Biomedical Engineering Reference
In-Depth Information
−29%, respectively) and body condition score
(0.0 vs. +0.4 points) were found in six 5-month-
old female Holstein-Friesian calves exposed to
hot conditions as compared with a control group,
kept under thermoneutrality conditions (Lacetera
et al.
1994
). Decrease in body growth and body
reserves between birth and puberty, especially
during the first few months, can be detrimental
for milk production during lactation and can
increase the replacement rate later (Chillard
1991
). High temperatures during late pregnancy
and the early postpartum period markedly modify
colostrum composition. Holstein-Friesian heifers
kept in a climatic chamber and exposed during
late pregnancy and the early postpartum period to
82 THI (daytime) and to 76 THI (night-time),
when compared to a counterpart maintained under
thermoneutrality conditions (65 THI), showed
lower colostrum net energy due to a reduction in
lactose, fat and protein content. In addition, the
analysis of protein fractions showed a reduction
in percentages of casein, lactalbumin, IgG and
IgA (Nardone et al.
1997
) explaining a lower
concentration of circulating Ig in summer calves
(Stott et al.
1976
) . An investigation on Israelian
Holstein cows in their third and fourth lactation
periods showed the antagonistic effect of heat
(−0.38 kg of milk/1°C) and photoperiod
(+1.157 kg of milk/per 1 h more of light) (Barash
et al.
2001
). On comparing milk production dur-
ing summer and spring in a dairy herd located in
central Italy, a lower milk yield (−10%), and
also lower casein percentages and casein number
in summer (2.18 vs. 2.58% and 72.4 vs. 77.7%
respectively), was found (Bernabucci et al.
2002
). The fall in casein was due to the reduction
in a
s
-casein and b-casein percentages. However,
no differences were observed between the two
seasons for
k
-casein, a-lactalbumin and b -lac-
toglobulin. The serum protein contents were higher
in summer than in spring. Although performance of
dairy cows is better during winter than during heat
stress, animals experiencing cold stress also reduce
milk yield. The drop starts around −4°C, and
marked yield depression occurs at −23°C. The
lower critical temperature is −40 or −45°C with a
daily milk yield of 36 kg (Broucek et al.
1991
) .
3
Behaviour of Cows
During Heat Stress
Several behaviour modifications in response to
heat stressing conditions have been observed in
dairy cattle. Dairy cattle change their behaviour
to reduce heat load in hot weather. Cows seek
shade, reduce feed intake, spend more time stan-
ding, spend more time near the water trough and
increase respiration rate as ambient conditions
become warmer. Animal behaviour can provide
insights into how and when to cool dairy cows.
In free stall barns, cows stay inside during the
hottest part of the day to obtain shelter from
intense solar radiation, while during the night,
cows go outside (Arave and Albright
1981
) .
High-producing dairy cows showed more eating
behaviour than low-producing cows during all
times of the day. The eating behaviour of low and
high-producing dairy cows decreased after 10:00 h
and through the day. High ambient temperature
decreases eating behaviour. The best recognised
effect of heat stress is an adaptive depression of
metabolic rate associated with reduced appetite
(Silanikove
2000
). Heat stress causes the rostral
cooling centre of the hypothalamus to stimulate
the medial satiety centre which inhibits the lateral
appetite centre and consequently lowers milk
production (Kadzere et al.
2002
) . The decrease
(26.2%) in milk production of high-yielding cows
is higher than that (15.2%) of low-producing dairy
cows during summer. Heat stress in high-produc-
ing lactating dairy cows results in a dramatic
reduction in rumination (Collier et al.
1982
) .
High-producing cows ruminated less compared
to low-producing cows. The underlying mecha-
nism in lower percentage of ruminating beha-
viour can be explained that high-producing dairy
cows coped with the adverse effect of heat stress
by ruminating less so that there is lower produc-
tion of metabolic heat (Kadzere et al.
2002
) .
High-producing cows tried to lower their body
temperature by not lying on the concrete floor
where heat from the sun was absorbed during
daytime. They preferred standing to minimise
surface area contact with the ground (escaping
