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Strategies for Managing Reproduction in the Heat-Stressed Dairy Cow¹

P. J. Hansen^* and C. F. Aréchiga^,2

^* Department of Dairy and Poultry Sciences, University of Florida, Gainesville 32611-0920 and Facultad de Medicina Veterinaria y Zootecnia, Universidad Aut;T 3;onoma de Zacatecas, Zacatecas, Mexico

² To whom correspondence should be addressed: P.O. Box 110920 (Phone: 352/392-5590; fax: 352/392-5595; E-mail: hansen{at}dps.ufl.edu).

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.

¹ This is manuscript No. R-06429 of the Florida Agric. Exp. Sta. The author's research described in this paper was supported by USDA-CBAG (Grants No. 92-34135-7432 and 95-34135-1860), USDA-NRICGP (Grant No. 96-35205-3728), the Florida Milk Checkoff Program and Hoffman-LaRoche. The author thanks his students and colleagues, especially William W. Thatcher, for input in formulating the ideas expressed in the paper.

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