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G. L. HUNTER

It has been postulated that the uterus in the sheep is either directly or indirectly responsible for the regulation of the life-span of the corpus luteum (cl) (see recent reviews by Ginther, 1967; Melampy & Anderson, 1968; Caldwell, Rowson, Moor & Hay, 1969). A local relationship between each uterine horn and the adjacent ovary has been suggested, since the lack of one of the uterine horns, achieved by partial (unilateral) hysterectomy, appears to prolong the life-span of cl in the ipsilateral ovary, but has no such effect on cl in the ovary adjacent to the remaining horn (Inskeep & Butcher, 1966; Moor & Rowson, 1966). An alternative explanation, suggested by Moor & Rowson (1966) and emphasized by Nalbandov & Cook (1968), is that it is the effect of the surgery itself rather

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G. L. HUNTER and L. E. CASIDA

Summary.

Six groups of rabbits were mated and used as follows: (1) embryos in one horn of the uterus, no embryos in the other; (2) pregnant, unilaterally hysterectomized; (3) pseudopregnant, unilaterally hysterectomized; (4) embryos present in both horns; (5) bilaterally pseudopregnant; and (6) bilaterally hysterectomized. At 26 days post coitum (p.c.) comparisons of the mean weights of corpora lutea in the ovaries adjacent to each horn showed no significant evidence of differences in local actions of the uterine horn on luteal growth and regression. The mean weights of corpora lutea in the two ovaries were similar in rabbits with one gravid horn and/or one non-gravid horn.

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L. S. Faillace and M. G. Hunter

A study was carried out to compare preovulatory ovarian events in prolific Meishan gilts (which have high levels of embryo survival) with contemporary white hybrid control gilts. Gilts of similar reproductive ages (second or third oestrous cycle) were observed three times a day for oestrous behaviour and ovaries recovered at a time estimated as within 7 h of ovulation (white hybrid, n = 13; Meishan, n = 16). Preovulatory follicles were recovered (n = 195, white hybrid; n = 252, Meishan), and oocytes were aspirated and fixed for later identification of meiotic stage, and follicular fluid was frozen for subsequent determination of progesterone. The number of presumed ovulatory follicles recovered per animal was similar to the expected ovulation rate (15.0, white hybrid; 15.75, Meishan; P > 0.1); however, follicles from Meishan gilts were smaller (8.1 versus 9.0 mm, P < 0.001) and contained less follicular fluid (139.9 versus 187.6 μl, P < 0.02) than did white hybrid gilts. Total follicular progesterone (88.7 ng per follicle, white hybrid; 77.4 ng per follicle, Meishan) and concentration of progesterone in follicular fluid (526 ng ml−1, white hybrid; 640 ng ml−1, Meishan) did not differ between the two breeds (P > 0.1). There were no breed differences in the extent of heterogeneity in follicular diameter, volume of follicular fluid, and total follicular progesterone and progesterone concentration (P > 0.1). However, classification of recovered oocytes into seven discrete meiotic stages revealed that more oocytes recovered from Meishan preovulatory follicles were in the more advanced stages of meiosis than were those recovered from white hybrid gilts (P < 0.001). In conclusion, it is suggested that the advanced oocyte maturation in follicles of Meishan pigs before ovulation may be important for ensuring the prolificacy of this breed.

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G. L. HUNTER and A. W. LISHMAN

Summary.

By means of ovarian examination and teasing with vasectomized rams, the intervals to first post-partum ovulation and oestrus were determined in a 23-factorial experiment with twenty-four German Merino ewes which lambed between 28th October and 15th November 1966. First ovulation occurred 32·1 ± 1·9 (8 to 45) days after lambing. Three, fifteen and six ewes had none, one or more than three silent ovulations respectively, before showing heat after a mean interval from lambing for all ewes of 57·1±4·5 days. The regression of post-partum interval to ovulation (Y) on ewe's weight 3 days post partum (X) was significant (P<0·01) ; for ewes weighing 83 to 162 lb, Y = 55·15–0·21X. The regression of post-partum interval to oestrus on ewe's weight was not significant, and neither were the effects of treatments, namely, weaning at 3 or 20 days post partum, supplementary feeding of 0 or 1 lb maize grain daily from 3 to 28 days post partum, and joining with vasectomized rams at 3 or 14 days post partum.

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S. D. PARSONS and G. L. HUNTER

Summary.

A series of experiments is reported in which the effect on the length of heat of varying degrees of association of the sexes was studied in Merino sheep. Length of heat in all ewes was determined by teasing with vasectomized rams at 4-hr intervals. When present throughout oestrus, mating behaviour of rams and also of stilboestrol-treated (`masculinized') ewes, reduced heat length in experimental ewes by some 30 to 50%. The sight, smell and sound of the rams were not sufficient to reduce the duration of oestrus. However, neither mounting nor intromission were necessary components of the male behaviour for the effect to be fully expressed. Oestrous periods synchronized by injections of progesterone were as effectively shortened by the continuous presence of the rams. The ewes were most sensitive to the effect of ram activity between 4 and 8 hr after the beginning of oestrus, but oxytocin treatment during this time did not reduce oestrous duration.

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S. D. PARSONS, G. L. HUNTER and A. A. RAYNER

Summary.

To investigate the relationship between oestrus and time of ovulation from the start of heat and the effect of the ram on this relationship, 200 ewes, half of which had been run with rams continuously during oestrus, were slaughtered at intervals after the end of heat. The method of probit analysis was used to fit multiple regression equations for percentage of ewes ovulated (as a probit) in terms of slaughter time and heat length. From these equations, estimates of mean ovulation times for various heat lengths were calculated. In the control group, which was teased at 4-hr intervals to determine lengths of heat, ovulation occurred, on the average, shortly after the end of oestrus. Although duration of oestrus was shorter when the sexes were continuously associated, ovulation occurred later. In addition, there was a positive correlation between heat length and time of ovulation in this group, but not in the control group.

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G. L. HUNTER and I. M. R. VAN AARDE

Summary.

A total of 131 2- to 6-year-old Mutton Merino ewes in two experiments were maintained in pens during the post-partum period and were exposed either to an artificial photoperiod of decreasing daylength, or to the naturally increasing photoperiod. After lambing in July, the ewes either were deprived of their lambs or they reared one lamb for 40 days. In both groups, the ewes were fed at one of four nutritional levels until the first post-partum oestrus had been recorded. The proportion of ewes showing first oestrus within 90 days of lambing was 44% for those subjected to the artificial photoperiod compared with 67% for those exposed to the natural photoperiod. First oestrus in the ewes which showed first oestrus more than 90 days after lambing was less delayed in those exposed to the artificial photoperiod. Nutritional level had no effect on the time of resumption of oestrus. In the natural photoperiod, 67% of lactating ewes and 68% of non-lactating ewes showed oestrus in 56·3 and 42·1 days, respectively, and a similar effect was observed in those exposed to the artificial photoperiod. In the natural photoperiod, 33% of 2-year-old, 53% of 3-year-old and 84% of mature ewes showed first oestrus within 90 days of lambing. It was concluded that, in most periods of the year, many ewes of this breed can be remated within 2 to 3 months of lambing and may thus be able to lamb three times in 2 years.

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L. S. Faillace, C. Biggs and M. G. Hunter

Experiments were carried out to study: the effects of season on age at puberty, the influence of reproductive age on ovulation rate, and the time interval from the onset of oestrus to ovulation in Chinese Meishan gilts. Gilts approaching puberty either in the spring (n = 88) or in the autumn (n = 40) were housed indoors under natural daylight conditions and observed daily for oestrous behaviour. Gilts approaching puberty in the spring were younger (P < 0.001) and more likely to reach puberty by 100 days of age (P < 0.01) than were those approaching puberty in the autumn. Ovulation rate was estimated in gilts at second (n = 22), third (n = 24), fourth (n = 18), fifth to ninth (n = 9) and tenth to twenty-first (n = 17) oestrous cycle and in primiparous Meishan sows (n = 12) by counting the number of corpora lutea or corpora albicantia at laparoscopy, laparotomy or at ovarian recovery following slaughter. Ovulation rate increased (P< 0.001) with reproductive age and approached that of primiparous sows only when gilts had experienced ≥ 10 oestrous cycles (19.2 versus 21.0). The time interval between the onset of oestrus and ovulation was studied in six naturally cyclic Meishan gilts and nine Meishan gilts administered hCG at the onset of oestrus. All gilts were observed six times a day for the commencement of oestrous behaviour and were subsequently examined by laparoscopy at 32 h following onset of oestrus and every 8 h until ovulation, which was at a maximum of 56 h. Relative to the onset of oestrus, gilts administered hCG ovulated earlier than did the control group (44.2 versus 48.7 h, P < 0.05) and at an interval of 42.2 h relative to the administration of hCG.

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G. L. HUNTER and I. M. R. VAN AARDE

Summary.

Seventy-two Mutton Merino ewes in a 3×23-factorial experiment lambed in July, November or March/April, suckled a lamb for 1 or 40 days, were fed after lambing at the rate of 110% or 60% of the estimated daily nutrient requirements of non-lactating and lactating ewes, respectively, and were run with (`joined') vasectomized rams from 20 or 30 days post partum. After the July lambing, the date of the first post-partum ovulation (estimated at laparotomy) was significantly earlier for ewes on the high plane of nutrition (48·2 versus 88·5 days after lambing), but the post-partum interval to the first oestrus was very variable (32 to 198 days, mean 102·9 days) and was not significantly influenced by the plane of nutrition. Following lambing in November and more especially after lambing in March/April, both ovulation (47·3 days and 27·6 days, respectively) and oestrus (65·5 days and 50·7 days, respectively) recurred more quickly than following lambing in July, but the difference in feeding treatment was not effective after these lambings. The resumption of sexual activity was not influenced by suckling or by the stage at which the ewes joined the rams. It is suggested that, if lactating and non-lactating ewes are fed to meet their respective nutritional requirements, the length of their post-partum anoestrous periods will not differ.

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G. L. HUNTER, G. P. BISHOP, C. E. ADAMS and L. E. ROWSON

Summary.

Two experiments were carried out involving the exportation by air from Great Britain of a total of three rabbit does carrying in their oviducts fertilized sheep ova for transfer to recipient ewes in South Africa. In October, 1960, ova were recovered in Cambridge from Border Leicester ewes which had been mated to a ram of the same breed. The ova were transported in two does to Pietermaritzburg, where they were transferred to Dorper ewes. In the second experiment early in December, 1960, fertilized ova were recovered from Welsh Mountain ewes (after they had been mated to Welsh Mountain rams) and finally transferred to German Merino ewes in South Africa. Following storage periods in the rabbits ranging (in the two experiments) from 101 to 128 hr, seventeen sheep blastocysts or late morulae were recovered in Pietermaritzburg and sixteen were transferred to a total of ten recipient ewes. Six pregnancies resulted and four live lambs were born to foster mothers in South Africa. The techniques are described in detail.