2,730 days
milked.
ency a
ion,
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To know if this can be a man
agement tool, we must look at
some factors that contribute to this
number. Freshening a large num
ber of cows in the period will cause
an increase in percent cows days in
milk.
Brining new heifers into the
herd at freshening will also show
an increase. Drying cows off early
or cows with extended dry periods
will cause a decrease in percent
cow days in milk.
In theory, a herd that milks all
cows for 305 days and then allows
them to dry for 60 days would
attain a percent cow days in milk o f
83.56 % ((305/365) X 100 =
83.56).
This would be a goal for herds if
we lived in a perfect world, but we
do not.
Currently the state average is 87
percent Days In Milk.
This means that the average PA
DHIA cows milks for 317 days and
then is put dry.
That is okay if we have the cow
bred on time and she stands dry for
60 days. What happens when she is
dry longer?
Now we see that the percentage
Days in Milk goes down and pro
fits along with it.
How do we use information like
% DIM?
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Lancaster Farming, Saturday, January 2, 1999-Al7
I wish I could say that there is a
number everyone should work
towards, but there is not
We can use this number to help
analyze our herd’s milking
performance.
When we evaluate management
changes that cause production
increases for milking cows, we
need to keep in mind that the parti
cular change is affected by the
makeup of the herd. This is how I
use percentage days in milk.
For instance, if a herd has pro
duction increase from 67 to 71
pounds and percentage days in
milk stays the same then we have a
true increase.
On the other hand, if % DIM
increases from 78 percent to 84
percent during this same increase,
then the real response to our efforts
is less than the 4-pound increase
that we had.
In this case, a 4-pound increase
is 6 percent, and coincides with a
6-percent increase in days in milk,
or freshness of the herd.
This is a truer evaluation of
management changes, than just
looking at milk response.
Another example has a herd pro
ducing 80 pounds per cow for three
straight months.
On the fourth month, the aver
age drops to 73 pounds. What
happened?
By looking at %DIM, we see no
change during the first three
months. On the fourth month, this
drops from 86 percent to 83
percent
My read here would be that,
even though we could predict that
production would drop due to
changes in freshness of the herd, it
is too severe a drop, and something
negative has occurred in our man
agement scheme.
The near 9-percent drop in pro
duction was partly due to our herd
makeup, but probably only a 2- to
3-pound loss should have been
expected.
We need to look at the 4- to
5-pound loss not due to herd
makeup.
This is my explanation of per
cent Days In Milk, and how to use
it to evaluate true herd
performance.
I am sure some of you feed peo
ple out there have had the misfor
tune of trying new products on
farms only to fall completely flat.
Use this %DIM information to
realistically predict herd produc
tion potential before you make
unattainable promises.
Again, the value of PA DHIA
records shines through.
ATTENTION TOBACCO
FARMERS
I want to thank you farmers and friends
for your support and friendship for the
past 2 years.
I want to let you know I will not be
working with or for Penn Leaf Tobacco
Co. or B.S. Beiler anymore. Watch for
my future ad. I want you to know I will
continue to support you.
Thank You
Jerry Winstead