Six experiments were conducted to study the mechanisms controlling estrogen-induced luteinizing hormone (LH) release in the cow. Estradiol-l7β (E2) induced a release of LH in ovariiectomized cows at doses ranging from 0.25–10.00 mg and estrone (E1) was less effective than E2. Exogenous progesterone (P4), administered s.c. in a silicone rubber implant for periods of 2–9 days prior to E2 injections, did not inhibit the E2-induced LH release even when a very low dose (0.25 mg) of E2 was given. However, when a corpus luteum (CL) was present and endogenous P4 was more than 1 ng/ml, E2 did not induce an LH release. Possible mechanisms for this discrepancy between the effects of exogenous and endogenous P4 are discussed. Suckling and/or lactation inhibited the E2-induced LH release for at least 2 weeks postpartum. This inhibition may partially explain postpartum anestrus.

Fourteen crossbred beef bulls were assigned at random to receive one of four sexual stimuli treatments. Treatments consisted of: (1) controls (four bulls), no visual or physical contact with any cows; (2) false mount (two bulls), allowed to mount an estrual cow; (3) mated (four bulls), allowed to mount an estrual cow with intromission and ejaculation; (4) electroejaculated (four bulls), no exposure to cows. Serum from blood samples taken at 15-min intervals from −15 min to 2 hr from sexual stimuli were radioimmunoassayed for luteinizing hormone (LH). Four bulls had elevated levels at the pretreatment bleeding, but none of the stimuli induced or were associated with LH releases. Basal levels of LH were consistent within bulls but varied considerably among bulls. Conclusion is that stimuli associated with mating do not cause a release of LH.

Eight 2 year old Hereford cows from days 8 to 12 of the estrous cycle were injected intramuscularly with 5 ml of corn oil containing 5 mg of estradiol-17beta (two cows), estrone (two cows), progesterone (two cows) or testosterone (two cows). Each cow treated with estradiol received 494 microc of estradiol-17beta-6, 7 H3 and each cow treated with estrone received 492 microc of estrone-6, 7 H3. Each cow treated with progesterone or testosterone received 400 muc of H3 compound labeled in the 7 position. Total urine was collected by urethral catheterization of the cows treated with estrogens. Blood samples for plasma and serum were collected via jugular cannulae. Blood and urine samples from estrogen-treated cows were collected hourly for the first 24 hr, at 2 hr intervals for the next 26 hr, at 4 hr intervals for the next 12 hr and at 12 hr intervals until background was reached. Blood samples were collected hourly from 1 to 8 hr after injection from progesterone or testosterone-treated cows. Plasma and serum levels of radioactive estradiol-17beta, estrone, progesterone and testosterone were similar. Blood levels of radioactivity peaked at 2 hr post-injection in cows receiving estradiol-17beta and at 3 hr in cows receiving estrone. Blood levels of labeled estradiol-17beta and estrone were nondetectable by 54 hr and 83 hr, respectively. Peak urinary excretion of radioactivity was reached at 7 hr for estradiol-17beta and at 14 hr for estrone and nondetectable levels were reached by 95 hr for estradiol-17beta and 14 hr for estrone. At these times, 15.5% of the total dose of radioactive estradiol-17beta and 17.5% of the injected estrone had been excreted in the urine. Peak blood and urinary excretion levels were reached earlier for radioactive estradiol-17beta than for estrone, and excretion of estradiol-17beta was completed more rapidly. No difference was found in plasma and serum levels for any steroid studies; thus, endogenous steroid titers in blood plasma and serum are not different in the cow.

Three experiments were conducted to examine different hormone treatments for their effectiveness in inducing or synchronizing a fertile estrus in prepuberal heifers. In experiments I and II, heifers which averaged 8.5 months of age and 249 kg in weight were given treatment E (5 mg E2β, experiment I), treatment PE (silastic rubber implant containing 2.1 g progesterone for 6 days and an injection of 5 mg E2β 24 hr after implant removal, experiments I and II) or treatment S-6 (6 mg SC-21009 progestogen implant for 6 days with an injection of 3 mg SC-21009 and 5 mg estradiol valerate at the time of implanting, experiment II). In experiment I, treatment PE increased the number of heifers showing estrus within 4 days (P.25) or 45 (P<12) days of breeding. In experiment III, heifers which averaged 13.7 months of age and 323 kg in weight were divided into puberal and prepuberal. The prepuberal heifers were given treatment S-9 (same as S-6 except implant period was 9 days) or SE (SC-21009 implant for 9 days with an injection of 5 mg E2β 24 hr after implant removal). Detection rate of the first estrus (after 4 days in prepuberal vs 22 days in puberal) was similar (P>.5) in all groups. Pregnancy rates were lower in SE heifers at the first estrus (P

Four hundred beef females from 1 to 12 years of age were randomly assigned by age, breed and planned artificial mating to receive either 10 sec of manual clitoral massage or no clitoral massage after artificial insemination for all services during the 1972 breeding season. In 1973, 783 beef females were assigned similarly to receive either clitoral massage at all inseminations, no clitoral massage at any insemination, clitoral massage at first insemination followed by no clitoral massage at second insemination or no clitoral massage at first insemination followed by clitoral massage at second insemination. Estrus was detected by technicians observing homosexual behavior and pregnancy by rectal examination in both years. Conception rates were higher (P <.05) in cows receiving clitoral massage (58.4%) than in cows not receiving clitoral massage (52.1%) when all inseminations were averaged over both years. Conception rates did not vary significantly among treatments in yearling heifers. An age × treatment interaction trend (P <.10) was found in conception rates and also was found in conceptions per service (P= .10). Conceptions per service tended to be higher in cows receiving manual clitoral massage (P <.15) but did not differ with treatment in yearling heifers. In this study, 10 sec of manual clitoral massage increased conception rate in cows but had no effect in yearling heifers.

ESTRUS was synchronized in 39 cows by feeding 180 mg medroxyprogesterone acetate (MAP)/head/day for 11 days and injecting 5 mg estradiol benzoate on day 2 of MAP feeding. At the postsynchronization estrus, the cows were assigned to five groups differing in intensity of experimentally administered mating stimulation. Cows in Groups I, II, III and IV were maintained in a lot with estrogenized cows and observed constantly to detect first standing estrus. Group V cows were maintained with an epididymectomized bull and observed constantly for first standing estrus; they were allowed a single service by the bull. At detection of standing estrus, cows were removed from the lots and were given the appropriate mating stimuli. Cows in Group I received no further stimulus, cows in. Group II received artificial insemination with no clitoral stimulation, cows in Group III received artificial insemination with 10 sec. of manual stimulation of the clitoris; cows in Group IV were immediately bred three times in succession by a bull and cows in Group V received no further stimulation. Blood samples were collected hourly for 24 hr. from the onset of estrus and at 2-hr. intervals thereafter until ovulation was detected by rectal palpation. Serum LH was determined using double antibody radio-immunoassay. The LH surge was 4.5 hr. earlier in relation to the beginning of estrus (P<.10) in Group II than in Group I and ovulation time was 4.3 hr. earlier in relation to the beginning of estrus (P<.005) in Groups III, IV and V (greater stimulation) compared to Groups I and II (lesser stimulation). In this study, cervical stimulation tended to hasten the LH surge and clitoral stimulation hastened ovulation.

Experiment I. LH levels rose from 3.1 ng/ml in 10 cows on day 1 following ovariectomy to 6.8 on day 28 (b=0.122, P< .01). Eighteen cows were injected IM with 10 mg estradiol-17β 25 days following ovariectomy. Nine were pretreated with 50 mg progesterone per day (IM) for 10 days from 14 to 25 days postovariectomy. Estrogen induced a significant LH release and estrus in all cows. Progesterone pretreatment had no effect on LH levels, acute LH release or expression of estrus. Experiment II. Eighteen ovariectomized cows were given one IM injection of 10 mg estradiol-17β. Six were given no other injections, six were injected with progesterone and six were injected with cortisol acetate. Twenty-five milligrams progesterone and 100 mg cortisol acetate were injected four times at 12-hr. intervals starting 12 hr. before the estrogen injection. Blood samples were taken by jugular venipuncture and estrus checked at −12 and 0 hr. from the estrogen injection and every 4 hr. from 12 to 40 hr. post-estrogen. All cows had an LH peak from 16 to 24 hr. post-estrogen injection. Progesterone or cortisol acetate treatment had no effect on interval to or height of LH peak. The proportion of cows detected in estrus was lower in progesterone-treated cows but the interval to estrus did not differ. Corticoid and estrogen levels were not affected by treatment but changed with time. Progestins decreased with time but progesterone-treated cows had elevated levels at 4 or 8 hr. after each progesterone injection.