**9. Effect of evaporative cooling system on synchronization of follicular development and ovulation**

The exposure of lactating cows to heat stress has been shown to cause a decrease in follicular growth and to reduce concentrations of serum estradiol [33]. In this study the cooled cows had a greater (P < 0.05) average diameter of the largest ovulatory follicle than the uncooled cows. Heat stress inhibited follicular growth and dominance during the preovulatory period. Abnormal ovarian function in heat stressed cows was manifested as a decrease in the proestrus rise in estradiol, and the smaller size of the second wave dominant follicle [33]. Circulating estradiol concentration during the preovulatory period is necessary to produce an LH surge and ovulation. In addition, a reduced estradiol peak may also alter aspects of the LH surge that could account for some types of anovulation in lactating cows [58]. A reduction of the endogenous LH surge by heat stress was reported in heifers [59]. It has been suggested that these differences are related to preovulatory estradiol levels because the amplitude of tonic LH pulses and GnRH-induced preovulatory plasma LH surges are decreased in cows with low plasma concentrations of estradiol but not in cows with high plasma concentrations of estradiol [60]. Therefore, the synchronization rate in the uncooled cows tended to lower than in the cooled cows. In addition, the ovulation rate in response to a second injection GnRH of Ovsynch was reported to be between 87% [61] and 91% [62] in cycling cows. Therefore a second GnRH injection after the PGF2 treatment might be used to improve the ovulation rate in dairy cows under heat stress.


**Figure 5.** Weekly changes in average body weight for the cooled and uncooled cows during the first 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Uncool Cool

**Week Postpartum**

**9. Effect of evaporative cooling system on synchronization of follicular** 

The exposure of lactating cows to heat stress has been shown to cause a decrease in follicular growth and to reduce concentrations of serum estradiol [33]. In this study the cooled cows had a greater (P < 0.05) average diameter of the largest ovulatory follicle than the uncooled cows. Heat stress inhibited follicular growth and dominance during the preovulatory period. Abnormal ovarian function in heat stressed cows was manifested as a decrease in the proestrus rise in estradiol, and the smaller size of the second wave dominant follicle [33]. Circulating estradiol concentration during the preovulatory period is necessary to produce an LH surge and ovulation. In addition, a reduced estradiol peak may also alter aspects of the LH surge that could account for some types of anovulation in lactating cows [58]. A reduction of the endogenous LH surge by heat stress was reported in heifers [59]. It has been suggested that these differences are related to preovulatory estradiol levels because the amplitude of tonic LH pulses and GnRH-induced preovulatory plasma LH surges are decreased in cows with low plasma concentrations of estradiol but not in cows with high plasma concentrations of estradiol [60]. Therefore, the synchronization rate in the uncooled cows tended to lower than in the cooled cows. In addition, the ovulation rate in response to a second injection GnRH of Ovsynch was reported to be between 87% [61] and 91% [62] in cycling cows. Therefore a second GnRH injection after the PGF2 treatment might be used to

week postpartum.

300

320

340

360

380

**Body weight (kg)**

400

420

440

460

**development and ovulation** 

improve the ovulation rate in dairy cows under heat stress.

**Table 4.** The effect of evaporative cooling and tunnel ventilation system on follicular development, time of ovulation and the response rates of synchronized cows to GnRH and PGF2. Results are expressed as mean±SEM.
