NEWS
first time, the heifer survival rate to first
calving is 80%. It is also important to
know at which age heifers are lost from
the herd as this affects the mortality
cost of heifers. A higher age at culling
increases the mortality cost of heifers,
mostly due to feeding cost which in-
creases daily. The mortality cost of lost
heifers should be included as part of the
rearing cost of the surviving heifers.
Milk yield per cow
The average milk yield of a dairy herd
is a common point of discussion among
dairy farmers. It is often regarded as
an indication of the standard of a dairy
herd, i.e. high milk yields being a “top”
herd while low milk yields are regarded
as a poor performing herd. However,
the emphasis should rather be the
amount of milk collected by the proces-
sor rather than the farmer’s herd esti-
mations. The reason for this is that not
all milk produced is sold. Some milk is
used in calf rearing and for household
purposes. Milk is also discarded being
contaminated when cows are treated
for mastitis and other infections. Each
treatment has a specific milk withdrawal
period. This is required to prevent milk
becoming contaminated by antibiot-
ics. The amount of milk cows produce
is also reduced by the level of subclini-
cal mastitis as indicated by herd (or bulk
tank) somatic cell counts. This also has
a negative effect on the milk price be-
cause of poor milk quality.
Milk income – feed cost
This indicator is based on the difference
between the milk income and feed cost
and is estimated as the gross margin
per cow per day. This figure changes
every time milk or feed prices change.
Similarly, the breakeven point of pro-
duction is also affected by feed and milk
prices. This refers to the minimum milk
yield per cow per day to cover the dairy
herd’s production cost of milk. Milk yield
must be higher when the feed price is
high or when the milk price is low. The
cost of feeding and management of
the non-producing animals in the herd
should also be included when estimat-
ing the production cost of milk. An easy
way to do this is compile a list of all the
different groups of animals in the herd,
i.e. lactating and dry cows, heifers at
different ages and young calves, the
diet (forages and concentrates) and
the feed intake per animal per day that
each group receives. Dairy farmers
should, on a monthly basis, compile
all the costs required to produce milk
as well as the herd milk income. Using
only the concentrate feeding cost can
be misleading as this may be less than
30% of the total cost of production per
litre of milk.
25
Days in milk
The average number of DIM is based on
the interval between the present date
and the calving down dates for all cows
currently in milk. This figure increases
when a greater proportion of lactat-
ing cows in the herd is in late lactation.
Cows calving down regularly keeps the
average number of DIM at a lower lev-
el. The number of DIM usually increase
because of reproductive problems in
getting cows pregnant. This results in
extending the number of days from
calving to conception or days open.
The lactation period then extends past
the normal 300-day lactation period as
cows are mostly milked until 50- 60 days
before the expected calving date. Be-
cause of the lactation curve, milk yield
is lower at this lactation stage and with
more cows in the herd, the average milk
yield of the herd is reduced. Milk yield
(and milk income) decreases when the
interval days open (or calving interval)
increases. Modelling lactation curves
for different calving intervals of 12, 13,
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.
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 percent-
age of first lactation cows can be lower
although it is greatly affected by culling
rate. At high culling rates the propor-
tion of cows in first lactation is higher
when aiming to maintain the number
of cows in the herd. Increasing the per-
centage 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 indi-
cators which can be estimated to moni-
tor herd management. These indicators
should be estimated on a monthly basis
and the progress (change) should be
presented as graphs. This would en-
able 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 cull-
ing programmes.
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 produc-
tion is reduced for cows at higher actual
ages while at the same lactation num-
ber 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 pro-
portion of older cows in the herd.
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),
Grassroots
Vol 19
No 1
March 2019