Pulsatile infusion of gonadotropin-releasing hormone initiates luteal activity in nutritionally anestrous beef cows

Thirty-five nonlactating Hereford and Hereford x Angus cows weighing 469 +/- 8 kg, with a body condition score (BCS) of 5.5 +/- .6 and exhibiting normal estrous cycles, were fed to lose 1% of BW per week (n = 27) or to maintain BW (n = 8). Blood samples were collected every 7 d to monitor luteal activity and BW and BCS were recorded every 2 wk. When concentrations of progesterone in plasma were < 1 ng/mL for three consecutive weeks, anestrous cows (BCS, 3.1 +/- .7; BW loss, 24 +/- 5%) were assigned to pulses (i.v.) of saline (2 mL every hour), GnRH-4 (2 micrograms of GnRH every 4 h), or GnRH-1 (2 micrograms of GnRH every hour). Cows were confined to stalls for 4 d and two jugular cannulas were inserted 2 d before infusion of GnRH. Infusions began on d 0 and continued through d 14. Progesterone was quantified in daily blood samples and LH was quantified in samples collected every 10 min for 4 h on d-1, 0, 2, 4, 6, 8, 10, 12, 14, and 15. On d 16, cows were returned to pasture and blood samples were collected every 2nd d through d 30. Pulsatile infusion of GnRH increased (P < .05) the amplitude of LH pulses but did not affect pulse frequency. During infusion of GnRH every hour or every 4th h, concentrations of LH in serum were increased compared with those in saline-treated cows (P < .001), and cows receiving GnRH hourly had greater (P < .005) LH than cows given GnRH every 4 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of exogenous thyroxine on timing of seasonal reproductive transitions in ewes

The thyroid gland is necessary for the transition from the breeding season to anestrus in the ewe, but the importance of seasonal changes in thyroxine in this process is not known. The objective of this experiment was to determine the effects of exogenous thyroxine during the breeding season on the time of the transition to anestrus and on the onset of the subsequent breeding season in ewes maintained on natural photoperiod. Cyclic Galway ewes were blocked on the basis of body weight and randomly allocated to the following groups: 1) untreated controls (n = 11), 2) placebo-injected controls (n = 9), and 3) ewes injected daily with thyroxine from the middle of the breeding season until the onset of anestrus (n = 10). Reproductive state was assessed from serum progesterone concentrations determined twice weekly until the breeding season had ended in placebo-injected controls, and until the onset of the subsequent breeding season in untreated and thyroxine-injected ewes. The mean (+/- SEM) date of onset of anestrus did not differ between untreated ewes (April 19 +/- 9 days) and placebo-injected controls (April 15 +/- 9 days; p > 0.05), but occurred earlier in ewes injected with thyroxine (February 28 +/- 6 days; p < 0.001). Thyroxine treatment during the breeding season, however, did not affect the onset of the subsequent breeding season. These data indicate that supplementary thyroxine during the breeding season can advance anestrus and thus shorten the duration of the breeding season.(ABSTRACT TRUNCATED AT 250 WORDS)

Female reproductive physiology and endocrinology of cattle

Maximizing reproductive efficiency is of major economic importance to dairy and beef producers. The development of new and improved methods of increasing reproductive performance depends on our understanding of the physiologic and endocrinologic mechanisms controlling the reproductive process. This article reviews the mechanisms associated with puberty, estrous cycles, pregnancy, parturition, and postpartum return to estrus.

GnRH-dependent and -independent components of FSH secretion after acute treatment of anoestrous ewes with ovine follicular fluid and a GnRH antagonist

Gonadotrophin and inhibin concentrations were measured in anoestrous ewes after acute treatment with either saline, ovine follicular fluid (oFF), GnRH antagonist, or oFF and GnRH antagonist in combination. The increase in mean LH concentrations observed in ewes treated with oFF alone, was not seen in either of the groups treated with GnRH antagonist, in which LH pulsatility was completely inhibited. This result suggests that the LH rebound that follows follicular fluid treatment is GnRH dependent. Blockade of GnRH had no effect on the suppression of FSH seen after follicular fluid injection, indicating that this component of FSH secretion is independent of short-term GnRH input. After this initial suppression, a rebound release of FSH was seen in the group treated with oFF alone. The addition of GnRH antagonist appeared to decrease the rebound, suggesting that this rebound release of FSH may have a GnRH-dependent component. Inhibin concentrations in both oFF-treated groups increased after oFF injection and then declined to pretreatment values. However, a second rise in inhibin concentration, concomitant with the FSH rebound in ewes receiving oFF alone, was seen in the group treated with oFF and GnRH antagonist. As this rise in endogenous inhibin concentration could also act to suppress the rebound release of FSH, it cannot be conclusively proved from this study that GnRH input is required for the generation of the rebound release of FSH after treatment with oFF.

Glucoprivation induces anestrus and lipoprivation may induce hibernation in Syrian hamsters

Previous results supported the notion that estrous cycles in Syrian hamsters are responsive to the general availability of metabolic fuels, rather than to either fatty acid or glucose availability per se. To test this idea, we monitored estrous cycles in hamsters that were fed ad libitum and treated with a range of doses of 2-deoxy-D-glucose (2-DG), an inhibitor of glucose utilization. Hamsters treated with 2-DG at doses ranging from 750-1,250 mg/kg showed normal estrous cycles, but higher doses (1,750 or 2,000 mg/kg) induced anestrus. While it is clear from these data that estrous cycles are affected by glucoprivation, it is not clear whether they are responsive to decreased fatty acid availability. Groups of hamsters were fed ad libitum and treated with a range of doses of methyl palmoxirate (MP), an inhibitor of fatty acid utilization. Some of the hamsters that received the highest doses became torpid and thus were not tested for lordosis. None of the euthermic, MP-treated hamsters became anestrous. Other experiments examined the role of glucose availability in fasted hamsters. Hamsters with a high body fat content were protected from fasting-induced anestrus. In contrast, fat food-deprived hamsters treated with low doses of 2-DG (750 mg/kg) became anestrous. Thus fatty acids mobilized from adipose tissue did not prevent fasting-induced anestrus when glucose utilization was blocked. One interpretation is that during fasting fatty acid utilization spares glucose for other tissues involved in the control of estrous cycles.(ABSTRACT TRUNCATED AT 250 WORDS)

Yearling bulls shorten the duration of postpartum anestrus in beef cows to the same extent as do mature bulls

The objective of this study was to determine whether yearling bulls, when pastured with cows, reduced the duration of postpartum anestrus to the same extent as did mature bulls. This experiment was conducted over a 3-yr period. Cows were stratified by parity group to achieve 37% 2-yr-old and 63% mature (> 2-yr-old) cows within each treatment group (approximately 50 cows per treatment per year). Cows were assigned in the order in which they calved to one of three treatment groups: 1) isolated from bulls (NBE; n = 158); 2) exposed to mature bulls that were > 3 yr of age (MBE; n = 154); or 3) exposed to bulls that were 1 yr of age (YBE; n = 152). Beginning the 2nd wk after calving, cows were pastured with either sterile bulls that were 1 yr (YBE) or > 3 yr of age (MBE) (three bulls per treatment group). Blood samples were collected twice weekly from late March until mid-July each year. Cows with serum concentrations of progesterone > 1 ng/mL for two consecutive sampling periods were assumed to have initiated estrous cycles after calving. Duration of postpartum anestrus in cows exposed to yearling bulls (YBE = 61.8 +/- 1.8 d) did not differ (P > .10) from duration of postpartum anestrus in cows exposed to mature bulls (MBE = 59.5 +/- 1.7 d). Duration of postpartum anestrus was shorter (P < .01) for cows exposed to bulls (MBE+YBE = 61.0 +/- 1.7 d) than for cows isolated from bulls (NBE = 72.3 +/- 1.8 d).(ABSTRACT TRUNCATED AT 250 WORDS)

Immunogold identification of the GH cells of goat in different physiological conditions

Growth hormone-containing cells (GH cells) in the anterior pituitary gland of the female goat during anoestrus, pregnancy and lactation were identified by the immunogold complex technique combined with electron microscopy. In the three physiological stages analyzed, most of the GH cells were characterized by a round to oval or triangular shape and the presence of numerous spherical and electron-dense secretory granules (300-1,200 nm). Granules with the largest diameters were observed during the lactating stage. In these cells, the rough endoplasmic reticulum (RER) and Golgi complex (GC) were usually poorly developed. Also in lactating animals, a low number of GH cells with more highly developed RER and GC with a few small secretory granules (230-360 nm in diameter) were also observed. This structural configuration suggests that these cells are more active than those with a lower development of cytoplasmic organelles, although their low frequency does not support a considerable increase in hormonal synthesis and release during lactation in comparison with anoestrus and pregnancy stages.

Biogenic amines and the activity of the hypothalamo-pituitary-ovarian axis in ewes

An important feature of communication between the central nervous system and the pituitary-ovarian axis is the pattern of pulsatile discharge of hypothalamic luteinizing hormone releasing hormone (LHRH). The discharge of LHRH is under control of noradrenergic, dopaminergic, and serotoninergic systems of the brain. These systems intervene between external and internal signals (e.g. photoperiod and gonadal steroids) and LHRH output. In the ewe, noradrenaline, dopamine and serotonin of the brain are implicated in the control of LHRH output, and hence luteinizing hormone (LH) release, during both the anoestrous and breeding seasons. These amines are involved in steroid-dependent and steroid-independent regulation of LHRH/LH discharge. An interplay of inhibitory versus excitatory influences of these amines on LHRH/LH release appears to govern the pattern of LHRH/LH output during the annual reproductive cycle and the ovulatory cycle. A concise overview of this topic will be provided.

Reduced postpartum anestrus of suckled beef cows treated with microencapsulated luteinizing hormone-releasing hormone analog

This study evaluated the effect of microencapsulated LHRH agonist (D-Trp6-LHRH) on gonadotropin release and occurrence of estrus in early postpartum beef cows. Angus cows (n = 54) were assigned randomly to two treatment groups at d 5 postpartum. Group 1 received a single i.m. injection of D-Trp6-LHRH (LHRH-A) encapsulated in poly-DL-lactide-coglycolide, calculated to release 15 micrograms of LHRH-A per day for 30 d (n = 23). Group 2 received vehicle only (control, n = 31). Blood samples (15-min intervals for 6 h) were obtained on d 5, 10, 20, 30, and 40 postpartum for evaluation of LH and FSH concentrations (n = 12 per group). Days to first postpartum estrus were reduced by treatment with LHRH-A (Group 1, 43.7 +/- 4.2 d vs Group 2, 55.9 +/- 4.7 d; P < .05). However, days to conception were similar between groups (68.9 +/- 7.9 vs 76.7 +/- 6.7 d, respectively). On the day of treatment, cows treated with LHRH-A had higher mean concentrations of LH and FSH than did controls (8.3 +/- 1.4 vs 2.0 +/- .4 ng/mL for LH and 211.0 +/- 8.6 vs 51.2 +/- 2.7 ng/mL for FSH (P < .05). There were no differences in mean concentrations of LH or FSH between treatment groups on d 10, 20, 30, and 40 postpartum. Cows given LHRH-A had more (P < .05) LH pulses on d 10 and 30 postpartum than did controls. This study demonstrated that microencapsulated D-Trp6-LHRH reduced the postpartum anestrous interval in suckled beef cows.

Factors affecting the reproductive performance of the weaned sow

The majority of sows return to estrus within 2 weeks after weaning. Swine practitioners attempt to optimize production by reducing the WEI. Some sows fail to resume estrous cyclicity after weaning; however, the endocrinologic pathogenesis of these anestrous sows is speculative. The average WEI is influenced by numerous factors, including season, environmental temperature, photoperiod, nutrition, stress, facility design, lactation length, and management practices. It is evident that the majority of these factors have a more profound influence on primiparous sows than on multiparous sows. Optimum protein and energy consumption by sows during lactation and after weaning and effective utilization of breeding facilities reduce the WEI. The precise roles of photoperiodic changes, elevated environmental temperatures, and stress in seasonal infertility remain poorly understood. Fortunately, current management techniques have reduced the WEI on most farms without instituting therapeutic measures.

Variation in the timing of the reproductive season among breeds of sheep in relation to differences in photoperiodic synchronization of an endogenous rhythm

Photoperiod may regulate seasonal reproduction either by providing the primary driving force for the reproductive transitions or by synchronizing an endogenous reproductive rhythm. This study evaluated whether breed differences in timing of the reproductive seasons of Finnish Landrace (Finn) and Galway ewes are due to differences in photoperiodic drive of the reproductive transitions or to differences in photoperiodic synchronization of the endogenous rhythm of reproductive activity. The importance of decreasing photoperiod after the summer solstice in determining the onset and duration of the breeding season was tested by housing ewes from the summer solstice in either a simulated natural photoperiod or a fixed summer-solstice photoperiod (18 h light:6 h dark; summer-solstice hold). Onset of the breeding season within each breed did not differ between these photoperiodic treatments, but Galway ewes began and ended their breeding season earlier than Finn ewes. The duration of the breeding season was shorter in Galway ewes on summer-solstice hold than on simulated natural photoperiod; duration did not differ between photoperiodic treatments in Finn ewes. The requirement for increasing photoperiod after the winter solstice for initiation of anoestrus was tested by exposing ewes from the winter solstice to either a simulated natural photoperiod or a winter-solstice hold photoperiod (8.5 h light:15.5 h dark). Onset of anoestrus within each breed did not differ between these photoperiodic treatments, but the time of this transition differed between breeds. These observations suggest that genetic differences in timing of the breeding season in Galway and Finn ewes do not reflect differences in the extent to which photoperiod drives the reproductive transitions, because neither breed requires shortening days to enter the breeding season or lengthening days to end it at appropriate times. These findings are consistent with the hypothesis that photoperiod synchronizes an endogenous rhythm of reproductive activity in both breeds and that genetic differences in timing of the breeding season reflect differences in photoperiodic synchronization of this rhythm.

Weight changes prepartum and presence of bulls postpartum interact to affect duration of postpartum anestrus in cows

Our hypothesis was that cows with greater body condition at parturition would respond to the presence of bulls (biostimulation) by having shorter periods of postpartum anestrus than cows with a lesser body condition. Multiparous cows in good body condition at the initiation of the study were either maintained on a high dietary regimen or switched to a low dietary regimen during the 90 d preceding parturition. After parturition, half of the cows from each group were placed in pastures with sterile bulls, and the remaining half were placed in pastures without bulls. This study was replicated during a 2nd yr. Blood samples were collected twice weekly and assayed for concentrations of progesterone to estimate time of onset of luteal function after parturition. There was a significant interaction (P less than .05) between body condition at parturition and presence of bulls postpartum on the duration of postpartum anestrus. Cows of lesser body condition that were in the presence of bulls after calving initiated estrous cycles 14 d earlier than cows with lesser body condition that were isolated from bulls. In cows with a greater body condition, presence of bulls after calving only shortened postpartum anestrus by 6 d. Thus, we reject our initial hypothesis.

Chronic food restriction amplifies the effect of lactation on the duration of postpartum anestrus in rats

We studied the effects of food intake and lactation on the duration of postpartum infecundability in rats. Sprague-Dawley rats were given free access to a purified diet (AIN-76A) or a similar diet (modified to contain twice the concentrations of vitamins and minerals) in amounts equal to 50% of that consumed by controls. At -65 d of age, animals were bred. At parturition, dams were allowed to nurse 4-5 pups, or all pups were removed. Thus, four groups were created: lactating control (n = 11), nonlactating control (n = 11), lactating food restricted (n = 8), and nonlactating food restricted (n = 10). Vaginal cytology was observed twice daily for the detection of proestrus, which occurred at 4.5 +/- 0.5 and 6.7 +/- 0.5 d (mean +/- SD) postpartum in nonlactating control and food restricted groups, respectively (P less than 0.005), and at 16.5 +/- 0.5 and 28.8 +/- 0.5 d postpartum in lactating control and food restricted groups, respectively (P less than 0.0001). The interaction between food intake and lactation was significant (P less than 0.0001). These results indicate that maternal food restriction exacerbates the effects of lactation in prolonging postpartum anestrus.

Is nutritional anestrus precipitated by subfunctional corpora lutea in beef cows?

Thirty-four multiparous, lactating, cyclic beef cows which calved in moderate body condition were used to determine effects of restricted nutrition on corpus luteum (CL) development and endocrine status. At 78 d postpartum, six cows were assigned to a control (CON) diet (26.0 Mcal ME), fed to increase bodyweight (BW) and body condition score (BCS), and the remaining 28 cows were fed to lose BW and BCS on a restricted (RES) diet (14.0 Mcal ME). Following a 40-d adjustment period on respective diets, estrous cycles were synchronized and cows bled daily for determination of progesterone (P4), luteinizing hormone (LH) and insulin (INS) beginning at the synchronized estrus. Ultrasonography was used to determine the ovulatory follicle and CL development. Control cows were maintained for one estrous cycle and were ovariectomized on day 11 of their second cycle. Ten cows on restricted diet (RES-C) continued to form a functional CL (P4 > 1.5 ng/ml at day 10 of an estrous cycle) through as many as 5 cycles, after which observations were discontinued. Fourteen cows on restricted diet (RES-A) were ovariectomized on day 11 of a cycle when a CL was identified by ultrasonography, but was subfunctional (P4 < 1.5 ng/ml on day 10 of that cycle). Four additional RES-A cows which had subfunctional CL were not ovariectomized but were bled for an additional 25 d. At ovariectomy, CL and ovarian weights were collected. Luteal tissue was prepared for evaluation of P4 synthesis, LH responsiveness in vitro, and for determination of P4 content and total LH receptors. Bodyweight and BCS increased in CON cows; whereas, RES cows lost BW and BCS (P < .05). In the cycle prior to ovariectomy, serum P4 and LH were not different in 18 RES-A cows which developed subfunctional CL in comparison to CON cows. Four RES-A cows not ovariectomized but bled for an additional 25 d neither exhibited estrus, ovulated, nor had P4 concentrations greater than .3 ng/ml. Serum INS was lower in RES-A cows during the cycle prior to ovariectomy than in CON cows (P < .05). During the 11-d period prior to ovariectomy, mean serum P4 and INS were lower in RES-A cows than in CON cows (P < .05); however, serum LH was not different. Furthermore, CL and ovarian weights, P4 content of CL, secretion of P4 by luteal tissue in response to LH in vitro and LH receptor number were not different between CON and RES-A cows. In conclusion, nutritional anestrus may be preceded by the formation of a CL with lower steroidogenic output in vivo. However, luteal tissue, collected from RES-A cows, did not appear to be subfunctional during in vitro incubation when substrate availability and gonadotropin support were equal between diets.

Ram influence on ovarian and sexual activity in anestrous ewes: effects of isolation of ewes from rams before joining and date of ram introduction

A 2-yr experiment was conducted to determine whether isolation of ewes from rams is necessary to achieve a high response to the ram effect and whether ewes respond as well in May as in June. The experiment was conducted at two locations, with the same four treatments at each location. The four treatments differed with respect to ewe proximity to rams before mating (isolated vs adjacent) and date of joining with novel breeding rams (May 15 vs June 15). The proximity treatment at one location was changed in the 2nd yr; teaser rams were joined with the ewes instead of being adjacent to them. Overall, 86% of the eligible ewes were judged to have responded to the ram effect. A period of isolation before mating did not increase response compared with ewes that remained adjacent to, or in contact with, rams (86 vs 85%). Response was greater (P less than .05) in June and in the 2nd yr (P = .05). A physiological response, different from that generally described, was identified. Ewes ovulated approximately 8 d (8.0 +/- .19 d) after joining with breeding rams. The subsequent ovulation, accompanied by estrus, occurred approximately 15 d later (15.3 +/- .29 d). Eighty-five percent (87/102) of the ewes sampled responded in this manner. However, 82% (31/38) of a sample of these ewes had at least one morphologically normal corpus luteum when examined by laparoscopy 4 d after joining. It seems that these corpora lutea were not completely functional with respect to progesterone production. The ram effect can be achieved without prior isolation of ewes from rams.(ABSTRACT TRUNCATED AT 250 WORDS)

Daily rhythms of follicle-stimulating hormone in adult anestrous and prepubertal female Turkish hamsters (Mesocricetus brandti)

We tested the hypothesis that daily FSH surges are correlated with the anestrous condition in adult Turkish hamsters. Blood samples were collected from anestrous hamsters on short (12L:12D) and long photoperiods (16L:8D) by cardiac puncture. Both photoperiod-induced anestrous (PIA) and spontaneously anestrous adult Turkish hamsters had a daily rhythm of FSH secretion with maximum hormone concentrations occurring late in the afternoon. We also hypothesized that daily FSH and progesterone surges are correlated with the reproductive response to short photoperiod in prepubertal female Turkish hamsters. Prepubertal hamsters reared on 16L:8D were sampled at 13, 15, 17, 19, and 21 days of age. Only at 19 days was a rhythm of FSH present. Prepubertal female hamsters reared on 12L:12D were sampled at 30, 35, 55, 75, 95, and 115 days of age or until estrous cycles were established. Those hamsters that delayed puberty in response to this short photoperiod had a daily rhythm of FSH and progesterone secretion. Conversely, no daily rhythms were observed in female hamsters that failed to respond to the inhibitory photoperiod with a delay in puberty. These results indicate that the daily rhythm of FSH secretion in the female Turkish hamster is correlated with reproductively inhibited physiological states.

Oestrus and LH responses to oestradiol during lactational anoestrus in Chinese Meishan and large white sows

To investigate endocrine mechanisms associated with the occasional occurrence of fertile oestrus during lactation in the high prolific Chinese Meishan (MS) breed, the incidence of oestrus and changes in plasma luteinizing hormone (LH) levels before and after oestradiol benzoate (OB, 15 micrograms/kg body weight) administration on day 22 was compared in 4 MS and 6 Large White (LW) sows. All sows exhibited oestrus in response to OB. Only 1 sow (MS) ovulated in response to OB, became pregnant and farrowed. Mean plasma LH levels before OB were low (MS: 0.38 +/- 0.06 ng LH/ml, LW: 0.29 +/- 0.04 ng LH/ml, ns). LH levels above 2 ng/ml (surge) occurred in 2/4 MS and 2/6 LW sows at 60 +/- 5 h after OB. The MS sow that ovulated had an LH surge level of 4.5 ng/ml plasma at 40 h after OB. These results indicate minor breed differences in the control of LH secretion during lactational anoestrus.

Mechanism controlling ovulation rate in ewes in relation to seasonal anoestrus

Three experiments were carried out during seasonal anoestrus in Finnish Landrace and Scottish Blackface ewes, to establish whether the differences between the breeds in ovulation rate are functional during the non-breeding season and are therefore independent of the mechanism controlling ovulation. In Expt 1, follicles greater than or equal to 2 mm in diameter were dissected from the ovaries of both breeds and incubated individually for 2 h to assess their ability to secrete oestradiol and testosterone. In both breeds, follicles producing greater than or equal to 500 pg oestrogen/ml/h (oestrogen-active) were readily identifiable from a population producing less (oestrogen-inactive). The number of oestrogen-active follicles in each breed was similar to the number of ovulations near the end of the breeding season. Oestrogen-active follicles also had more luteinizing hormone (LH) receptors and larger diameters than oestrogen-inactive follicles. There were, however, no significant differences between the two follicle types in follicular fluid or in-vitro testosterone concentrations. In Expt 2, seasonally anoestrous Scottish Blackface ewes were unilaterally ovariectomized; the second ovary was removed 7 days later. Follicles from both ovaries were processed as described for Expt 1; oestrogen-active follicles were categorized according to their ability to produce greater than 500 pg/ml/h. There were twice as many oestrogen-active follicles in the second ovary as in the first ovary; the number of oestrogen-active follicles in the second ovary was also similar to the total number of oestrogen-active follicles in both ovaries of the Scottish Blackface ewes in Expt 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of calcium soaps of fatty acids on postpartum reproductive function in beef cows

Twelve multiparous Simmental cows (584 kg) were used to determine the influence of calcium soaps of fatty acids (CSFA) incorporated in a range supplement on postpartum reproductive characteristics. Cows were assigned randomly to receive a control [C; containing grain sorghum (GS) and soybean meal (SBM)] or CSFA-based (containing Megalac [a source of CSFA], GS, and SBM) supplement. Supplements plus prairie hay were individually fed. Diets were isonitrogenous and met the NEm requirement for heavy-milking beef cows in early lactation. Supplement feeding and daily blood collection began at parturition. Calves were removed permanently from cows at 25 +/- 2 d postpartum. Duration of first postpartum estrous cycles was determined by both visual observations and changes in concentrations of progesterone in serum. Concentrations of LH in serum (15-min intervals for 6 h) were determined 12 h before and 48 and 96 h after calf removal. Concentrations of progesterone and estradiol-17 beta in serum were determined daily. Cows receiving CSFA had higher (P = .06) mean concentrations of LH than those receiving C (1.47 vs 1.12 +/- .13 ng/ml). Concentrations of estradiol-17 beta were lower (P less than .02) and serum progesterone were higher (P less than .02) between d 6 and 8 of the induced cycle in CSFA-fed cows. Plasma cholesterol was greater (P less than .01) in cows fed CSFA although plasma triglyceride concentrations were similar between treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolactin short-loop feedback and prolactin inhibition of luteinizing hormone secretion during the breeding season and seasonal anoestrus in the ewe

In seasonally breeding mammals, plasma prolactin (PRL) concentrations vary on an annual basis with levels high in summer and low in winter. In this study of the ewe, we determined, first, whether PRL secretion is regulated by short-loop feedback and, second, whether the high summer levels of PRL are due to a change in sensitivity or loss of this feedback loop. Because the high summer levels of PRL coincide with the period of seasonal anoestrus in the ewe and could therefore be involved in the seasonal suppression of gonadotrophins, the effects of intracerebroventricular PRL on pulsatile LH secretion were also determined. Ovary intact ewes received intracerebroventricular injections of ovine PRL (oPRL; 50 micrograms) or anti-PRL serum. From 3 to 13 h after central administration of oPRL, plasma PRL concentrations were significantly reduced compared with the vehicle-treated controls. In contrast, following injection of anti-PRL serum, plasma PRL levels increased significantly. To determine whether there was a seasonal change in sensitivity to PRL feedback, a series of experiments were conducted in July and November when PRL concentrations are high and low, respectively. At each time of year, ovariectomized oestradiol-implanted ewes were injected intracerebroventricularly with 10 and 50 micrograms oPRL with control animals receiving the vehicle. At both times of year there was clear evidence of PRL short-loop feedback with no indication that sensitivity was reduced in the July trial. Luteinizing hormone (LH) pulse frequency, pulse amplitude and mean LH were not affected by intracerebroventricular oPRL at either time of year.(ABSTRACT TRUNCATED AT 250 WORDS)

Poor reproductive response of anestrous Suffolk ewes to ram exposure is not due to failure to secrete luteinizing hormone acutely

Twenty Polypay-sired ewes (Group P) and 14 Suffolk ewes (Group S) were bled at 48-h intervals for 10 d beginning on April 6, 1989, and the serum was assayed for progesterone to determine which ewes were anestrous; 9/20 Group P ewes were anestrous, compared with 14/14 Group S ewes (P less than .001). Catheters were placed into the jugular vein of anestrous ewes from both breed groups (eight of Group P, seven of Group S), and samples of serum were collected at 12-min intervals for 4 h. Then, the ewes were exposed to mature, intact rams, and additional samples were taken at 12-min intervals for 4 h after ram exposure. The serum was later analyzed to determine the secretion of LH in response to ram introduction. After the acute bleeding period, all Group P and Group S ewes were commingled and exposed to a ram continuously for 42 d. Samples of serum were collected thrice weekly and analyzed for progesterone to monitor ovulatory response to ram introduction through the 42-d period. In addition, breeding activity and lambing data were recorded. When all Group P ewes were compared to Group S ewes, a greater proportion (P less than .001) of Group P ewes were mated (20/20 vs 3/13; one Group S ewe died during the 42-d mating period) by the end of the 42-d period and more (P less than .001) ewes lambed in the fall (17/20 Group P vs 2/13 Group S).(ABSTRACT TRUNCATED AT 250 WORDS)

Uterine influences on the formation of subnormal corpora lutea in seasonally anestrous ewes

The hypothesis that subnormal luteal function after induced ovulation in anestrous ewes was the result of uterine influences exerted during the periovulatory period was tested. Crossbred ewes (n = 27) in seasonal anestrus were induced to ovulate by administration of 12 doses of 250 ng of LHRH at 2-h intervals, followed immediately by a bolus injection of LHRH (250 micrograms; d 0). Ewes were unilaterally hysterectomized on either d -3 (PRELHRH) or 2 (POSTLHRH). Daily blood samples were collected and assayed for progesterone (P4) and 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM). All ewes were slaughtered on d 10, and corpora lutea (CL) were collected, weighed, and assayed for concentration of P4. All ewes that ovulated exclusively in the ovary ipsilateral to the remaining uterine horn had a transient increase in plasma P4 of 2 to 3 d (short luteal phase). In ewes with at least one CL in the isolated ovary, elevated plasma P4 was maintained after hysterectomy but was consistently lower (P less than .05) in POSTLHRH ewes than in PRELHRH ewes. Concentrations of PGFM did not differ between treatments. The CL ipsilateral to the remaining uterine horn weighted less (P less than .01) and contained less P4 (P less than .01) than contralateral CL. These data confirm the hypothesis that premature regression of subnormal CL is uterine-dependent in a local fashion. Presence of the uterus during the follicular and(or) early luteal phase inhibited subsequent luteal function in seasonally anestrous ewes.

Effects of steroid administration on pituitary luteinizing hormone and follicle-stimulating hormone in ovariectomized pony mares in the early spring: pituitary responsiveness to gonadotropin-releasing hormone and pituitary gonadotropin content

These experiments tested the hypothesis that administration of steroid hormones to ovariectomized (OVX) mares during the vernal transition to the breeding season would influence LH and FSH secretion. Circulating gonadotropin concentrations, response to exogenous GnRH, and pituitary gonadotropin content were monitored. Experiments 1 and 2 were conducted, beginning 10 March, and 3 February, respectively, utilizing a total of 30 long-term OVX pony mares. In experiment 1, mares were administered vehicle (n = 5) or estradiol-17 beta (E2, n = 5, 5 mg/3 ml sesame oil), twice daily for 16 days. Blood samples were collected daily for assessment of circulating LH and FSH concentrations. On Day 10 of treatment, 400 micrograms GnRH were administered to all mares. LH increased significantly over days of treatment in the estradiol-treated group, but pituitary response to GnRH tended to be less than in control mares. Circulating FSH tended to decline over days of treatment in estradiol-treated mares, and the pituitary response to GnRH was significantly reduced. Pituitary LH, but not FSH, was increased on Day 16 of treatment with estradiol. In experiment 2, 20 OVX mares received, twice daily, vehicle (n = 5), E2, n = 5; 5 mg), progesterone (P4, n = 5; 100 mg), or progesterone plus estradiol (P4/E2, n = 5; 100 + 5 mg). Treatment continued for 14 days. GnRH (100 micrograms) challenges were administered on Days 6 and 13 of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Nutritional anestrus in beef cows: effects of body condition and ovariectomy on serum luteinizing hormone and insulin-like growth factor-I

The objectives of this experiment were to determine if a postcastration increase in concentrations of LH occurs in nutritionally anestrous beef cows and to examine the relationship between body energy reserves and secretion of LH and insulin-like growth factor-I (IGF-I). Nonpregnant, nonlactating, Hereford cows were fed to maintain (M) body weight (BW), body condition score (BCS), and normal estrous cycles (n = 5) or were fed a restricted (R) diet for 26 wk to lose BW and BCS and to become anestrus (n = 10). At 5-7 wk after the initiation of anestrus, R cows were randomly allotted to be ovariectomized (OVX) via flank incision (n = 5) or to remain intact (INT, n = 5). OVX was performed when R cows became anestrous. All M cows were OVX. Serum was collected frequently the day before and during the first 10 days after OVX, and concentrations of progesterone, estradiol, LH and IGF-I were quantified. On Day 10 after OVX, 1 mg of estradiol was injected into 3 cows from each group and serum was collected for 30 h. After OVX, there was a treatment-by-day effect for mean serum LH and IGF-I concentrations. Concentrations of LH increased (p less than 0.01) and concentrations of IGF-I decreased (p less than 0.05) in M-OVX cows when compared with R-OVX and R-INT cows. Concentrations of LH and IGF-I were similar for R-OVX and R-INT cows. The number of LH pulses was similar for M and R cows.(ABSTRACT TRUNCATED AT 250 WORDS)