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D. R. LAMOND

Summary.

Three groups of Merino ewes (a total of 150) kept in yards were studied for a period of 1 year. Vasectomized rams were with one group and occurrence of oestrus was noted. The presence and number of corpora lutea were observed by laparotomy at 6-week intervals. The annual rhythms in these characteristics were determined.

A second group of ewes received progesterone treatments at intervals of 6 weeks. Half the ewes were with vasectomized rams continuously but rams were with the others for only 1 week after the final progesterone injection. The period from the final injection to the onset of oestrus was determined. Introduction of rams increased the number of ewes showing oestrus at the beginning (late summer) and end (late winter) of the breeding season. The period to onset of oestrus was related to dose and frequency of progesterone injections, the time of the year, and introduction of rams. Periods to onset of oestrus became progressively longer after mid-winter (June) and a proportion of progesterone-treated ewes did not show oestrus in the 1 week period after cessation of progesterone injections in July, at a time when all untreated ewes were experiencing oestrous cycles. The implications of the seasonal change in the effects of progesterone on the ewes are discussed in relation to the sexual season.

A third group of ovariectomized ewes received progesterone followed by oestrogen, also at intervals of 6 weeks. The number of ewes showing oestrus during the period January to June was dependent on the dose of progesterone independently of the effect of dose of oestrogen. The relationship did not hold at other times. In general, there was considerable heterogeneity in mean responses and slopes of the dose-response lines.

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D. R. LAMOND

Summary.

An experiment was carried out to obtain quantitative information on time of ovulation after hormonal treatments at four times of the year. Four groups of Merino ewes were used in a 4 × 2 × 3×2 factorial experiment, the factors being:

  • Four times in one year: July 1960, October 1960, January 1961, April 1961.
  • Two doses of pms: 300 i.u., 900 i.u.
  • Three doses of hcg: 300 i.u., 600 i.u., 1200 i.u.
  • Administration of hcg at two different periods after pms: 24 hr, 48 hr.

All ewes were given 12·5 mg of progesterone in oil every 2 days for 2 weeks. pms was given at the time of the final injection of progesterone. The time of ovulation was determined at laparotomy at various periods after the final hormone injection.

There were no significant differences between times of year in either number of ovulations occurring during progesterone treatments or in numbers of multiple ovulations following cessation of hormone treatments.

Seasonal differences in numbers of ewes not ovulating and in the time of ovulation in the remainder were observed.

The results cannot be explained by a simple theory of seasonality in ovulating hormone production, but point to a more fundamental problem of a neuro-hormonal nature.

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D. R. LAMOND

Summary.

Hereford heifers in yards were given progesterone for periods of 2 weeks, at intervals of 7 weeks over a period of 1 year. Seasonal changes in the effects of progesterone were observed. During the late winter and spring months the ovarian cycles were not suppressed by doses of progesterone that were satisfactory at other times of the year. The interval from the cessation of treatments to onset of oestrus was also shorter in the late winter/spring period.

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D. R. LAMOND

Summary.

Injections of progesterone were given to ewes to suppress ovarian cycles; pregnant mare serum gonadotrophin (pms) was administered within 2 days of the final injection of progesterone. Oestrus, numbers of ovulations and follicle growth were observed during the week following treatment. In some experiments, ewes were mated to fertile rams and fertility and fecundity were recorded. Experiments were carried out at different stages of the breeding season.

Relationships between methods of suppression of ovarian cycles (dose and frequency of injections of progesterone) and methods of ovarian stimulation (dose and time of administration of pms in relation to the final progesterone injection) for each of the responses were examined. The most important finding was that for comparable dosage levels of pms, the numbers of ovulations were greater after progesterone injections on alternate days than after daily injections. Stage of the breeding season and introduction of rams influenced the results.

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D. R. LAMOND

On theoretical grounds, one might expect to find a good deal of variation between cows in amounts of progesterone and oestrogen secreted by the ovary during the follicular phase of the oestrous cycle. Thus, there is variation in the rate of regression of corpora lutea (cl) in terms of size and progesterone content (Mares, Zimbelman & Casida, 1962; Niswender, Kaltenbach, Shumway, Wiltbank & Zimmerman, 1965), and limited data suggest that content is correlated with amounts of progesterone secreted into the ovarian vein (Gomes & Erb, 1965). Although little is known about oestrogen content of ovarian vein blood, considerable variation has been observed in numbers of Graafian follicles during the follicular phase (Rajakoski, 1960; Choudary, Gier & Marion, 1968; Lamond, unpublished). It seemed likely that variation in size of Graafian follicles might be associated with variation

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D. R. LAMOND and R. G. GADDY

Because of much interest in multiple births of calves, there is a need to investigate the problems associated with gestation of two to four embryos. This paper relates number of cl to plasma progesterone concentrations in maiden cows of beef breeds.

In Exp. 1, carried out in September 1969, oestrous cycles of eighteen Angus and Hereford cows, 18 to 20 months old, were suppressed with 0·5 mg melengestrol acetate (MGA—The Upjohn Co., Kalamazoo)/cow/day for 18 days. Four days before the end of MGA treatment, 1500 to 3000 i.u. pmsg were given to induce multiple ovulation. The cows were fasted for 3 days after the pmsg injection to reduce excessive follicular development (Lamond, 1970). Thirteen cows which came into oestrus on the 3rd day after cessation of hormonal treatments were

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B. M. BINDON and D. R. LAMOND

Summary.

The uterine weight and ovulation responses of immature intact and hypophysectomized mice from two lighting regimes have been studied after injection of gonadotrophin. The results indicate diurnal variation in ovarian response: the response to gonadotrophin was greater when it was administered during the second half of the light phase of the daily light cycle than at any other time.

Pregnant mice were fasted for 36- to 48-hr periods beginning at 06.00 hours or 18.00 hours on the 2nd, 3rd or 4th day after mating. Diurnal variation in the proportion of pregnancies failing was demonstrated. The effect was not attributable to differences in loss of body weight during fasting, and could be prevented by injections of human chorionic gonadotrophin (hcg).

The numbers of eggs ovulated by adult di-oestrous mice treated with hcg were studied after 36- or 48-hr fasts commencing at 06.00 hours or 18.00 hours 2 days before an injection of hcg. Fasting reduced numbers of ovulations, but the effect was influenced by the time of day when the fast began. Reversal of the lighting regime resulted in a reversal of the pattern.

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B. M. BINDON and D. R. LAMOND

Summary.

Experiments were conducted in pregnant mice in an attempt to clarify the role of the pituitary gland in implantation in this species. Hypophysectomy was performed at various times during Days 2 to 5 of pregnancy and replacement studies with human chorionic gonadotrophin (hcg) or progesterone were carried out.

Results indicated that pituitary involvement in the initiation of implantation was confined to the early part of Day 3. Changes in the photoperiod altered the time of pituitary involvement, thus exposing a possible source of error in between-laboratory comparisons in this type of study.

Pregnancy was maintained in hypophysectomized mice by daily injection with hcg. The minimum daily dose was between 1·1 and 3·3 i.u. Embryos maintained by hcg were smaller than those in hypophysectomized mice injected with progesterone.

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D. R. LAMOND and B. M. BINDON

Summary.

Progesterone was administered to Merino ewes in February (early breeding season) and May (mid-breeding season) for 2 weeks. The period from the final progesterone injection to the onset of oestrus, the duration of oestrus, and fertility after hand mating were observed. The approximate time of ovulation was also determined in the February experiment.

The duration of oestrus was 10 to 50 hr; ovulation took place near the end of oestrus. The period from the last injection to the onset of oestrus, and the fertility of the ewes were influenced by dose, interval between consecutive progesterone injections, the time of day that the injections were given, and the stage of the breeding season. Variability in onset of oestrus (indicating the degree of synchronization of oestrus) was least when the final injections were given in the morning.

The results clearly show that normal levels of fertility may be achieved after synchronization of oestrus in Merino ewes with progesterone.

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D. M. HENRICKS, J. R. HILL Jr, J. F. DICKEY and D. R. LAMOND

Summary.

Doses of 0, 1600 or 3200 i.u. PMSG were administered on Day 16 of an oestrous cycle to twenty-four beef heifers. Jugular blood was collected at 08.30 and 17.30 hours each day until oestrus and at 0, 3, 6, 9 and 12 hr after the beginning of oestrus. The plasma was analysed for total oestrogen, progesterone and LH concentrations. The heifers were killed 3 to 8 days after mating, and their ova and ovaries were examined to determine the incidence of fertilization, numbers of CL, and numbers of follicles greater than 10 mm diameter. The low dose of PMSG caused less variation in number of ovulations and fewer large follicles than the high dose, though the mean number of fertilized ova was similar. The hormonal patterns were related to the dose of PMSG. Plasma progesterone levels remained elevated for a longer period and decreased more rapidly in the animals given 3200 i.u. than in the others. Total oestrogen began to rise about 48 hr before oestrus in all groups and reached highest levels and remained elevated for longest periods in the animals given 3200 i.u. PMSG. The LH peaks occurred at 0 hr, 3 hr, or 6 hr after the beginning of oestrus in cows treated with 3200 i.u., 1600 i.u. or nil PMSG, respectively.