Breeds
Parameters
Holstein
40
Jersey
25
Ayrshire
32
Guernsey
30
ADG* 0 to 12 months of age (kg/day)
0.60-0.70
0.40-0.45
0.50-0.575
0.45-0.50
ADG* 12 to 24 months of age (kg/day)
0.70-0.75
0.50-0.55
0.60-0.65
0.55-0.60
Live weight at 15 months of age (kg)
320-365
215-240
270-300
245-265
Live weight at 24 months of age (kg)
515-570
355-390
435-480
390-435
Birth weight (kg)
Table 1. The expected growth rates and target live weights of heifers from different dairy breeds at specific
development stages i.e. at conception (15 months of age) and at first calving (24 months of age)
* ADG: average daily gain
milk production will be adversely
affected. In Table 1 a summary of
the expected growth rate and target live weights of heifers of various
dairy breeds is provided.
Research has shown that the size of
the pelvic opening in heifers calving
down at a live weight of 584 kg,
while differing in age at first calving, is similar. In this study, two
groups of heifers were fed different
diets to reach similar live weights at
first calving although at different
ages, i.e. 22.4 and 24.6 months of
age. Heifers calving down earlier
were on a higher feeding level. Following first calving, all cows were
treated the same. During first lactation, the milk yields of cows that
differed in growth rate as heifers
were similar. However, their production per day of life differed being 10.0 vs. 9.3 kg for the heifers
calving down at 22.4 and 24.6
months of age, respectively. The
reproduction performance of cows
during first lactation was also the
same regardless of age at first calving. The main benefits for an earlier
age at first calving were a reduction
Lactation
number
in the feeding cost of heifers and a
higher milk yield per day of life.
Both these factors have important
economic implications.
Feed is converted to body weight
more efficiently when animals are
on a higher growth rate. This is because a smaller portion of the feed
is being used for maintenance requirements while a larger portion is
used for growth (live weight gain).
Heifers have to grow as quickly as
possible in order to reach sexual
maturity to be serviced for the first
time. Therefore, for heifers, live
weight at first service (and conception) should be reached at the earliest possible age. However, when
heifers conceive at an early age
while not having reached expected
live weights, their milk yield during
first lactation will be affected negatively. Also, the possibility of calving
problems (dystocia) may occur. It is
for this reason that it is recommended that heifers should preferably not calve down earlier than
22 months of age. Research indicated that heifers, which had calved
after 22 months of age, produced
an additional 73 ℓ of fat-corrected
milk per month during their first
lactation. However, this higher milk
yield declined when age at first
calving is beyond 29 months of
age. There is actually no reason why
heifers should calve down later
than 27 months.
The size and body condition of heifers at first calving have a significant
effect on their production during
their first three lactation periods
(Table 2). From these results, it can
be concluded that heavier heifers at
first calving produce more milk during their first three lactation periods. There are mainly two reasons
for this, (i) heavier heifers have
more body fat reserves to supply in
their energy needs during the first
three months of the first lactation
and (ii) because heifers are closer to
their mature live weights, less feed
is being used during first lactation
for growth towards mature live
weight. Usually in early lactation,
cows do not consume enough feed
for the natural increase in milk yield
towards peak production. For this
reason they require a sufficient
Extra number of litres of milk produced per lactation for each 1 kg of higher live weight at first
calving
Per kg
Per 50 kg
Cumulative total
1
8,7
435
435
2
7,6
380
815
3
6,6
330
1145
Table 2. The effect of heifer live weight at first calving on milk production during the first three lactations