14 and 15 months, average DIM were 155, 170,
186 and 201 days and average milk yields were
30.0, 28.9, 27.8 and 26.8 kg/day, respectively.
Missing one heat cycle (21 days) amounts to a
milk loss of approximately 0.8 kg milk per cow
per day.
Lactation number
The average lactation number of all the cows
in the herd provides an indication of the
age of a dairy herd. Heifers calve down for
the first time at about two years of age after
which cows should calve down every year.
This means that a cow in fourth lactation is
at least six years old. Actual age at the end of
fourth lactation can be higher when age at
first calving is later than 24 months of age and
calving interval is longer than 12 months. The
efficiency of production is reduced for cows at
higher actual ages while at the same lactation
number because of more unproductive days
over the cows’ lifetime.
The reason why the age (average lactation
number) of a dairy herd is important is
because the lactation milk yield of cows
increases from first to fifth lactation after
which it decreases although not declining
to the same production level as during first
lactation. The efficiency of a dairy herd
increases when there are a greater proportion
of older cows in the herd.
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First lactation cows
The percentage of first lactation cows in
a dairy herd provides an indication of the
replacement rate in a dairy herd as well as the
culling rate in a dairy herd. For an expanding
herd, i.e. increasing in size (the number of
cows in the herd), the percentage of first
lactation cows can be higher especially when
sexed semen is being used. For a stable herd,
i.e. not increasing in size, the percentage of
first lactation cows can be lower although it is
greatly affected by culling rate. At high culling
rates the proportion of cows in first lactation
is higher when aiming to maintain the number
of cows in the herd. Increasing the percentage
of first lactation cows in a herd would reduce
the total (and average) milk yield of the
herd because of the lower milk yield of first
lactation cows.
In closing
There are several key performance indicators
which can be estimated to monitor herd
management. These indicators should be
estimated on a monthly basis and the progress
(change) should be presented as graphs.
This would enable estimating trendlines
over time to show progress or deterioration.
These trendlines could be used as a basis for
changing the standard of management and/or
breeding programmes which may include sire
selection and cow culling programmes.
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