Δευτέρα, 24 Μαΐου 2010

Strategies for Managing Reproduction in the Heat-Stressed Dairy Cowl

P. J. Hansen" and C. F. ArBchiga'y2
*Department of Dairy and Poultry Sciences, University of Florida, Gainesville 3261 1-0920 and
+Facultad de Medicina Veterinaria y Zootecnia, Universidad Autonoma de Zacatecas Zacatecas, Mexico


Establishment and maintenance of
pregnancy is difficult in lactating dairy cows exposed
to heat stress because of reductions in estrous
detection rate and the proportion of inseminated cows
that maintain pregnancy. The most common approach
to ameliorate heat stress in developed countries has
been to alter the cow's environment through provision
of shade, fans, sprinklers, and so on. Nonetheless,
seasonal variation in reproductive function persists.
Increased understanding of bovine reproductive function
and its alteration by heat stress has led to
additional strategies for reducing deleterious consequences
of heat stress on reproduction. These include
hormonally induced timed artificial insemination,
which can reduce losses in reproductive efficiency
caused by poor detection of estrus, and embryo
transfer, which can increase pregnancy rate by
allowing embryos to bypass the period when they

are most sensitive to elevated temperature (Le., in the
first 1 to 2 d after breeding).
Other efforts are directed
toward developing methods to protect the embryo from
harmful actions of elevated temperature. Approaches
being studied include manipulation of embryonic
synthesis of heat shock proteins and use of antioxidants
to reduce free radical damage associated with
heat stress. It may also be possible to reduce the
magnitude of hyperthermia caused by heat stress.
This might be possible physiologically, for example by
feeding of agents that affect thermoregulatory systems,
or genetically by selecting for specific traits
conferring thermal resistance. Finally, the development
of bovine somatotropin as a lactational promotant
means that it may be possible to extend
lactations beyond 305 d and voluntarily discontinue
inseminations during periods of heat stress.

Conclusions and Implications

hot periods of the year has reduced the magnitude of
effects of heat stress on reproductive function but
large declines in herd pregnancy rate still occur
during periods of hot weather. Nonetheless, a combination
of new reproductive technologies and innovative
application of existing technologies offers
prospects for achieving additional improvements in
reproductive performance during heat stress.

Poor estrus detection. This problem can be solved
through the use of TAI programs (Arechiga et al.,
1998a; de la Sota et al., 1998). Under this paradigm,
high embryonic mortality following breeding will
continue to limit herd pregnancy rates

Increase in embryonic survival through embryo
transfer. Pregnancy rates in summer can be improved
through the use of embryo transfer (Putney et al.,
1989a; Drost et al., 1994; Ambrose et al., 1997). For
this technology to be widely adopted, further improvements
must be made in the techniques for in vitro
production of embryos, embryo freezing, and timed
embryo transfer so as to lower costs and simplify
animal handling

Alteration of the biochemical properties of the embryo
to protect it from exposure to elevated temperature.
Cultured embryos can be made more thermotolerant
to heat shock by exposure to a mild heat shock (Ealy
and Hansen, 1994; Arechiga et al., 1995; Arechiga and
Hansen, 1998). Additional challenges will be to
identify the molecules responsible for this phenomenon
of induced thermotolerance and develop strategies
for achieving thermoprotection in vivo. Antioxidants
have reduced effects of heat shock on cultured
embryos (Malayer et al., 1992; Ealy et al., 1992;
Arechiga et al., 1994, 1995) and, in one experiment
(Arechiga et al., 1998a), increased herd pregnancy
rate in summer. Nonetheless, additional studies
evaluating effectiveness of antioxidant administration
are warranted before use of antioxidant supplementation
can be recommended

Genetic modification. Genetic selection has been a
traditional method to reduce effects of environment on
livestock by development of animals that are genetically
adapted to hot climates. There are additional
possibilities for meeting this goal. Identification of
specific genes that control traits related to thermotolerance
make it possible to select for thermal
resistance without inadvertently selecting against
milk yield. Examples of such traits include coat color
(King et al., 1988; Hansen, 1990; Becerril et al., 1994)
and genes controlling hair length (Olson et al., 1997).
Possibly, there are also genes controlling cellular
resistance to heat shock (Kamwanja et al., 1994).
Integration of marker-assisted selection (Meuwissen
and Van Arendonk, 1992; Dodgson et al., 1997;
Kappes, 1999) into animal breeding systems should
make selection for traits conferring thermotolerance
more rapid. Crossbreeding schemes are likely to be
important in certain dairy production systems
(Madelena et al., 19903 and use of in vitro production
of embryos makes maintenance of the purebred herds
required to produce crossbreds less of a limitation to
these breeding programs (Rutledge, 1997).

Pharmaceuticals and nutraceuticals. There are
prospects for development of pharmaceuticals or
feedstuffs (nutraceuticals) that regulate heat production
and heat loss mechanisms; perhaps fungal culture
extracts represent the first example of such preparations
(Huber et al., 1994). In addition, an existing
pharmaceutical, BST, can prove useful for extending
lactation (Van Amburgh et al., 1997) and avoiding
inseminations during hot periods of year

Taken together then, it is evident that reproductive
dysfunction caused by heat stress can be reduced by
1) reducing the magnitude of heat stress that drives
cattle into hyperthermia, 2) changing the cow physiolog~
callyo r genetically so that the degree of hyperthermia
caused by heat stress is reduced, 3 ) manipulating
the cow’s reproductive system to bypass phases of the
reproductive process that are particularly sensitive to
heat stress or 4) use of lactational promotants like
BST to avoid breeding cows during seasons of the year
when heat stress can compromise reproduction.
Although not all of these approaches will prove
feasible under practical conditions, it should be
manifest that there are myriad opportunities for
solving what is the one of the most severe environmental
limitations to cattle reproduction

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