starting and peak milk yields and milk yield
persistency for cows was similar, being 30 and
50 kg/day and milk yield decreasing at 5% per
month, respectively. Following each lactation
period, a dry period of 60 days was included for
each cow. To utilize age effects, the starting milk
yield at the second and third lactation period
was 12 and 14%, respectively, higher than at
first lactation. The production performance of
cows was estimated up to the end of the third
lactation using standard (300-day) lactation
periods as a control. In the following figure
the effect of calving interval on three lactation
curves for Holstein cows is shown.
Total days in milk were the same for the three
cows. The average daily milk yield differed,
being 43.2, 42.8 and 41.8 kg/day, for cows
showing calving intervals of 365, 395 and 425
days, respectively. At a milk price of R5.00 per
litre, milk yield losses amounted to R1940 and
R6350 per lactation. Increasing starting and
peak milk yields resulted in higher milk income
losses.
To compare three standard lactation periods
to two extended lactation periods would entail
a calving interval of 12 vs. 17 months. This
decreases the milk income even more as more
days in milk are at the end of the lactation
period when milk yield is low.
Other studies have shown similar results. A
budgeting procedure using a spreadsheet
programme to determine the effect of calving
interval on income over feed and variable costs
showed that in the USA recommendations for a
12- to 13- month calving interval appear to be
justified.
Calving interval explained
Although seemingly difficult to achieve, a calving
interval of 365 days is theoretically possible.
The gestation period of cows is approximately
AgriKultuur |AgriCulture
275-280 days. After calving and without any
uterine or other infections, the uterus may be
fully recovered within 20 to 40 days. This leaves
about 45 to 65 days during which cows could
be inseminated. As the average heat interval
is 21 days, cows therefore have at least two to
three opportunities to be serviced to become
pregnant within 85 days after calving.
Therefore, cows that show an early return to
oestrus, conceive from a minimum number of
services and which stay pregnant to the next
calving, are regarded as highly fertile. For this
reason, the intervals between the calving date
and first service date and days open (calving
date to conception date) as well as the number
of services per conception are important traits
indicating fertility in dairy cows. These interval
traits can also be used as an indication of the
standard of reproduction management within
a dairy herd. Interval traits can be further used
as binary traits (yes = 1 and no = 0) to determine
the proportion of cows being serviced before
80 days after calving or the proportion of
cows conceiving within 100 and 200 days after
calving.
In closing
Increasing the voluntary waiting period increases
the interval from calving to conception as well
as the lactation period and calving interval. This
also reduces the lifetime milk yield and milk
income of cows. This is because the extension
of the lactation period occurs during the late
lactation stage when milk yield is lower. For a
higher lifetime milk yield cows should be in milk
more times during the early lactation stage
when milk yield and feed efficiency is higher.
Reproduction management affects the fertility
of dairy cows. Ways to improve fertility in dairy
herds depends on: (i) a short term management
strategies such as providing a clean, dry and
sunny calving down area, monitoring cows post
calving for early detection of uterine infections,
checking the heat cycling performance of
cows within the first 80 days after calving; (ii)
a medium term managerial strategy such as
having an action plan for cows more than 150
days in milk not confirmed pregnant, putting a
heat detection programme in place, ongoing
checking of AI techniques (or performance) of
inseminators, checking semen quality and (iii)
a long term genetic strategy by using sires for
AI with breeding information such as daughter
pregnancy rates and productive life estimated
breeding values.
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