cost which increases 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 processor
rather than the farmer’s herd estimations. 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 antibiotics. The amount of
milk cows produce is also reduced by the level
of subclinical mastitis as indicated by herd (or
bulk tank) somatic cell counts. This also has a
negative effect on the milk price because 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 production 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.
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The cost of feeding and management of the
non-producing animals in the herd should also
be included when estimating 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.
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 lactating cows in the herd is in
late lactation. Cows calving down regularly
keeps the average number of DIM at a lower
level. 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.
Because 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,
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