dione, which serves as substrate for estradiol synthesis by the granulosa cells. Impaired functioning of theca cells in the autumn may be due to previous exposure of the animals
to HS in the preceding summer. In contrast, aromatase activity of granulosa cells in the autumn was unaffected by HS. The hypothesis that changes in follicular steroidogenesis
in the autumn were due to a delayed effect of HS on follicular function was reinforced by a study in which cows were heat-stressed during days 2–6 of the cycle and
Ž medium-size follicles were examined on day 3 of the subsequent cycle Roth et al.,
. 1997 . In previously HS cows, granulosa and theca cells obtained from medium follicles
produced one-third and one-fourth of the quantities of estradiol and androstenedione, respectively, compared with non-HS cows. Collectively, the above results show im-
paired steroidogenic capacity of follicles from cows previously subjected to HS.
Low autumn fertility could be related to a delayed effect of HS on oocyte function in Ž
. cows previously heat-stressed during the summer. In a recent study Roth et al., 1999 ,
Ž .
follicles 3–8 mm were aspirated during four consecutive estrous cycles in the autumn from lactating cows previously subjected to summer HS. The percentage of grade I
Ž .
Ž .
best oocytes was low in the first cycle early autumn; 28 and rose later in cycles 3 Ž
. and 4 late autumn; 55 . The percentage of eight-cell-stage embryos developed in vitro
following oocyte maturation and activation, rose by 50 in late autumn compared with early autumn. Furthermore, enhanced removal of impaired follicles by frequent follicle
Ž .
aspiration days 4, 7, 11 and 15 of the cycle led to a more rapid emergence of healthy Ž
. oocytes in the autumn Roth et al., 1999 .
7. Oocyte quality, embryonic development and uterine function
Various aspects of the effects of HS on oocyte quality and embryonic development Ž .
include the following: 1 the deleterious effects of heat exposure during different stages of oocyte maturation and early embryo development, on the impaired function of
Ž . oocytes and embryos, in both in vitro and in vivo systems; 2 the increase in the heat
Ž . tolerance of the embryo with age; 3 the production of heat-shock proteins by the
Ž . embryo, and their potential function in protecting the embryo during HS; and 4 the
possible use of antioxidants to increase embryo resistance to thermal stress. Relevant Ž .
aspects of the effect of HS on uterine environment and endometrial function include: 1 production of heat-shock proteins by the endometrium during HS, and its implications;
Ž . Ž .
2 reduced production of interferon-g by the conceptus; and 3 increased production and release of PGF
from the endometrium during HS, and its implications for
2 a
pregnancy recognition and CL maintenance. The reader is referred to recent studies Ž
. Edwards and Hansen, 1997; Rocha et al., 1998 and reviews that cover these subjects
ŽThatcher and Collier, 1986; Thatcher and Hansen, 1992; Hansen, 1997; Hansen et al., .
1992; Zavi, 1994 .
8. Fertility studies
A prerequisite for improving summer fertility is to lower the level of hyperthermia that lactating cows develop daily during the summer. A lactating cow experiences mild
or severe hyperthermia, depending on HS intensity, milk yield and efficiency of the cooling system used. During the last two decades, new cooling systems have been
introduced in dairy farms. They are based on wetting the cow with water to cool it directly by evaporation from the skin, or on evaporative cooling of the air surrounding
Ž the cows under shades Bucklin et al., 1991; Berman and Wolfenson, 1992; Armstrong,
. 1994; Huber, 1996 . Collectively, most studies show that such systems achieve some
improvement of fertility in commercial farms, compared with non-cooled controls, but the improvement does not match winter fertility. The reason for this is the inability of
the various cooling procedures totally to eliminate hyperthermia during the summer. Not surprisingly, however, complete elimination of HS by intensive and frequent use of the
sprinkling and ventilation cooling system under experimental farm conditions was able
Ž to restore the summer conception rate to that recorded in winter Wolfenson et al.,
. Ž
. 1988 . The following describes hormonal and other treatment strategies that were
tested during summer HS as means to improve fertility. Administration of GnRH during early stages of estrus, timed to coincide with the
endogenous LH surge, may induce an enhanced LH surge, and may also improve the synchronization of the time intervals between estrus, LH surge, ovulation and insemina-
tion. Injection of 100 mg of GnRH into lactating cows at detection of estrus during late Ž
summer in Mississippi, increased their conception rate from 18 to 29 Ullah et al., .
1996 . The rise in fertility was suggested to be related to an increase in concentration of plasma progesterone during the first 30 days after AI. In agreement with this result,
conception rate of lactating cows that were injected with GnRH at the first signs of standing estrus during summer and autumn months in Israel, increased by about 16.6
Ž .
above that of untreated control cows Kaim et al., unpublished data . Ž
. In contrast to the findings of Ullah et al. 1996 , two other studies did not show
improvement of fertility following post-AI supplementation of progesterone: induction of an accessory CL by a single injection of 3000 IU of hCG on day 5 or 6 after
insemination in summer in Florida, did not improve the fertility of heifers or lactating Ž
. cows Schmitt et al., 1996 ; and exogenous supplementation of progesterone by insertion
of a progesterone-containing CIDR into lactating cows on day 7 post-AI for 11 days in Ž
. summer in Israel also failed to improve fertility Wolfenson et al., 1994 . Since post-AI
treatment with progesterone has been shown to be effective in improving the fertility of Ž
. non-HS cattle Robinson et al., 1989; Sianangama and Rajamahendran, 1992 , it has
been suggested that the increases in progesterone in HS studies might have occurred after day 7, whereas most of the damage to the conceptus in severely HS cattle occurs
Ž .
between estrus and day 7 of pregnancy Putney et al., 1988; Ealy et al., 1993 . Free radicals have been suggested to be partly responsible for the detrimental effect
of elevated temperature on cellular membrane integrity, and for compromising the cellular function of steroidogenic tissues and embryos, since these have been found to be
Ž .
sensitive to free radical damage Hansen, 1997; Arechiga et al., 1998b . Administration Ž
. Ž
of the antioxidant, vitamin E 3000 IU at the time of AI, or injection of vitamin E 500 .
Ž .
mg and selenium 50 mg at 30 days postpartum, had no beneficial effect on pregnancy Ž
. rate during summer or winter in Florida Ealy et al., 1994; Arechiga et al., 1998b .
Ž .
Similarly, cows supplemented with b-carotene 400 mgrday starting 15 days or more Ž
. before AI did not improve their reproductive function Arechiga et al., 1998a .
A different approach that has been tested recently is the incorporation of the timed-AI program in the system of summer fertility management. Injection of GnRH induces a
programmed recruitment of an ovulatory follicle; 7 days later, a PGF injection
2 a
regresses the CL and permits final maturation of the ovulatory follicle; 48 h later, a Ž
second GnRH injection induces ovulation and 16 h later, cows are inseminated Burke et .
al., 1996; Pursley et al., 1998 . This program eliminates the need for estrus detection. Ž
The timed-AI program was tested during summer condition in Florida de la Sota et al., .
1998 . Overall pregnancy rate at 120 days postpartum was greater for treated than for Ž
. control cows inseminated at estrus 27 and 16.5, respectively , and the number of
days open was less and the number of services per conception was greater for timed-AI than for control cows. The timed-AI protocol improved reproductive management, but it
does not protect the embryo from the detrimental effects of high temperatures.
9. Conclusions
