ABSTRACT 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 (i.e., 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.
Implications Designing dairy cattle housing to cool cows during 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 (Aréchiga 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; Aréchiga et al., 1995; Aréchiga 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; Aréchiga et al., 1994, 1995) and, in one experiment (Aréchiga 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., 1990) 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 physiologically or 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.
Key Words: Heat Stress, Embryo, Estrus, Fertility, Cattle
© 1999, by the American Society of Animal Science and the American Dairy Science Association. All rights reserved.
AM Symp. 36-50