Intravaginal impedance and sexual behavior of ovariectomized goats given estrogen alone or in combination with progesterone.

Intravaginal impedance (IVI) fluctuates during the goat estrous cycle. To understand which ovarian steroids are responsible for IVI changes and whether IVI variations are associated with precopulatory and copulatory behaviors, 8 ovariectomized females were assigned to 4 treatments in a 4 x 4 Latin square replicated over four 8-d periods. The treatments were as follows: progesterone plus estradiol-17beta (P4 + E2), oil plus estradiol-17beta (E2), progesterone plus oil (P4), or oil (OIL). Daily IVI measurements at the vagino-cervical junction were taken at 1 and 70 KHz. Progesterone was given on d 2 and 3. Estradiol was given in the evening of d 5. On d 1 to 8, goats were group-exposed to a sexually experienced male and observed for the expression of sexual behaviors. On d 6 and 7, IVI was less when goats received P4 + E2 or E2 compared with goats given P4 or OIL (P < 0.05). Impedance measured at 1 kHz tended to remain lower on d 8 in P4 + E2-treated females compared with those given P4 or OIL (P < 0.055). Like previous results, P4 + E2 or E2 treatment induced behavioral estrus; 5 of 8 P4 + E2-treated and 5 of 8 E2-treated females were sexually receptive on d 6. On d 7, although IVI remained low and 2 of 8 P4 + E2-treated goats and 4 of 8 E2-treated goats remained sexually receptive, no additional females were in estrus. No IVI decreases and no estrous behavior were observed in goats given P4 or OIL. This experiment demonstrated that E2 initiates the periestrous drop in IVI, and P4 may delay baseline return.

Castration increases pulsatile luteinizing hormone release, but fails to diminish mounting behavior in sexually experienced bulls.

We tested the hypothesis that mounting and chemoinvestigatory behaviors are testosterone-dependent in bulls. Eighteen bulls were divided into three treatment groups: intact (I), castrated (C) and castrated+testosterone (T). Sexual behaviors of all bulls were tested with an unrestrained receptive female 1 week prior to and weekly for 4 weeks after castration. Mounts with intromissions, aborted mounts and flehmen responses were quantified for each test period. In addition, patterns of LH and testosterone secretion were assessed at these times. Neither mounts with intromissions nor aborted mounts were affected by treatment. In contrast, numbers of flehmen responses were lower in C bulls than in the other groups following castration. Before castration, concentrations of LH were not different among groups and LH pulse frequency was approximately one pulse per hour. Castration resulted in a 2-fold increase in mean concentrations of LH and a 6-fold increase in LH pulse frequency. Neither mean concentration of LH nor LH pulse frequency changed in I or T bulls. The data fail to support the hypothesis that mounting behavior is T-dependent, but supports the hypothesis that this steroid hormone regulates flehmen behavior in sexually experienced bulls.

Effects of melengestrol acetate on reproductive behavior and concentrations of LH and testosterone in bulls.

We investigated the use of an orally active progestin (melengestrol acetate; MGA) to suppress reproductive activity in yearling beef bulls. Twenty-four crossbred bull calves were given a daily dose of 0, 0.5, 1.0, or 2.0 mg MGA for 99 d. Pulsatile patterns of LH and concentrations of testosterone and MGA were characterized on d 8, 36, 63, and 92 of the experiment. Numbers of aborted mounts, mounts with intromission, total mounts, and flehmen responses were assessed on d 15, 43, 71, and 99. Plasma concentrations of MGA were proportional to dose of MGA. Melengestrol acetate did not consistently affect mounting behavior in a dose-related manner, but, on d 99, number of total mounts for MGA-treated bulls was lower (P = 0.07) than that for control bulls. On d 15, MGA suppressed (P = 0.07) numbers of flehmen responses in a dose-dependent manner, but this effect was not sustained throughout the experiment. On d 8, concentrations of testosterone in control bulls were higher (P = 0.02) than in MGA-treated bulls, but this effect was not observed at other time periods. Overall, MGA caused slight decreases in mean concentrations of LH (P = 0.09) and LH pulse frequency (P = 0.06). Scrotal circumference was not affected by MGA. None of the behavioral traits was correlated with mean concentrations of LH or LH pulse frequency. Mounting activity was not correlated with testosterone concentrations, but number of flehmen responses was positively correlated with testosterone concentrations (P = 0.01). These results fail to support the hypothesis that progestins impair male sexual behavior or fertility in males.

Melengestrol acetate blocks the preovulatory surge of luteinizing hormone, the expression of behavioral estrus, and ovulation in beef heifers.

We tested the hypothesis that melengestrol acetate (MGA), an orally active progestin, blocks estrus and the preovulatory surge of luteinizing hormone (LH) in beef heifers. Cycling yearling Angus heifers were divided randomly into two groups: MGA-treated (n = 6) and control (n = 5). All heifers received injections of prostaglandin F2alpha (PGF) on d -25, -11, and 0 to synchronize estrus. Following the last PGF injection on d 0, heifers were fed either 0.5 mg MGA in a carrier or the MGA carrier each day for 8 d. At 4-h intervals on d 1 through 6, all heifers were observed for expression of estrous behavior, and blood samples were collected and assayed for LH. Daily blood samples were collected at 0800 on d 1 through 10 and assayed for circulating progesterone concentrations. All control heifers exhibited estrus and a preovulatory surge of LH. In each case, this was followed by increases in circulating concentrations of progesterone indicative of ovulation and normal luteal function. In contrast, none of the MGA-treated heifers exhibited estrus, LH surges, or evidence of ovulation. The results of this experiment show that MGA prevents ovulation in cattle by inhibiting the preovulatory surge of LH.

Effects of melengestrol acetate on onset of puberty, follicular growth, and patterns of luteinizing hormone secretion in beef heifers.

Our objective was to test the hypothesis that short-term (8 days) treatment of prepubertal heifers with melengestrol acetate (MGA) and subsequent steroid withdrawal would stimulate LH secretion and follicular growth. Angus heifers were divided randomly into two groups; MGA-treated (n = 8) or control (CON; n = 9). Puberty was determined by monitoring circulating concentrations of progesterone and ovarian morphology during a 14-day period following MGA withdrawal. LH secretory patterns were assessed upon initiation of MGA (Day 0), during MGA (Day 7), and 1 day after withdrawal of MGA (Day 9). All MGA-treated heifers, versus four CON heifers, exhibited corpora lutea and luteal phase concentrations of progesterone within 10 days after treatment (p = 0.01). Mean LH and LH pulse frequency increased (p = 0.005 and 0.0001, respectively) between Days 0 and 9 in MGA-treated heifers. In CON heifers, mean LH concentrations and pulse frequencies did not change. During the same period, diameter of the largest follicle increased in MGA-treated animals (p = 0.003) but did not change in the CON heifers. On the basis of these results, we suggest that MGA withdrawal enhances onset of puberty by stimulating pulsatile LH secretion that accelerates follicle growth to the preovulatory stage.