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  • Author: R. G. ZIMBELMAN x
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R. G. ZIMBELMAN and L. W. SMITH

Summary.

The effects of oral melengestrol acetate (mga) on ovarian activity have been studied in 264 dairy and beef heifers. A total of ninety-eight heifers received 0·4 mg of mga daily for short periods of time (18 to 32 days) and ovarian activity was determined by rectal palpation. As the incidence of a detectable corpus luteum decreased from 76 % to 10 %, the incidence of a detectable follicle increased from 56 % to 91 %. Follicular size also increased with time. Samples of cervical mucus were rated according to fern pattern, which was interpreted as due to an oestrogenic influence, and which occurred upon regression of the corpus luteum even though oestrus and ovulation were inhibited by mga treatment.

A total of 166 beef heifers was used in long-term studies (105 to 116 days) of mga treatment. Significant (P<0·05) increases were found in the weight of follicular fluid, primarily of the largest follicle from the pair of ovaries. Also indicative of increased oestrogenicity was an increased adrenal weight in puberal, intact mga-treated heifers. The adrenal weights of spayed heifers tended to decrease with mga treatment. The optimal dose for increased follicular activity appears to be near 0·4 mg mga daily/heifer.

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L. W. SMITH and R. G. ZIMBELMAN

Summary.

Sixty-four normally cycling Holstein heifers were randomly assigned into four equal groups (three treatment and one control). Each of the three treatment groups received 0·5 mg of melengestrol acetate (MGA®) daily and a single injection of either oestradiol cypionate (ECP®), human chorionic gonadotrophin (hcg) or pituitary luteinizing hormone (plh) between Days 22 and 26 following oestrus or Days 8 to 12 of MGA feeding. The control animals received neither MGA nor an injection. Heifers from each treatment group were spayed (overiectomized) on Days 2, 5, 8 and 11 from the injection and control heifers were spayed on Days 1, 4, 7 and 10 from oestrus. The corpora lutea (cl) and the largest follicles on the ovaries of each heifer were observed for size and colour.

Oestrus was observed in only the ECP-treated group within 24 hr following the injections (nine of sixteen animals). Ovulation occurred following the injection in all of the sixteen animals in the hcg group and in fifteen of the sixteen animals in each of the ECP and plh groups. All the control heifers ovulated following oestrus.

Normal cl were present in all the treated heifers spayed by Day 5. On subsequent days, increasing numbers of heifers had cl which were regressing.

The percentages of heifers spayed on Day 8 and Day 11 after injection with regressing cl were: 0% and 50% for the ECP group; 25% and 100% for the hcg group; and 75% and 50% for the plh group. All the heifers in the control group had normally developing cl regardless of the day of spaying.

The follicles from those animals which had regressing cl were generally larger than those from control heifers.

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R. G. ZIMBELMAN and L. W. SMITH

Summary.

Dairy heifers with normal oestrous cycles were treated with progestagen for 15 to 18 days beginning at the 15th day of the cycle. Daily oral doses of 0·25 to 8 mg of melengestrol acetate (6α-methyl-6-dehydro-16-methylene-17-acetoxyprogesterone: mga) inhibited oestrus and ovulation in all heifers except one receiving 0·5 mg. Daily intravenous injections of 0·4 mg inhibited ovulation in eight out of eight heifers. Lower doses by either route suppressed oestrus but did not uniformly inhibit ovulation. Orally, mga was about 300 to 900 times as potent as map (medroxyprogesterone acetate; Provera*) but only about ten to fifteen times as potent when both were compared by intravenous injection.

Groups of eight to ten heifers were fed doses of 0·2 to 2·0 mg daily beginning without regard to the stage of the oestrous cycle. Of seventy-two heifers, sixty-nine did not ovulate during treatment. The average interval from last feeding to oestrus or ovulation ranged from 2·7 days at 0·2 mg to 6·3 days at 2·0 mg. The conception rates for various groups varied from 25 to 88 % at first service. Of the total, 42 % conceived with one service and 82 % with two services.

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L. W. SMITH and R. G. ZIMBELMAN

Summary.

Sixty-three dairy and beef heifers were given single intravenous (i.v.) injections of 2, 5 or 10 mg of oestradiol cypionate (ECP®) during, on the last day, or on Day 1 or Day 2 after last feeding of melengestrol acetate (MGA®). Thirty-five heifers, which were used as controls, were fed MGA for intervals corresponding to those given ECP. The dairy heifers were checked for pregnancy by rectal palpation 35 to 45 days after service, but the beef heifers were slaughtered following breeding. Reproductive tracts were grossly examined for abnormalities and the oviducts were flushed for ova. Data were collected from both groups on the occurrence of oestrus, ovulation and on conception rate.

ECP increased the overall incidence of oestrus. Ovulation, however, did not appear to be greatly affected by ECP, although there was a slight tendency for ovulation to occur sooner after oestrus in the heifers given ECP. ECP appeared to have a detrimental effect on the conception rate at the first controlled breeding, but the conception rate at subsequent breedings appeared normal. The reduced conception rate at first service following MGA plus ECP treatment appeared to be due to multiple causes: ovulation failure, ovum loss, fertilization failure and embryonic death. ECP would appear to have a favourable effect only when oestrus and/or ovulation failure was the primary cause of reduced conception rate.

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L. W. SMITH and R. G. ZIMBELMAN

Summary.

The following compounds were used in an effort to induce oestrus and ovulation in Holstein dairy heifers during melengestrol acetate (MGA®) treatment: oestradiol cypionate (ECP®), oestradiol-17β, Carbestrol®, human chorionic gonadotrophin (hcg), pituitary luteinizing hormone (plh), pregnant mare serum gonadotrophin (pmsg), oxytocin, neostigmine, amphetamine, nafoxidine hydrochloride and clomiphene citrate. MGA feeding began on Day 15 of the oestrous cycle and continued for 15 to 20 days. All the above compounds were administered between Day 8 and Day 13 of MGA feeding. The compounds were given by various routes of administration. Only the oestrogens (ECP, oestradiol-17β and Carbestrol) effectively induced oestrus and ovulation, their effectiveness seeming to depend on the method of administration and the type of vehicle in which they were administered. ECP was effective at 5, 10 and 20 mg when injected intravenously (i.v.) in an oil vehicle. Oestradiol-17β was effective at 10 mg when injected intramuscularly (i.m.) in an oil vehicle. Both were less effective when the vehicle was propylene glycol. Carbestrol was effective only when given orally at 40 mg and 60 mg levels.

The gonadotrophins (hcg, plh and pmsg) did not produce consistent results. When the follicles disappeared, a subsequent corpus luteum did not always appear to develop.

The other compounds tested had no observable effect on oestrus or ovulation.