BARD Project No. IS-1475-88; Duration: 3 years; Budget: $250.000

Utero-Ovarian-Conseeptus Response to Heat Stress in The Dairy Cow and its
Involvement in Low Summer Fertility

D. Wolfenson, R. Meidan, A. Berman                           W.W. Thatcher, M. Drost
Faculty of Agriculture                                                            University of Florida
The Hebrew University of Jerusalem                                   Gainesville, FL,U.S.A.
Rehovot, Israel

Final Report ABSTRACT

In a series of experiments in dairy cattle, we examined the multifactorial problem of heat stress on the functions of: ovarian follicles - in particular the dominant, preovulatory follicle - the corpus luteum, pituitary gonadotrophins, and on uterine prostaglandin F2a secretion. Attempts to improve summer fertility in cattle by hormonal therapy also are reported.

Initially, basic information was gained in the absence of stress, with attention focused on control of follicular development and function: LH appeared to maintain dominance of the large follicle; plasma progesterone concentration, via its effect on tonic LH secretion, caused turnover or persistence of that follicle; hormonal secretion by the dominant follicle (estradiol, progesterone, androstenedione, oxytocin, PGF2a, IGF-1) characterized its functionality in relation to age and stage of development. Emergence of fresh, healthy preovulatory follicles was induced using a GnRH agonist.

The dominant follicle was highly susceptible to heat stress. Using ultrasonography, we found that follicular dominance was suppressed, and that the preovulatory follicle emerged earlier, resulting in a more aged follicle at ovulation. Theca cells of the dominant follicle secreted much less androstenedione; granulosa cell viability, aromatase activity and inhibin secretion were suppressed drastically. Late summer, and even autumn, were more stressful in terms of estradiol production than early summer, suggesting chronic delayed impairment of follicular function.

Corpora lutea (CL) in summer secreted less progesterone and more PGF2a than during the winter. This suggested a temporal impairment in CL function that can be related to the above described damage to the ovulatory follicle as a possible carry over effect. Low progesterone secretion may affect normal embryonic development in utero. Pulsatile LH, as well as GnRH-induced surges of LH and FSH, were only significantly suppressed during heat stress in cows with low plasma estradiol concentrations. Low plasma concentration of gonodotrophins may alter several ovarian functions. At the uterine level, heat-stressed pregnant heifers secreted more PGF2a than cooled heifers. This could be detrimental to the life span of the corpus luteum of pregnancy.

Injection of a GnRH agonist (buserelin) on days 8 or 12 post-insemination in summer-stressed cows did not affect their fertility. Intravaginal supplementation of progesterone from day 8 post-insemination for 12 days yielded equivocal results. Treatment might be beneficial but more needs to be done to  substantiate this effect.

The detailed and important information gained in this project could serve to minimize the closed loop of events leading to low summer fertility. Efforts which combine an efficient cooling system and hormonally programmed reproductive management may increase summer conception rates in dairy cattle.