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In this study we have examined luteal function in non-lactating and late lactation dairy cows on day 5 of the cycle, during the period of the post-ovulatory progesterone rise. Comparison of luteal progesterone content and in vitro synthetic capacity with circulating plasma progesterone demonstrated that circulating progesterone concentration is a function of total luteal activity rather than the activity of individual units of tissue. Incubation of luteal tissue in vitro demonstrated stimulatory activity of LH and IGF-I, and to a greater degree IGF-II, on luteal progesterone synthesis. Finally the study showed no effect of double ovulation on luteal function. Occurrence of double ovulation in 35% of animals was not associated with any difference in luteal function or plasma progesterone concentrations.

Infusion of leptin during the ovine follicular phase has been shown to increase progesterone secretion during the subsequent luteal phase. In this study, we have assessed the effects of infusing leptin during the early luteal phase. Infusion of leptin (2.5 microg/h) into the ovarian artery of ewes with ovarian autotransplants (n=5) on day 3 of the luteal phase for 12h did not affect progesterone estradiol or LH concentrations compared to control ewes (n=5). These results suggest no direct effect of leptin on ovarian function at this stage of the estrous cycle.

Data was collated from a number of studies on various aspects of luteal function in non-lactating dairy cows to allow comparisons to be made between single and double ovulating animals. In these studies, estrous cycles had been synchronized and animals slaughtered on day 5 or 8. The overall incidence of double ovulations was 28.3%. Double ovulation was associated with smaller individual corpora lutea but no difference in total weight of luteal tissue or any aspect of luteal tissue function or plasma concentrations of progesterone. Furthermore, in a sub set of animals, there was no difference in preovulatory follicle characteristics or plasma concentrations of estradiol around ovulation. These results demonstrated a high incidence of double ovulation in non-lactating cows that had no influence on circulating progesterone concentrations.

A delayed rise in post-ovulatory progesterone is associated with poor embryo development in the cow, although the underlying cause of this aberrant luteal function is poorly understood. The objective of this study was to develop a novel model, in which a delayed progesterone rise could be induced by manipulating the dynamics of the follicular phase. Luteolysis was induced in 20 dairy cows in the presence of either a larger follicle > 10 mm (LF, n = 11) or a smaller follicle < 10 mm (SF, n = 9) and transrectal ultrasonography was performed to determine follicle and CL growth and timing of ovulation. Plasma progesterone and oestradiol were analysed 3x daily. Cows were slaughtered on either day 4 (n = 4 per group) or day 7 (SF, n = 5; LF, n = 7) after ovulation. The pre-ovulatory follicle was larger in the LF group than the SF group at luteolysis (13.5 +/- 0.4 mm versus 6.7 +/- 0.7 mm, P < 0.001) and ovulation (16.7 +/- 0.3 mm versus 13.6 +/- 0.6 mm, P < 0.001). The LF group experienced a shorter follicular phase and ovulated 36 h earlier than the SF group (P < 0.001). At luteolysis, plasma oestradiol concentrations were greater in the LF group (P < 0.001), although peak concentrations were not different (P > 0.05). Moreover, higher progesterone concentrations were observed in the LF group during the early luteal phase (P < 0.05). Luteal weights were positively correlated with plasma progesterone concentrations on day 5 (P < 0.05) but not day 8. In conclusion, a model has been developed which has shown that the dynamics of follicle development during the pre-ovulatory period is an important determinant of subsequent CL development and function.

Productivity in farm species is controlled by many factors, including ovulation rate. In cattle, single ovulations occur most frequently and in sheep (and goats) the number of ova released can range from one to many depending upon the breed, whilst the pig is polyovular. The processes of recruitment and selection determine the number of ovulatory follicles in all these species with FSH and subsequently LH playing major roles. GnRH-agonist models in which endogenous gonadotrophin secretion is suppressed and exogenous LH and/or FSH are administered at specific concentrations in defined patterns, are useful in all three species for elucidating the precise roles of specific hormones in stimulating follicular development. Differences in the hypothalamic-pituitary-ovarian feedback response lead to the differences in the number of ovulatory follicles, as does the pool of antral follicles from which the ovulatory ones are selected. Precocious development of follicles is also associated with more ovulations, as is the case with the Booroola due to the single gene acting through bone morphogenetic proteins (BMPs). It is well established that ovulation rate can also be influenced by exogenous hormone administration and by environmental factors such as nutrition. It has become apparent that these nuritional effects are mediated by a direct action at the level of the ovary, involving insulin, insulin-like growth factors (IGF) I and II and their binding proteins among other factors. These factors can also affect the quality of the oocyte and consequently embryo development and survival. Recently, the regulation of follicular angiogenesis has been shown to be important for the development of ovulatory follicles, particularly vascular endothelial growth factor (VEGF) which is produced primarily by the granulosa cells within the ovary and can be stimulated by gonadotrophins. Administration of VEGF has been shown to stimulate pre-antral follicular growth and increase the number of pre-ovulatory follicles. In summary both extra- and intra-ovarian factors are involved in the control of ovulation rate. Manipulation of the angiogenic process may also provide new opportunities for regulating the quality and number of follicles that ovulate.

This study examined the effect of feeding either a maintenance (1.35 kg day(-1)) or high (3.5 kg day(-1)) plane of nutrition for 19 days after oestrus on oocyte maturity and both reproductive and metabolic hormone concentrations in gilts. Blood samples were collected each day from oestrus until slaughter on day 19 and during two pulse bleeds (15 min samples for 8 h) conducted on day 12 and day 18. After slaughter, oocytes were recovered from the presumed ovulatory population of follicles, matured in vitro for 46+/-2 h with 10% of their own follicular fluid, and then fixed and stained to determine the stage of nuclear maturation of the oocyte. Gilts fed the high diet had a higher proportion of oocytes that reached metaphase II than gilts fed the maintenance diet (88.3+/-2.71% versus 68.2+/-6.48%; P=0.013). Circulating concentrations of insulin-like growth hormone I (IGF-I) and the number of LH pulses were lower (P<0.05) in gilts fed the maintenance diet compared with gilts fed the high diet on day 12 and day 18. Mean oestradiol and progesterone concentrations were higher (P<0.05) for gilts fed the maintenance diet compared with gilts fed the high diet. Leptin concentrations were also higher on day 19 in gilts fed the high diet (2.16+/-0.26 ng ml(-1) (n=9) versus 3.20+/-0.32 (n=11), P=0.025). The results of this study indicate that improved oocyte quality (increased proportion of oocytes that reached metaphase II) is associated with a number of changes in reproductive and metabolic hormones. Further studies are required to indicate which hormonal mechanism may, in turn, lead to increased embryo survival and eventual litter size.

This study compared the dynamics of follicle development during the follicular phase in Meishan sows (n = 10) with a high ovulation rate, with those in contemporary Large-White hybrid sows (n = 12) (ovulation rates: 27.7 and 17.6, respectively; p < 0.01). Ovaries were recovered on Day 16 or 19 of the cycle, and all follicles > or = 1 mm in diameter were dissected. Fluid was aspirated from all those > or = 2 mm, and the granulosa and theca cells were incubated to estimate aromatase activity and estradiol production. The total number of follicles dissected was higher in Meishan than in Large-White hybrid animals (p < 0.001), and whereas the number decreased (p < 0.01) between Days 16 and 19 in Large-White hybrid sows, there was no significant change in Meishan animals. The mean diameter of Meishan follicles recovered on each of Days 16 and 19 was smaller (p < 0.001) than that of those recovered from Large-White hybrids. Total estradiol production in vitro was higher (p < 0.01) by follicles > or = 2 mm from Meishan sows. There was an interaction (p < 0.01) between day of the cycle, breed, and follicle diameter in aromatase activity in both granulosa cells and theca tissue, and in follicular fluid estradiol concentrations in follicles > or = 2-< 6 mm. The largest three size categories of follicles recovered on Day 19 from each breed were classified as preovulatory (29.0 and 17.7 for Meishan and Large-White hybrid, respectively), and the mean diameter was smaller (p < 0.001) in Meishan animals. Follicular fluid estradiol concentrations and total estradiol synthesis by granulosa and theca combined were higher (p < 0.05) in Meishan than in Large-White hybrid sows. In conclusion, high-ovulating Meishans maintained a higher number of follicles during the follicular phase, which overall synthesized more estradiol in vitro, than did Large-White hybrid animals.

The present study was designed to investigate the hormone profiles (oestradiol, LH, FSH, inhibin, progesterone) in high ovulating Meishan sows (MS; n = 9) and in contemporary Large-White hybrid control sows (LW; n = 9) during the follicular phase, the periovulatory period and the early luteal phase. Ovulation rate was higher in MS than LW animals (23.7 and 16.6 respectively; P < 0.001) and overall was correlated with the area of the oestradiol peak (P < 0.05) and inhibin concentrations (P < 0.05). Both the duration of and the area of the oestradiol peak were greater in MS than LW (P < 0.01; P < 0.02), as were inhibin concentrations both before and after the LH surge (P < 0.05). Neither basal nor peak concentrations of LH or FSH differed between the breeds (P > 0.05), although FSH concentrations were correlated with the area under the oestradiol peak (P < 0.05). Finally, the time-interval from the onset of the LH surge until the rise in plasma progesterone was shorter in MS than LW (54.5 and 74.3 h respectively; P < 0.01). In conclusion, these results show for the first time that the higher ovulation rate in MS is associated with enhanced oestradiol and inhibin secretion with no significant difference in LH or FSH concentrations. The more rapid luteinization response to the LH surge by MS in terms of plasma progesterone concentrations may be important in ensuring the high level of embryo survival in this breed.

The superovulation of red deer hinds with eCG is commonly associated with premature luteal regression. This study was an investigation of the endocrine mechanisms regulating luteal function after superovulation. Four groups of hinds (n = 7-8 per group) were treated with progesterone-impregnated intravaginal controlled internal drug-releasing (CIDR) devices for 12 days to synchronize estrus (CIDR device withdrawal = Day 0). Group 1 served as controls; group 2 received an i.m. injection of 0.25 mg estradiol benzoate (EDB) at 72, 84, 96, and 108 h after removal of the device; group 3 received an i.m. injection of 1200 IU eCG at -72 h; group 4 received both EDB and eCG treatments. Oxytocin-induced prostaglandin F2 alpha (PGF2 alpha) release was assessed on Day 4 by oxytocin challenge. Ovarian response was determined by laparoscopy on Day 14. Plasma steroid profiles were determined from thrice-weekly plasma samples collected from Day -13 to Day 35 (progesterone) and Days 0 to 14 (estradiol). EDB increased the incidence of premature luteal regression in monovulatory and eCG-treated animals (p < 0.05) and reduced the number of CL (p < 0.05) in eCG-treated animals. EDB and eCG each elevated plasma concentrations of estradiol and increased the incidence of significant oxytocin-induced PGF2 alpha release. These results support the hypothesis that eCG causes premature luteal regression by inducing prolonged estrogen secretion that sensitizes the endometrium to oxytocin, thereby eliciting PGF2 alpha release during the early luteal phase.

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 > or = 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 glits were observed six times a day for the commencement of oestrous behaviour and were subsequently examined by laparoscopy at 32 h following onset of oestrous and every 8 h till ovulation, which was a maximum of 56 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Attainment of puberty, cycle lengths, ovulation rate and endocrinology during the periovulatory period were studied in Meishan (MS) and European Large-White hybrid (LW) gilts. The mean age at onset of puberty of 115 days in MS (n = 20) gilts was younger (P < 0.001) than the 235 days in LW (n = 23). In the MS population studied, ovulation rate was not different (P > 0.1) during the third and fourth oestrous cycles, nor were there differences (P > 0.1) in the mean cycle length over the first three cycles. Overall changes in plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH) and oestradiol did not differ significantly (P > 0.1) between the breeds (MS, n = 6; LW, n = 5) during the periovulatory period, but plasma inhibin concentrations were significantly (P < 0.05) higher in the MS. The time intervals from the oestradiol peak concentration and the onset of the LH surge until the onset of behavioural oestrus were significantly different (P < 0.005) between the breeds, with oestrus occurring earlier in the MS. However, no difference (P > 0.1) was found between the groups when the intervals from the peak oestradiol concentration to the onset of the LH surge were compared. These results indicate differences between the breeds, particularly in terms of the age of attainment of puberty and the timing of the onset of behavioural oestrus relative to the oestradiol and LH surges.

Follicular growth in sheep and pigs is under both endocrine and paracrine control, and the local factors involved include steroids, growth factors and other regulatory peptides such as the putative follicle regulatory protein. These factors can act either directly on ovarian cells, or by modulating the response to circulating gonadotrophins. It is suggested that during the early stages of development in sheep and pigs, follicles pass through a stage at which they are modulated by growth factors, after which endocrine control mechanisms become more important. There is evidence that interactions between follicle-stimulating hormone, insulin-like growth factor-1 and oestradiol further enhance development and steroidogenesis. Interfollicular relationships in polyovular pigs may differ from those in sheep, in that the more mature follicles may act to enhance development of the less mature in the selected population. Although it is unclear whether similar mechanisms operate in both hyperprolific pigs and prolific breeds of sheep, Meishan pigs and Booroola ewes do exhibit similar follicular characteristics compared with those of nonprolific breeds. The significance of this in relation to prolificacy requires further investigation.

In order to investigate the development and possible heterogeneity in the luteal population following superovulation, anoestrous ewes were induced to ovulate using progestagen priming followed by injections of pregnant mare serum gonadotrophin (1000 IU) and hCG (1000 IU). Ovaries were recovered from ewes on each of days 2, 4, 6, 8, 10, 12 and 15, and the weight, progesterone content, 125I-labelled hCG binding and progesterone synthesis in vitro of the individual corpora lutea measured. The results obtained showed that plasma progesterone concentrations on the day of slaughter were significantly correlated with time (P less than 0.05), total weight of luteal tissue (P less than 0.001) and number of corpora lutea (P less than 0.05). The number of corpora lutea recovered per animal ranged from two to 12 and was significantly (P less than 0.05) correlated with the day after hCG injection until day 10. There was much variation between individual corpora lutea, particularly in terms of weight and progesterone content, although both were significantly (P less than 0.001) correlated with day of recovery until day 10. 125I-labelled hCG binding was significantly (P less than 0.001) correlated with time until day 15. There was a significant (P less than 0.001) effect of age of the tissue on progesterone production in vitro, with output declining throughout the luteal phase. These results show that the number of corpora lutea induced by superovulation in anoestrous ewes was very variable, and suggest that ovulation may have continued to occur during the luteal phase.(ABSTRACT TRUNCATED AT 250 WORDS)