amount of body reserves to enable
them to produce high levels of milk,
generally described as milking off
their backs. The benefit of a higher
live weight at first calving lasts up
to third lactation.
The reason why heifers that are
growing too quickly, produce less
milk during first lactation, is because fatty tissue rather than udder
tissue is deposited in the udder.
This may occur before or during
puberty. The composition of the
udder tissue of Holstein heifers that
were fed at different feeding levels
is shown in Table 3.
LL: Low live weight gain during the
first year and gestation
LH: Low live weight gain during the
first year and high live weight gain
during g estation
HL: High live weight gain during the
first year and low weight gain during gestation
These results show that Holstein
heifers growing at a low rate during
the first year of life (0.58 kg/day)
and a high live weight gain (0.84
kg/day) during gestation, had the
largest udders and more milk secreting tissue (parenchyma) than
heifers growing at different rates. A
high growth rate in heifers, at
about 1.0 kg per day, during the
first 12 months of age, although
having heavier udders, resulted in
the lowest milk secreting tissue in
their udders. On the other hand,
heifers, at a low growth rate during
gestation, also produced less milksecreting tissue. Therefore, it is gen-
erally recommended that the live
weight of Holstein and Jersey heifers should, before being served, not
be higher than 0.75 kg/day and
0.45 kg/day, respectively.
It should be noted that a combination of live weight, condition score
and live weight gain (growth rate)
is critical when rearing heifers to
reach their full genetic potential. It
is therefore essential that heifers are
weighed regularly in order to monitor their growth. Various methods
are available for weighing heifers. A
scale should preferably be used, as
weigh bands that are available, are
not very accurate. It is also difficult
to estimate live weight of heifers
with the naked eye.
Feeding level
Parameters
LL
LH
HL
Weight of udder (kg)
6.4
9.0
6.8
Milk secreting tissue (kg)
4.0
5.4
3.2
Milk secreting tissue (%)
63.7
61.0
49.6
Udder as % of live weight
1.53
1.82
1.39
Live weight gain up to 12 months of age (kg)
0.58
0.58
1.03
Live weight gain during gestation (kg)
0.68
0.84
0.58
Table 3. The composition of the udder tissue of Holstein heifers fed at different feeding levels and slaughtered at 250 days of gestation