by several factors. Genetically, it is difficult to
change because it is lowly heritable. Cows not
becoming pregnant are culled because of poor
fertility; however poor fertility may also result
from poor management. Cows may come on
heat but are not inseminated owing to poor
heat observation therefore missing heats and
cows not becoming pregnant. This means that
poor management could induce infertility while
veterinary intervention could improve fertility.
Both these practices have direct and indirect
cost implications for farmers.
up at 50-60 days before the next expected
calving date. Extending the voluntary waiting
period also results in a longer calving interval.
In some case farmers put in no effort to service
cows before at least 120 days in milk.
Extending calving intervals
Because of high milk yield levels, it has become
a standard practice in high producing, TMR-
based dairy herds to service cows later into the
lactation period, i.e. after the live weight nadir
point. After this point live weight increases
because of a positive energy balance. Maximum
feed intake has been reached and milk yield
is decreasing following peak milk yield. Some
research has shown a positive response in
conception when cows are inseminated at this
stage of the lactation period. The reasoning for this is based on a common
adage that cows producing milk during two
extended lactation periods would be more
profitable than cows producing during three
standard lactation periods. This follows that
a younger herd (two lactation periods) would
be better than an older herd, i.e. three plus
lactation periods. This assumption goes
against the general consensus in the literature
that calving intervals should not exceed 12-13
months while a considerable effort should be
put into extending the lifetime and productive
lifetime of dairy cows. Reducing the lifetime of
cows increases the proportion of first lactation
cows in a dairy herd. This come at a double
expense because of a larger number of heifers
to be reared to first calving as well as a lower
herd milk yield as the first lactation milk yield of
cows is lower than that of older cows.
However, extending the voluntary waiting
period to first insemination also extends the
interval between calving and conception (days
open). The lactation period is also extended
(past 300 days) especially when cows are dried Modelling lactation milk yield
Lactation curves were developed to demonstrate
and compare the effect of calving interval, i.e.
12, 13 and 14 months, on the total and average
daily milk yield of Holstein cows. The lactation
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