Ž .
fully functional CL Peters et al., 1994 . The pattern of tonic LH secretion also
Ž influenced the rate of growth and turnover of dominant follicles in cyclic cows Savio et
. al., 1993 . The effects of HS on tonic LH as well as on GnRH-induced preovulatory
surge concentrations were found to be dependent on the concentration of estradiol in Ž
. plasma Gilad et al., 1993 . Chronic HS during summer decreased the mean and
amplitude of tonic LH as well as of GnRH-induced preovulatory plasma LH concentra- tion surge in cows with low concentrations of plasma estradiol. In contrast, neither tonic
LH concentration nor GnRH-induced LH concentration surge was altered in cows with high concentrations of plasma estradiol. Similar responses were recorded in cows
Ž .
acutely heat-stressed for 16 h in a hot chamber during the winter Gilad et al., 1993 . GnRH-induced plasma FSH surge was lower in lactating cows during both chronic
exposure to HS in summer and an acute 16 h exposure to thermal stress in a hot Ž
. Ž
. chamber Gilad et al., 1993 . These reductions were evident as with LH only in cows
Ž .
with low concentrations of plasma estradiol. In contrast as discussed above , in HS cows that were not treated with GnRH, there was a pronounced increase in plasma
concentrations in the FSH surge that preceded the second-wave dominant follicle, and in the preovulatory FSH surge that was associated with decreased plasma inhibin concen-
Ž .
trations Roth, 1998 . In agreement with the latter study, serum FSH concentrations and content of GnRH in the hypothalmus have been found higher in summer than in winter
Ž .
in primiparous sows Armstrong et al., 1986 . The reason for the discrepancy in
Ž .
responses to HS between cows treated with GnRH lower plasma FSH and intact cows Ž
. higher plasma FSH is unclear.
6. Delayed effects of HS
Autumn fertility of dairy cows is lower than in winter, although ambient temperatures drop and cows are no longer exposed to thermal stress. For example, in Florida, the
Ž .
autumn conception rate October–November of Holstein cattle was found to be around Ž
. 35–40, compared with above 50 in winter January–March; Badinga et al., 1985 .
Ž .
Conception rates of high-yielding dairy cows in Israel Herd Book data, 1992–1997 drop from 45 in winter to around 20 during the summer, and to 23 and 29 in
October and November, respectively. These figures clearly show a delayed influence of summer HS on autumn fertility.
Follicular dynamics was altered in lactating cows that had been heat-stressed by Ž
. direct solar radiation during the preceding estrous cycle Roth et al., 1997 . During the
Ž .
first wave of the subsequent cycle, fewer medium-size class 2 follicles developed in previously HS cows than in the controls, and the rate of decline in their number was
slower. Another study showed that cows heat-stressed for 7 days in a hot chamber had an increased proportion of small and large follicles just after the heat exposure
Ž
. Guzeloglu, 1998 .
Ž .
In a seasonal study, Wolfenson et al. 1997 found that estradiol concentration in the follicular fluid of first-wave dominant follicles collected during the autumn on day 7 of
the estrous cycle was about one-third of that in winter. This low concentration in autumn follicles resulted from a marked reduction in production by the theca cells of androstene-
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
