Abstract
BackgroundLetrozole, a non-steroidal aromatase inhibitor, prevents the body from producing its own estrogen. The objectives of the present study were to test the hypotheses that letrozole treatment, initiated prior to selection of the preovulatory dominant follicle, will induce the growth of more than one follicle to a pre-ovulatory size, and will delay ovulation.MethodsPost-pubertal beef heifers were given two luteolytic doses of PGF (12 h apart) and monitored by ultrasonography for ovulation. Five to eight days later, ovarian follicular wave emergence was synchronized by ultrasound-guided transvaginal follicular ablation (Day 0=wave emergence) and a luteolytic dose of PGF was given 60 and 72 h later. On Day 1, heifers were divided randomly into two groups (n=15/group) and an intravaginal device containing 1 g of letrozole or a blank device (control) was inserted. The intravaginal devices were removed on Day 7, or at the time of ovulation, whichever occurred first. Transrectal ultrasonography and blood sample collection were performed daily from the day of ablation to 12 days after subsequent ovulation.ResultsThe mean (+/-SEM) interval from device placement to ovulation was longer in letrozole-treated animals compared to controls (6.1+/-0.25 vs 5.1+/-0.26 days, respectively; P<0.01). Single dominant follicles were present in both groups. The day-to-day diameter profiles of the dominant follicles of the ovulatory wave were larger (P<0.05) and the maximum diameters greater in letrozole-treated heifers (14.6+/-0.51 vs 12.4+/-0.53 mm, respectively; P<0.01). The diameter profile of the corpus luteum (CL) that formed after treatment did not differ between groups; however, plasma progesterone concentrations were higher (P<0.01) in heifers treated with letrozole. Estradiol concentrations were reduced following letrozole treatment (P<0.05), although a preovulatory rise of estradiol occurred in both groups.ConclusionsAdministration of letrozole with an intravaginal device during growth of the ovulatory follicle delayed ovulation by 24 h and resulted in the formation of a CL that secreted higher levels of progesterone. A sustained-release intravaginal device may be useful for the development of an aromatase inhibitor-based protocol to control ovulation for herd synchronization and to enhance fertility by increasing circulating progesterone concentrations during the first 7 days post-ovulation in cattle.
Highlights
Letrozole, a non-steroidal aromatase inhibitor, prevents the body from producing its own estrogen
The day-to-day diameter profile of the dominant follicle during treatment and the maximum diameter of the ovulatory follicle were larger in the letrozole-treated group (P=0.05 and P=0.01; respectively (Figure 1; Table 1; respectively)
Previous studies of the effects of letrozole on ovarian function in cattle were focused on the non-ovulatory portion of the estrous cycle in cattle [15,16,17]; the effect of letrozole treatment on pre-ovulatory follicles was the subject of the present study
Summary
Letrozole, a non-steroidal aromatase inhibitor, prevents the body from producing its own estrogen. Estrogen-based protocols, as a treatment for synchronizing ovulation in cattle, have modernized breeding practices and allowed producers to reliably control the timing of ovulation, enabling efficient use of time, labour and resources by allowing pre-scheduled insemination. The European Union has already banned the use of estrogenic products in food producing animals [7,8,9,10]. In 2007, New Zealand and Australia banned use of estrogens in lactating dairy animals [10]. These policies have created a void in methods to control reproductive function for breeding management in cattle
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