Coincident secretion of follicle-stimulating hormone and luteinizing hormone in early postpartum beef cows: effects of suckling and low-level increases of systemic progesterone

Exposure to progesterone before an ovulation synchronization protocol increases the follicular diameter and fertility of multiparous suckled Bos taurus cows

The objective of this study was to evaluate the effect of administering injectable progesterone (P4i) before a timed artificial insemination (TAI) protocol on the follicular growth, ovulation, and pregnancy rate of Bos taurus suckled cows. The effect of P4i administration before the TAI on the pregnancy rate (P/AI) was evaluated in 576 suckled Bos taurus cows at 30-90 days postpartum. In addition, the effect of P4i administration before TAI on follicular dynamics was evaluated in subgroup of 401 suckled Bos taurus cows. On Day -10 (D-10), cows were divided into two experimental groups (Control and P4i). In this moment, P4i cows received i.m. 150 mg of injectable long-action progesterone. After that, both experimental groups received a synchronization protocol (Day 0; D0) that consisted of administration i.m. of 2 mg of estradiol benzoate and a progesterone intravaginal insert on D0. On Day 8 (D8), the progesterone insert was removed, and the cows received 500 μg of cloprostenol, 400 IU of eCG, and 1 mg of estradiol cypionate. TAI was performed 48 h after the removal of the progesterone insert. The ultrasound exams were performed in a subgroup of cows on Days 0, 8, 10 and 12 to evaluate the diameter of the largest follicle, rate of follicular growth and risks of single and double ovulation. The pregnancy diagnosis was performed 30 days after TAI in all cows to determine the pregnancy rate. The diameter of the largest follicle, on D10 (P = 0.84), rate of follicular growth (P = 0.14), ovulation rate (P = 0.40) and double ovulation rates (P = 0.23) did not differ between experimental groups. The pregnancy rate was greater in the P4i group [Control 46.2 % (133/288) vs. P4i 55.6 % (160/288); P = 0.03]. The diameter of the largest follicles (LF) on D0 (Control 11.6 ± 0.2 vs. P4i 13.3 ± 0.3) was greater (P = 0.01) in the P4i group. In conclusion, injectable progesterone before the ovulation synchronization protocol increased the diameter of the largest follicle on the D0 and the pregnancy rate in multiparous Bos taurus suckled beef cows.

Interaction of pre- and postnatal nutrition on expression of leptin receptor variants and transporter molecules, leptin transport, and functional response to leptin in heifers†

Perinatal nutrition modulates the hypothalamic neurocircuitries controlling GnRH release, thus programming pubertal maturation in female mammals. Objectives of experiments reported here were to test the hypotheses that prenatal nutrition during mid- to late gestation interacts with postnatal nutrition during the juvenile period in heifer offspring to alter expression of leptin receptor (LepR) variants (ObRa, ObRb, ObRc, ObRt), and lipoprotein transporter molecules (LRP1 and 2) in the choroid plexus, leptin transport across the blood-brain barrier, and hypothalamic-hypophyseal responsiveness to exogenous ovine leptin (oleptin) during fasting. Nutritional programming of heifers employed a 3 × 2 factorial design of maternal (high, H; low, L; and moderate, M) × postnatal (H and L) dietary treatments. Results (Expt. 1) demonstrated that prepubertal heifers born to L dams, regardless of postnatal diet, had reduced expression of the short isoform of ObRc compared to H and M dams, with sporadic effects of undernutrition (L or LL) on ObRb, ObRt, and LRP1. Intravenous administration of oleptin to a selected postpubertal group (HH, MH, LL) of ovariectomized, estradiol-implanted heifers fasted for 56 h (Expt. 2) did not create detectable increases in third ventricle cerebrospinal fluid but increased gonadotropin secretion in all nutritional groups tested. Previous work has shown that leptin enhances gonadotropin secretion during fasting via effects at both hypothalamic and anterior pituitary levels in cattle. Given the apparent lack of robust transfer of leptin across the blood-brain barrier in the current study, effects of leptin at the adenohypophyseal level may predominate in this experimental model.

Comparison of Gonadotropin-Releasing Hormone versus Estrogen-Based Fixed-Time Artificial Insemination Protocols in Grazing Bos taurus Suckled Beef Cows

Fixed-timed artificial insemination (FTAI) protocols for beef cattle in South America are primarily based on estradiol esters and intravaginal progesterone-releasing devices (IVPD). The objective of this study was to determine the optimal gonadotropin-releasing hormone (GnRH)-based protocol as an alternative to the use of estrogen-based protocols in grazing Bos taurus suckling beef cows. All cows received an IVPD on the day of protocol initiation and prostaglandin F2α (PG) plus equine chorionic gonadotropin (eCG) treatments at the time of IVPD removal. In Experiment 1, cows (n = 235) were randomly assigned to one of four treatments: (i) 7-day estradiol = 2 mg of estradiol benzoate (EB) at IVPD insertion on Day 9 and 1 mg of estradiol cypionate (ECP) at IVPD removal on Day 2; (ii) 7-day GnRH = 10 µg of GnRH at IVPD insertion on Day 10, IVPD removal on Day 3 and GnRH at FTAI; (iii) 7 & 7 estradiol = PG at IVPD insertion on Day 16, EB on Day 9 and ECP at IVPD removal on Day 2; (iv) 7 & 7 GnRH = PG at IVPD insertion on Day 17, GnRH on Day 10, IVPD removal on Day 3 and GnRH at FTAI. In Experiment 2, cows (n = 462) were randomly assigned to one of four treatments: (i) 6-day estradiol = EB at IVPD insertion on Day 9, IVPD removal on Day 3 and GnRH at FTAI; (ii) 7-day estradiol; (iii) 7-day GnRH; (iv) 7 & 7 GnRH. In Experiment 1, plasma progesterone concentrations and percentage of cows with a corpus luteum (CL) at IVPD removal, and pregnancy per AI (P/AI) were greater for cows subjected to GnRH-based protocols compared with cows subjected to estrogen-based protocols (p < 0.01). In Experiment 2, cows subjected to the 7 & 7 GnRH protocol had the greatest P/AI (p < 0.01). In summary, GnRH-based FTAI protocols resulted in similar or greater P/AI compared to estrogen-based FTAI protocols in grazing postpartum Bos taurus suckled beef cows. The greatest P/AI was attained with the 7 & 7 GnRH protocol.

Enkephalin-δ opioid receptor signaling partly mediates suppression of LH release during early lactation in rats

Gonadal function is often suppressed during lactation in mammals including rodents, ruminants, and primates. This suppression is thought to be mostly due to the inhibition of the tonic (pulsatile) release of gonadotropin-releasing hormone (GnRH) and consequent gonadotropin. Accumulating evidence suggests that kisspeptin neurons in the arcuate nucleus (ARC) play a critical role in the regulation of pulsatile GnRH/gonadotropin release, and kisspeptin mRNA (Kiss1) and/or kisspeptin expression in the ARC are strongly suppressed by the suckling stimuli in lactating rats. This study aimed to examine whether the central enkephalin-δ-opioid receptor (DOR) signaling mediates the suckling-induced suppression of luteinizing hormone (LH) release in lactating rats. Central administration of a selective DOR antagonist increased the mean plasma LH levels and baseline of LH pulses in ovariectomized lactating mother rats compared to vehicle-injected control dams on day 8 of lactation without affecting the number of Kiss1-expressing cells and the intensity of Kiss1 mRNA signals in the ARC. Furthermore, the suckling stimuli significantly increased the number of enkephalin mRNA (Penk)-expressing cells and the intensity of Penk mRNA signals in the ARC compared to non-lactating control rats. Collectively, these results suggest that central DOR signaling, at least in part, mediates the suppression of LH release induced by suckling stimuli in lactating rats via indirect and/or direct inhibition of ARC kisspeptin neurons.

Kisspeptin and lactational anestrus: Current understanding and future prospects

Lactational anestrus, characterized by the suppression of pulsatile gonadotropin-releasing hormone (GnRH)/luteinizing hormone (LH) release, would be a strategic adaptation to ensure survival by avoiding pregnancy during lactation in mammals. In the present article, we first provide a current understanding of the central regulation of reproduction in mammals, i.e., a fundamental role of arcuate kisspeptin neurons in mammalian reproduction by driving GnRH/LH pulses. Second, we discuss the central mechanism inhibiting arcuate Kiss1 (encoding kisspeptin) expression and GnRH/LH pulses during lactation with a focus on suckling stimulus, negative energy balance due to milk production, and the role of circulating estrogen in rats. We also discuss upper regulators that control arcuate kisspeptin neurons in rats during the early and late lactation periods based on the findings obtained by a lactating rat model. Finally, we discuss potential reproductive technology for the improvement of reproductive performance in milking cows.

Impact of polyherbal formulation oral administration on the estrus response, luteal activity, and oxidative stress in postpartum dairy cows with ovarian subfunction

Background and Aim:

The reproductive performance of dairy cows is of a high economic value to maintain efficient and sustainable productivity. Postpartum anestrus is one of the major infertility problems that cause limitation of dairy cow reproductive performance and productivity. The previous studies have reported using a polyherbal formulation for different purposes, including reproductive performance. This study was conducted to examine the efficacy of polyherbal formulation administration on the reproductive performance of cows and to establish the oral administration of polyherbal formulations as a safe, effective, and economic treatment for dairy cows with postpartum anestrus due to nutritional disorders, negative energy balance, high milk production, and/or heat stress.

Materials and Methods:

A total of 14 dairy cows with postpartum anestrus were randomly divided into two groups, which were subjected to oral administration of distilled water that served as the control (Ctrl; n=5) or polyherbal treatment (polyherbal treated; n=9) for two shots of treatment (each for 3 successive days) with a 10-day interval. Blood sampling and ultrasonography were performed before treatment and after the first and second shots of treatment. Progesterone (P4) assay was also performed.

Results:

The estrus induction rate was 66.7% after the first shot of treatment in the polyherbal-treated group, which increased significantly to 88.9% after the second shot of treatment, compared with the Ctrl group (20%). The estrus response was confirmed by ultrasonography and P4 hormone assay, wherein the polyherbal treatment significantly increased the P4 concentration in the polyherbal-treated group after the second shot of treatment compared to that before treatment and in the Ctrl group at all time points. On pregnancy diagnosis, the treated dairy cows showed conception rates of 66.7% and 20.0% in the polyherbal-treated and Ctrl groups, respectively. In contrast, the concentration of malondialdehyde, an oxidative stress marker, and the total antioxidant capacity remained unchanged between both groups before and after treatment.

Conclusion:

The polyherbal formulation containing tubers of C. rotundus, M. pterygosperma, rhizome of Z. officinale, and A. cepa has the potential to induce estrus response and luteal activity in dairy cows and is a possible treatment for ovarian inactivity in dairy farms.

A protocol for initiating oestrus and ovulation earlypost partumin dairy cows

The objective of the present study was to evaluate the efficacy of a prostaglandin F2α(PGF2α) and progesterone (P4) treatment in initiating oestrus and ovulation post partum (PP) in Holstein-Friesian cows. Using four herds, the treatment protocol consisted of a single intra-muscular injection of PGF2α(Estrumate) between days 12 to 14 PP followed 48 h later by progesterone treatment via intra-vaginal CIDR insertion for a period of 7 days. Milk samples for progesterone determination were collected three times weekly from 7 to 65 days PP. The ovarian activity and reproductive performance of treated (T, no. = 153) animals and untreated control cows (C, no. = 315) were compared. Treatment was effective (P< 0·001) in reducing the mean interval to PP commencement of luteal activity from 29·62 (s.e. 0·82) days to 22·09 (s.e. 0·70) days. The mean interval to first PP oestrus in the T animals was significantly reduced (P< 0·001) from 55·62 (s.e. 1·58) days to 44·91 (s.e. 1·44) days and the incidence of silent ovulation in cycles between days 21 to 65 PP was reduced (P< 0·001) from 56·28% to 42·27%. In two herds under one management regime and with a similar block-calving pattern (no. = 280 animals), the treatment protocol was beneficial to the overall reproductive performance as there was a significant shortening in the mean interval to first PP service (75·82 (s.e. 1·93) v. 80·86 (s.e. 1·32) days) and in the interval to PP conception (83·07 (s.e. 2·49) v. 88·90 (s.e. 1·95) days), bothP< 0·05.

Impacts of meloxicam administration before temporary calf weaning on physiological and reproductive responses of beef cows

The objective of this experiment was to evaluate temperament, physiological, and reproductive variables in beef cows assigned to an estrus synchronization + timed AI protocol including eCG administration, 48-h temporary calf weaning (TCW), or TCW + meloxicam administration. A total of 943 lactating, multiparous, nonpregnant Nelore cows, allocated into 8 groups of approximately 120 cows each, were assigned to the experiment. Groups were maintained in individual pastures and assigned to the following estrus synchronization + timed AI protocol: a 2-mg injection of estradiol benzoate and an intravaginal progesterone-releasing device (CIDR) on d 0, a 12.5-mg injection of PGF on d 7, CIDR removal in addition to a 0.6-mg injection of estradiol cypionate on d 9, and timed AI on d 11. Within each group, cows were randomly assigned on d 9 to 1) TCW from d 9 to 11 (TCW-CON; = 317), 2) no TCW and a 300-IU injection of eCG on d 9 (NOTCW; = 311), and 3) TCW-CON in addition to meloxicam administration (intramuscular; 0.5 mg/kg BW) on d 9 (TCW-MEL; = 315). Cow BW and BCS were assessed on d 0. On d 9 and 11, blood samples were collected, and cow temperament was evaluated via chute score and exit velocity. Pregnancy status was verified 30 d after timed AI via transrectal ultrasonography. No treatment differences were detected ( ≥ 0.23) for cow age, days postpartum, BW, and BCS on d 0 of the estrus synchronization + timed AI protocol. No treatment effects were detected ( ≥ 0.41) for any of the temperament variables evaluated. A treatment × day interaction was detected ( = 0.02) for serum cortisol concentrations, which were similar ( = 0.55) between treatments on d 9 but greater ( ≤ 0.05) in TCW-CON and TCW-MEL compared with NOTCW cows on d 11. No treatment effects were detected ( = 0.90) for serum haptoglobin concentrations, which decreased from d 9 to 11 in all treatments (day effect; < 0.01). No treatment differences were detected ( = 0.84) for pregnancy rates to timed AI. In summary, TCW during estrus synchronization did not impact temperament or serum haptoglobin concentrations in beef cows but increased serum cortisol concentrations compared with cows not assigned to TCW, although such an outcome was not sufficient to impact pregnancy rates to timed AI. Moreover, administration of meloxicam did not alleviate the TCW-induced increase in serum cortisol concentrations and failed to benefit pregnancy rates to timed AI in beef cows.

Secretion of luteinizing hormone into pituitary venous effluent of the follicular and luteal phase mare: novel acceleration of episodic release during constant infusion of gonadotropin-releasing hormone

We tested the hypothesis that continuous infusion of native GnRH into mares during the estrous cycle, at a dose of 100 μg/h, would elevate circulating concentrations of LH without disrupting the endogenous, episodic pattern of LH release. Ten cyclic mares were assigned to one of two groups (n = 5/group): (1) Control (saline) and (2) GnRH in saline (100 μg/h). On experimental day 0 (3 to 6 d after ovulation), osmotic pumps containing saline or GnRH were placed subcutaneously and connected to a jugular infusion catheter. Blood samples were collected from jugular catheters daily and at 5-min intervals from catheters placed in the intercavernous sinus (ICS) for 8 h on experimental day 4 (luteal phase; 7 to 10 d after ovulation), followed by an additional 6-h intensive sampling period 36 h after PGF(2α)-induced luteal regression (experimental day 6; follicular phase). Treatment with GnRH increased (P < 0.001) concentrations of LH by 3- to 4-fold in the peripheral circulation and 4- to 5-fold in the ICS. Continuous GnRH treatment accelerated (P < 0.01) the frequency of LH release and decreased the interepisodic interval during both luteal and follicular phases. Treatment with GnRH during the luteal phase eliminated the low-frequency, long-duration pattern of episodic LH release and converted it to a high-frequency, short-duration pattern reminiscent of the follicular phase. These observations appear to be unique to the horse. Further studies that exploit this experimental model are likely to reveal novel mechanisms regulating the control of gonadotrope function in this species.

Treatments to Optimize the Use of Artificial Insemination and Reproductive Efficiency in Beef Cattle under Tropical Environments

Bos indicus cattle, the preferred genetic group in tropical climates, are characterized by having a lower reproductive efficiency than Bos taurus. The reasons for the poorer reproductive efficiency of the Bos indicus cows include longer lengths of gestation and postpartum anestrus, a short length of estrous behavior with a high incidence of estrus occurring during the dark hours, and puberty at older age and at a higher percentage of body weight relative to mature body weight. Moreover, geography, environment, economics, and social traditions are factors contributing for a lower use of reproductive biotechnologies in tropical environments. Hormonal protocols have been developed to resolve some of the reproductive challenges of the Bos indicus cattle and allow artificial insemination, which is the main strategy to hasten genetic improvement in commercial beef ranches. Most of these treatments use exogenous sources of progesterone associated with strategies to improve the final maturation of the dominant follicle, such as temporary weaning and exogenous gonadotropins. These treatments have caused large impacts on reproductive performance of beef cattle reared under tropical areas.

Efeito da frequência de amamentação sobre a taxa de ovulação após tratamento com desmame temporário + GnRH, sobre a incidência de luteólise prematura no primeiro ciclo estral pós-parto e sobre o desempenho da progênie de vacas Nelore

Avaliou-se o efeito da restrição na frequência de amamentação sobre o diâmetro folicular no dia 0 (DFOL), sobre a taxa de ovulação (TO), e sobre a incidência de luteólise prematura no primeiro ciclo estral pós-parto (ILP) de vacas Nelore multíparas, em anestro, submetidas à amamentação ad libitum (controle; n= 115) ou amamentação uma vez ao dia (restrito; n= 109), entre os dias -14 e 9 do experimento, e estudou-se o efeito desses tratamentos sobre o peso à desmama da progênie dessas vacas. Induziu-se ovulação com remoção de bezerros entre os dias -2 e 0 e aplicação de 100μg de GnRH no dia 0. Somente animais que ovularam foram mantidos no experimento (n= 125). A ocorrência de luteólise prematura foi avaliada por meio da dosagem da concentração sérica de progesterona nos dias 5 e 9. A TO não foi influenciada pelos tratamentos (55,8%; P>0,1), e as vacas do tratamento restrito apresentaram maior DFOL (10,90±0,26 vs. 10,18±0,21mm; P<0,05) e menor ILP (21,4% vs. 43,5%; P<0,05). Os bezerros do tratamento controle foram mais pesados (162,32±2,08 vs. 155,91±4,12kg; P<0,05). Conclui-se que a restrição na frequência de amamentação em vacas Nelore reduz a ILP, porém com possível efeito negativo no desenvolvimento dos bezerros.

Effects of acclimation to human interaction on performance, temperament, physiological responses, and pregnancy rates of Brahman-crossbred cows

The objective of this study was to evaluate, over 2 consecutive years, the effects of acclimation to human interaction on performance, temperament, plasma concentrations of hormones and metabolites, and pregnancy rates of Brahman-crossbred cows. A total of 160 Braford and 235 Brahman x British cows were assigned to the 2-yr study. Approximately 45 d after weaning (August 2006) in yr 1, cows were evaluated for BW, BCS, and temperament (chute score, pen score, and exit velocity), stratified by these measurements in addition to breed and age, and randomly allocated to 14 groups (Braford = 8; Brahman x British = 6). Groups were randomly assigned to the control or acclimation treatment. In yr 2, cows were reevaluated within 45 d after weaning (August 2007) for BW, BCS, and temperament, stratified, and divided into 14 groups similarly as in yr 1, but in such a way that cows received the same treatment assigned in yr 1. Cows were acclimated to human interaction from August to January, and the acclimation process consisted of the same person visiting groups twice weekly and offering approximately 0.05 kg of range cubes per cow (as-fed basis). In January of both years, cow temperament, BW, and BCS were reassessed and cows were exposed to a 90-d breeding season. Blood samples were collected at the beginning of the acclimation period (August) and breeding season (January) for determination of plasma cortisol, IGF-I, and acute phase proteins. A treatment x breed interaction was detected during yr 1 (P < 0.01) for pregnancy analysis because acclimated Braford cows conceived earlier and at a greater percentage (P < 0.01) compared with control cows. According to values obtained at the beginning of breeding and pooled across treatments and breeds, IGF-I concentrations and BCS affected quadratically (P < 0.05), and concentrations of ceruloplasmin and haptoglobin decreased linearly (P < 0.05), the probability of pregnancy during both years. Temperament and cortisol concentrations decreased the probability of pregnancy linearly (P < 0.05) during yr 1 (only Braford cows for cortisol analysis) and affected the probability of pregnancy quadratically (P < 0.05) during yr 2. Results from this study indicate that acclimation did not affect cow temperament and physiological responses but did increase pregnancy rates of Braford cows during yr 1. Further, measurements and physiological responses associated with temperament influenced the probability of cows becoming pregnant during the breeding season.

Remoção temporária de bezerros em dois momentos do protocolo de sincronização da ovulação GnRH-PGF2α-BE em vacas Nelore pós-parto

Os efeitos da remoção temporária de bezerros (RB) sobre características foliculares e taxas de ovulação e de prenhez de vacas Nelore foram avaliados em dois experimentos quando da inseminação artificial em tempo fixo (IATF), utilizando-se um protocolo à base de GnRH-PGF2α-BE. No experimento 1, 139 vacas Nelore, lactantes, cíclicas ou acíclicas, receberam um protocolo hormonal envolvendo a aplicação de 100±g de análogo GnRH no dia zero (D0-GnRH), 25mg de PGF2α no dia 7 (D7-PGF2α) e 1,0mg de benzoato de estradiol no dia 8 (D8-BE). Os animais foram aleatoriamente distribuídos para serem submetidas ou não à RB (48 horas) antes da aplicação de GnRH (RB1) ou após a aplicação de PGF2α (RB2). Avaliaram-se o diâmetro folicular (DF) nos dias 0 (D0) e 9 (D9) do protocolo e as taxas de ovulação (T0), advindas dos tratamentos. As vacas acíclicas que receberam RB1 apresentaram maiores DF no D0 (P<0,05) e taxa de ovulação (P<0,05) quando da aplicação de GnRH, em relação às que não receberam RB1. Nas vacas cíclicas, não houve efeito da RB1 sobre as variáveis analisadas. As vacas que receberam RB2 apresentaram maiores DF no D9 (P<0,05) e taxa de ovulação (P<0,01) quando da aplicação de BE, em relação às que não receberam a RB2. No experimento 2, 376 vacas da raça Nelore, lactantes, foram submetidas ao mesmo protocolo hormonal e aos mesmos tratamentos do experimento 1, mas foram inseminadas 24 a 30 horas após a aplicação de BE no D8, visando avaliar a taxa de prenhez após a IATF. A taxa de prenhez foi maior nos animais que receberam as duas RB em relação às que não receberam RB (29,8% vs 10,6%; P<0,05). A remoção de bezerros associada ao protocolo hormonal aplicado pode ter aumentado as taxas de prenhez devido às maiores taxas de ovulação, em resposta ao GnRH ou ao benzoato de estradiol.

Shortening the postpartum anoestrous interval in suckled crossbred dual purpose cows using progestagen intravaginal sponges plus eCG and PGF(2alpha)

One hundred and twenty-six suckled crossbred cows (Bos taurus x Bos indicus), with body condition score >or=3 (1-5 point scale), were employed in the present study to evaluate the effectiveness of intravaginal progestin-releasing sponges (IVS) for shortening anoestrous interval. Fifty-four cows were assigned to control group. Seventy-two cows were treated with IVS impregnated with 250 mg of medroxy-acetate-progesterone (MAP) as follows: day 0, IVS plus 5 mg of 17beta-E and 50 mg of MAP i.m.; day 6, 500 IU of equine chorionic gonadotropin and 25 mg prostaglandin F(2alpha) i.m.; day 8, IVS withdrawal and day 9, 1 mg 17beta-E i.m. Cows were also grouped according to postpartum days (dpp) at treatment: MAP <70 days (n = 25); control <70 days (n = 22); MAP >70 days (n = 47); control >70 days (n = 32). From IVS removal, cows were detected in oestrus and inseminated. Cows not detected in oestrus were timed artificial insemination 72 h after sponge removal. Treatment effect on oestrous rate (ER), conception rate (CR), pregnancy rate (PR) and treatment to conception intervals (TCI) and calving to conception intervals (CCI) were evaluated. The ER, CR and PR were analysed using PROC LOGISTIC, while TCI and CCI with PROC GLM of SAS. The groups MAP <70 days and MAP >70 days showed higher (p < 0.01) ER than control <70 days and control >70 days (84.0% and 76.6% vs 31.8% and 31.3% respectively). The PR was higher (p < 0.01) in MAP <70 days vs control <70 days (64.0% vs 22.7%) and also higher (p < 0.05) in MAP >70 days vs control <70 days (40.4% vs 18.8%). The TCI and CCI were shorter (p < 0.01) in MAP <70 days vs control <70 days (36.0 and 95.8 days; 95.3 and 158.6 days respectively). In conclusion, only cows treated with IVS before 70 dpp had a CCI shorter than 100 days, consequently this treatment shortened postpartum anoestrous interval in crossbred dual purpose cattle.

Treatment with an intravaginal progestagen and estradiol to prevent prolonged anestrus in crossbred zebu cows under tropical conditions

To study the effectiveness of a treatment with an intravaginal progestagen in combination with estradiol-17beta (E-17beta) on estrus induction, prevention of prolonged anestrus and fertility in noncycling crossbred zebu cows, a trial was conducted in a farm in Zulia state, Venezuela. Fifty noncycling suckled crossbred cows (B. taurus x B. indicus) with 60 dpp were randomly allotted to one of the following treatments: MAP+E-17beta (n=22), intravaginal device (250 mg of MAP) during 7 days + 50 mg of MAP and 5 mg of E-17beta im (day 0) and 1.5 mg of E-17beta im (day 8) (Pregnaheat-E; VIATECA-Venezuela); CG (n=28), control group without hormonal treatment. The MAP+E-17beta group showed higher (P<0.05) estrus and synchronization rates (59.1 and 36.4 %) than CG (25.0 and 3.5 %). Cows receiving MAP had higher (P<0.05) overall pregnancy rate than CG (45.5 and 17.8 %, respectively). A decreased (P<0.05) anestrus rate was observed in MAP+E-17beta (31.8%) compared to CG (57.1%). Calving-conception interval was shorter (P<0.05) in MAP+E-17beta (132.5+/-16.8 days) than in CG (178.9+/-14.7 days). In conclusion, treatment with MAP+E-17beta at 60 days postpartum was effective to induce estrus, prevent prolonged anestrus, and reduce the calving-conception interval, improving reproductive performance in dual purpose crossbred Zebu cows.

Ovarian, hormonal, and reproductive events associated with synchronization of ovulation and timed appointment breeding of Bos indicus-influenced cattle using intravaginal progesterone, gonadotropin-releasing hormone, and prostaglandin F2α1

The objectives of this study were to 1) compare cumulative pregnancy rates in a traditional management (TM) scheme with those using a synchronization of ovulation protocol (CO-Synch + CIDR) for timed AI (TAI) in Bos indicus-influenced cattle; 2) evaluate ovarian and hormonal events associated with CO-Synch + CIDR and CO-Synch without CIDR; and 3) determine estrual and ovulatory distributions in cattle synchronized with Select-Synch + CIDR. The CO-Synch + CIDR regimen included insertion of a controlled internal drug-releasing device (CIDR) and an injection of GnRH (GnRH-1) on d 0, removal of the CIDR and injection of PGF2alpha (PGF) on d 7, and injection of GnRH (GnRH-2) and TAI 48 h later. For Exp. 1, predominantly Brahman x Hereford (F1) and Brangus females (n = 335) were stratified by BCS, parity, and day postpartum (parous females) before random assignment to CO-Synch + CIDR or TM. To maximize the number of observations related to TAI conception rate (n = 266), an additional 96 females in which TM controls were not available for comparison also received CO-Synch + CIDR. Conception rates to TAI averaged 39 +/- 3% and were not affected by location, year, parity, AI sire, or AI technician. Cumulative pregnancy rates were greater (P < 0.05) at 30 and 60 d of the breeding season in CO-Synch + CIDR (74.1 and 95.9%) compared with TM (61.8 and 89.7%). In Exp. 2, postpartum Brahman x Hereford (F1) cows (n = 100) were stratified as in Exp. 1 and divided into 4 replicates of 25. Within each replicate, approximately one-half (12 to 13) received CO-Synch + CIDR, and the other half received CO-Synch only (no CIDR). No differences were observed between treatments, and the data were pooled. Percentages of cows ovulating to GnRH-1, developing a synchronized follicular wave, exhibiting luteal regression to PGF, and ovulating to GnRH-2 were 40 +/- 5, 60 +/- 5, 93 +/- 2, and 72 +/- 4%, respectively. In Exp. 3, primiparous Brahman x Hereford, (F1) heifers (n = 32) and pluriparous cows (n = 18) received the Select Synch + CIDR synchronization regimen (no GnRH-2 or TAI). Mean intervals from CIDR removal to estrus and ovulation, and from estrus to ovulation were 70 +/- 2.9, 99 +/- 2.8, and 29 +/- 2.2 h, respectively. These results indicate that the relatively low TAI conception rate observed with CO-Synch + CIDR in these studies was attributable primarily to failure of 40% of the cattle to develop a synchronized follicular wave after GnRH-1 and also to inappropriate timing of TAI/GnRH-2.

Body condition and suckling as factors influencing the duration of postpartum anestrus in cattle: a review

Prolonged postpartum anestrus is a main factor limiting reproductive efficiency in cattle, particularly in Bos indicus and Bos taurus/Bos indicus cows from tropical regions, because it prevents achievement of a 12 month calving interval. During anestrus, ovulation does not occur despite ovarian follicular development, because growing follicles do not mature. Although many factors affect postpartum anestrus, nutrition and suckling are the major factors influencing the resumption of postpartum ovarian cycles, as they affect hypothalamic, pituitary and ovarian activity and thus inhibit follicular development. Under-nutrition contributes to prolonged postpartum anestrus, particularly among cows dependent upon forages to meet their feed requirements and it apparently interacts with genetic, environmental or management factors to influence the duration of anestrus. The nutritional status or balance of an animal is evaluated through body condition score (BCS), as it reflects the body energy reserves available for metabolism, growth, lactation and activity. There is a converse relationship between energy balance and time to resumption of postpartum ovarian activity; inadequate nutrient intake results in loss of weight and BCS and finally cessation of estrous cycles. Suckling interferes with hypothalamic release of GnRH, provoking a marked suppression in pulsatile LH release, resulting in extended postpartum anestrus. The effects of suckling on regulation of tonic LH release are determined by the ability of the cow to identify a calf as her own or as unrelated. Vision and olfaction play critical roles in the development of the maternal-offspring bond, allowing the cow to identify her own calf, and abolition of both senses attenuates the negative effects of suckling on LH secretion. Thus, the maternal-offspring bond is essential for prolonged postpartum suckling-induced anovulation, and the suppressive influence of suckling is independent of neurosensory pathways within the teat or udder.

Ability of intravaginal progesterone inserts and melengestrol acetate to induce estrous cycles in postpartum beef cows

Postpartum anestrous interval in beef cows is a major factor contributing to reproductive failure during a defined breeding season. Our objectives were to determine the ability of a controlled internal drug-releasing device (CIDR, 1.9 g of progesterone), a normal dose of melengestrol acetate (MGA, 0.5 mg x cow(-1) x d(-1)), or a high dose of MGA (4.0 mg x cow(-1) x d(-1)) to induce ovulation and to eliminate short estrous cycles. Multiparous beef cows (n = 100) were equally assigned to one of four treatments: CIDR, normal MGA, high MGA, or control by age, days postpartum, body condition, and body weight. All cows were fed carrier (0.9072 kg x cow(-1) x d(-1)) with (normal MGA, 0.55 mg/kg; high MGA, 4.41 mg/kg) or without MGA for 7 d (d -6 to 0). On d -6, CIDR were inserted and then removed on d 0. Estrous behavior was monitored continuously from d -6 until 29 using HeatWatch electronic mount detectors. Blood was collected on d -13, and three times weekly from d -6 to 29. Treatment influenced (P = 0.03) the percentage of cows that were detected in standing estrus. Beginning on d 2, more CIDR-treated cows had exhibited standing estrus compared with high MGA-treated or control cows, but CIDR- and normal MGA-treated cows did not differ. The percentage of CIDR-treated cows that had ovulated was greater (P < 0.05) than the percentage of normal MGA-treated, high MGA-treated, or control cows beginning on d 4. The percentage of cows that exhibited standing estrus before the first postpartum ovulation (CIDR = 65%, normal MGA = 57%, high MGA = 35%, control = 30%) did not differ (P = 0.09) among treatments. Luteal life span following the first ovulation postpartum and the percentage of cows with a normal luteal life span (i.e., progesterone > 1 ng/mL for > or = 10 d) was greater (P < 0.01) in CIDR-treated cows (14.0 +/- 0.8 d; 20/20, 100%) compared with normal MGA-treated (6.2 +/- 1.0 d; 3/13, 23%), high MGA-treated (9.6 +/- 1.0 d; 8/14, 57%), or control cows (6.1 +/- 0.9 d; 4/17, 24%), and greater (P < 0.03) in high MGA-treated cows than in normal MGA-treated or control cows. In the present study, treatment of early postpartum suckled beef cows with CIDR induced ovulation and initiated estrous cycles with a normal luteal life span in more cows than did treatment with MGA. Treatment with MGA (normal or high dose) did not induce ovulation earlier than in control cows, but a high dose of MGA increased the percentage of cows with normal luteal life spans following the first ovulation postpartum.

Physiological classification of anovulatory conditions in cattle

Evaluation of follicular growth patterns by ultrasound combined with measurement of circulating reproductive hormones has allowed designation of three functionally critical follicular sizes during the final stages of follicular growth: emergence (-4 mm), deviation (-9 mm), and ovulation (variable from 10 to 20 mm). Classification of anovulatory conditions on the basis of these three critical points is logical and provides for rational diagnosis and treatment of the underlying physiological condition. In extreme undernutrition, there is growth of follicles to emergence but not to deviation; however, the underlying pathophysiology is not defined because of relatively few scientific investigations of this condition. Anovulatory conditions with growth of follicles to deviation but not to ovulatory size have been extensively studied. Undernutrition and/or suckling can cause this anovulatory condition. It is characterized by a greater negative feedback effect of estradiol on GnRH/LH pulses than found in normally cycling cows. Another anovulatory condition that is common in high producing lactaing dairy cows is characterized by growth of follicles to larger than ovulatory size, such as is observed in cows with follicular cysts. This condition is characterized by an insensitivity of the hypothalamus to the positive feedback effects of estradiol. Thus, these last two common anovulatory conditions appear to be primarily due to changes in the responsiveness of the hypothalamus to estradiol. Treatments that increase circulating progesterone concentrations can help in the treatment of these two conditions by potentially altering GnRH/LH pulses and allowing the final stages of follicular growth or resetting the hypothalamic responsiveness to the positive feedback effects of estradiol.

Uptake of beta-carotene by ovarian and uterine tissues and effects on steroidogenesis during the estrous cycle in cats

OBJECTIVES

To determine uptake of beta-carotene by ovarian and uterine tissues and influence of dietary beta-carotene on steroidogenesis and production of uterine protein during the estrous cycle in cats.

ANIMALS

56 female cats.

PROCEDURE

Cats were fed diets containing 0, 0.4, 2, or 10 mg of beta-carotene daily for 8 weeks prior to detection of estrus. At time of observed estrus, all cats were manually induced to ovulate. Blood samples were obtained at estrus and every 2 days until day 14 after ovulation. On that day, cats underment laparotomy, and the ovaries and uterus were removed. Uterine contents were flushed, and luteal and endometrial tissues were obtained.

RESULTS

Concentrations of beta-carotene in plasma and luteal and endometrial tissues increased in a dose-dependent manner. Concentrations of plasma progesterone were higher between days 6 and 10 after ovulation in cats fed diets containing beta-carotene and continued to increase through day 14 after ovulation in cats fed a diet containing 10 mg of beta-carotene. Plasma concentration of estradiol-17beta also was higher between days 0 and 4 after ovulation in cats fed diets containing beta-carotene. Cats fed a diet containing 10 mg of beta-carotene had the highest plasma estradiol concentration. Total uterine protein concentration was higher in cats fed beta-carotene, compared with values for cats fed an unsupplemented diet.

CONCLUSION AND CLINICAL RELEVANCE

Cats readily absorb beta-carotene. Increased concentrations of progesterone, estradiol, and uterine protein may provide more optimal ovarian function or a better uterine environment for embryonic survival and development.

Postpartum acyclicity in suckled beef cows: a review

Prolonged postpartum acyclicity in suckled beef cows is a source of economic loss to beef cattle producers. Duration of postpartum acyclicity is influenced by suckling status, nutritional status, calving season, age, and several other factors. Although uterine involution begins and ovarian follicular waves resume soon after parturition, dominant follicles of these waves fail to ovulate, due to a failure to undergo terminal maturation. As a result, postpartum anovulatory dominant follicles are smaller than the ovulatory follicles in cyclic cows. Failure of postpartum dominant follicles to undergo terminal maturation is due to absence of appropriate LH pulses, a prerequisite for follicular terminal maturation prior to ovulation. Absence of LH pulses early post partum is primarily due to depletion of anterior pituitary LH stores, although GnRH pulses are also absent during this period due to suckling. Following replenishment of LH stores between Days 15 and 30 post partum, absence of LH pulses is due to continued sensitivity of the hypothalamic GnRH pulse-generator to the negative feedback effect of ovarian estradiol-17beta, which results in absence of GnRH pulses. This negative feedback effect of estradiol-17beta is modulated by suckling which stimulates release of endogenous opioid peptides from the hypothalamus. As the postpartum interval increases, sensitivity of the GnRH pulse-generator to the negative feedback effect of ovarian estradiol-17beta decreases. This is followed by an increasing frequency of GnRH discharges and LH pulses, terminal follicular maturation, ovulation, and continued cyclicity. The first ovulation post partum is usually followed by a short cycle due to premature luteolysis because of premature release of PGF2alpha from the uterine endometrium, which is possibly intensified by the suckling-induced oxytocin release from the posterior pituitary. A model for the postpartum ovulatory acyclicity and for the resumption of cyclicity is presented.

Comparative effects of early weaning or once-daily suckling on occurrence of postpartum estrus in primiparous beef cows

We concurrently evaluated early weaning and once-daily suckling as options for shortening postpartum interval to estrus in primiparous cows calving in good body condition. Over 2 consecutive years, a total of 77 primiparous cows were assigned to 1 of 3 treatments: control (ad libitum suckling); once-daily suckling (for 30 d prior to start of breeding); or early weaning (calves weaned 30 d prior to start of breeding). All cows had a body condition score (BCS) of > or = 5 at parturition and were maintained in good condition throughout the breeding season. Cows were observed for estrus twice daily and blood samples for subsequent measurement of progesterone were collected at 48-h intervals. For data analysis, cows were divided into early- (> or = 85 d post partum) and late-calving (< 85 d post partum) groups at the onset of the breeding season. All calves were weighed at birth, at the onset of treatment, at weekly intervals until the start of the breeding season, and within 45 d of 205 d of age. Early weaning or once-daily suckling shortened the postpartum interval to first estrus of late-calving primiparous cows by 17 (P < 0.01) and 12 d (P < 0.08), respectively, compared with control group cows. However, among early-calving cows, neither early weaning nor once-daily suckling influenced the postpartum interval. Postpartum intervals of once-daily suckled cows and early-weaned cows were similar (P > 0.10).

Effects of calf removal at parturition on postpartum ovarian activity in Zebu (Bos indicus) cows in the humid tropics

To assess endocrine and morphological responses of ovaries to total weaning at parturition, 6 Zebu (Bos indicus) cows 5 years or older were investigated. Following parturition, blood samples were collected daily during the first month and twice weekly thereafter until day 60 to determine concentrations of progesterone (P4) and prostaglandin F2 alpha metabolite. It took between 25 to 32 days to complete uterine involution. The prostaglandin metabolite remained elevated for a mean period of 14.2 days (range, 4-21) postpartum. Five of the animals resumed cyclicity with a short estrous cycle starting between days 7 to 34 and lasting between 7 and 14 days. No estrous behavior was recorded prior to the short estrous cycles, but subsequent normal-length estrous cycles were all preceded by signs of estrus. In the 1 animal that resumed cyclicity with an estrous cycle of normal length on day 37 (length 20 days), the cycle was preceded by estrous behavior. Progesterone concentrations reached a mean maximum of 4.8 nmol liter-1 during the short estrous cycles, and prostaglandin metabolite concentrations peaked while P4 concentrations were decreasing. P4 concentrations reached a mean maximum of 12.2 nmol liter-1 during the estrous cycles of normal length. The interval from parturition to the first estrous cycle of normal length varied between 16 and 48 days, and the length of the cycle was 18 to 22 days. Starting 2 days postpartum, ovaries from 5 of the cows were scanned by ultrasonography every second day until day 30 postpartum. Medium-sized follicles were detected between days 4 to 7 postpartum in 4 of the scanned cows that later had short estrous cycles. The time between parturition and the appearance of the first dominant follicle was 7.6 days (range 6-10 days). The interval between parturition and the appearance of the first ovulatory-sized follicle was 10.2 days (range 8-13 days). In 3 of the scanned cows this ovulatory-sized follicle ovulated. We conclude that cyclic ovarian activity in Zebu cows can start early in the postpartum period in the absence of offspring, and that short luteal phases, not preceded by estrous behavior, may play an important role in establishing normal postpartum ovarian activity.

Resumption of follicular activity and interval to postpartum ovulation after exogenous progestins

In Experiment 1, treatment of dairy cows with progesterone using an intravaginal device from d 5 to 15 postpartum failed to alter onset or duration of the first estrous cycle. Compared with controls in Experiment 2, concentrations of FSH increased, and those of LH decreased, in cows treated with either norgestomet or progesterone for 6 d. Relationships of various measures of LH, but not FSH, were all positive with increasing days postpartum, and those of FSH produced positive polynomial regressions as days to first ovulation decreased. No significant changes in follicular status occurred during or after progestin treatments; however, a consistent relationship was observed between increasing diameter of dominant follicles and serum concentrations of estradiol-17 beta in most cows. Appearance of the ovulatory dominant follicle generally was detected 4 to 5 d before ovulation regardless of treatment and was first detected more frequently during treatment with progestins when concentrations of FSH were increased compared with controls. Concentrations of progesterone were higher on d 7 to 10 following removal of progestin treatments. Appearance of the first dominant follicle was detected during periods of increased average minimum concentrations of FSH, increased variation in LH secretion, and was related to subsequent duration of the first estrous cycle.

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.

Effects of transcutaneous thermal and electrical stimulation of the teat on pituitary luteinizing hormone, prolactin and oxytocin secretion in ovariectomized, estradiol-treated beef cows following acute weaning

The objective of the study was to determine if chronic electrical or thermal stimulation of sensory neurons on the surface of the teat is able to activate pathways that suppress the weaning-induced increase in luteinizing hormone (LH) secretion in beef cows. Treatment groups (n = 5 per group) consisted of: 1) control suckled (CS); 2) weaned plus electrical stimulation of the teat (ESTT); 3) weaned plus electrical stimulation of the tail (ESTL); 4) weaned plus thermal stimulation of the teat (TTT); 5) weaned plus thermal stimulation of the tail (TTL) and 6) weaned (WN). Cows were ovariectomized on Day 5 post partum (PP) and were treated with estradiol-17beta to maintain a constant tonic baseline. Beginning on Days 17 to 21 post partum, cows were suckled by their own calf (control), weaned or weaned and electrically or thermally stimulated for 10 minutes every 6 hours for 4 days. Chronic transcutaneous electrical and thermal stimulation of the teat or tail failed to impede the unambiguous rise (P < 0.001) in LH pulse frequency and amplitude following weaning. Positive and negative feedback of estradiol on LH secretion was not affected by treatments. Relatively consistent episodes of oxytocin and prolactin release were observed following control-suckling, but responses to electrical and thermal stimulation were inconsistent. Chronic electrical or thermal stimulation of teat-specific or nonspecific loci did not attenuate heightened secretion of LH after weaning. The results are further evidence against a role for mammary somatosensory neurons in the suckling-mediated inhibition of LH secretion.

Effects of suckling on oestradiol benzoate induced LH release in post-partum, ovariectomised cows

Twelve suckling and 12 non-lactating post-partum cows were treated with a progesterone-releasing pessary for 7 or 14 d followed by an injection of 500 micrograms oestradiol benzoate (ODB) 24 h after pessary removal or no injection. Suckling had no significant effect on plasma progesterone concentrations (plasma P4) or plasma luteinising hormone concentrations (plasma LH) during pessary insertion. After pessary removal plasma LH rose in response to ODB. The extent of the rise was similar in suckling and non-lactating cows treated with progesterone for 14 d but was significantly lower in non-lactating than suckling cows (P less than 0.025) treated for 7 d. These findings indicate that suckling increases the responsiveness of plasma LH in post-partum cows following progesterone and oestrogen treatment compared to non-lactating cows and that extended treatment with progesterone can remove this difference.

Response of prepubertal and × heifers to melengestrol acetate with or without gonadotropin-releasing hormone

The effectiveness of treatments to induce estrus in prepubertal beef heifers was evaluated. Angus x Hereford (n = 148) and Brahman x Hereford (n = 148) heifers were sorted after weaning by body weight into light and heavy weight blocks. Heifers were assigned to diets, calculated to reach a target weight of 55% or 65% of their projected mature weight by the start of breeding. Cyclicity was determined after a 160-d observation period and from concentrations of progesterone in serum determined 10 d before and on the day that treatments began to induce puberty. The remaining nonpubertal heifers, with concentrations of progesterone in serum of less than 1 ng/ml (0 or 10 d before treatment), were assigned randomly within breed and nutrition group to either a melengestrol acetate + saline (MGA+S) or MGA + gonadotropin-releasing hormone (MGA+GnRH) treatment. Prepubertal Angus x Hereford heifers (n = 11) and Brahman x Hereford heifers (n = 49) were fed 0.5 mg MGA for 7 d. Forty-eight hours after MGA, heifers were injected with 500 ug s.c. GnRH or 5 ml of saline. Blood samples were collected from all prepubertal heifers every 3 d after GnRH or saline for 30 d. There was no difference between treatments in the proportion of heifers that exhibited estrus by Day 7 after treatment. However, a larger (P<0.05) proportion of MGA+S-treated heifers exhibited estrus within 14 d after treatment than MGA+GnRH-treated heifers (87 vs 63%). Among heifers that exhibited estrus during that time period, the proportion with increased progesterone was higher (P<0.10) for the MGA+GnRH group than for the MGA+S group (71 vs 41%, Day 7; 79 vs 54%, Day 14). There was no difference in conception rate at first service between treatment groups. Thirty-seven and 53%, respectively, of the MGA+S and MGA+GnRH-treated heifers had short estrous cycles after treatment, and 44 and 50%, respectively, of those short cycles were repeated. Pregnancy rates at the end of 45 d were numerically higher for MGA+S heifers than for MGA+GnRH treated counterparts (63 vs 53%).

Patterns of tonic luteinizing hormone release and ovulation frequency in suckled anestrous beef cows following varying intervals of temporary weaning

Thirty postpartum Brahman crossbred cows were utilized to determine the effects of varying intervals of temporary weaning on tonic LH secretion and ovulation. Cows were assigned randomly on day 17-21 postpartum to one of five groups: 1) Suckled Ad libitum, 2) 48-hr weaned, 3) 72-hr weaned, 4) 96-hr weaned, or 5) 144-hr weaned. The mean maximal rise in LH pulse frequency due to weaning occurred within 2 days and averaged 221 percent of time 0 values. The frequency of LH pulses was greater (P less than .06) in weaned than in suckled controls. This temporal increase was self-limiting, displaying an acute rise followed by a variable rate of decline in all groups. However, pulse frequency remained elevated relative to suckled controls for the longest period of time for weaning durations of 96 and 144 hr (P less than .10). In 48-hr and 72-hr weaned cows, a rapid reversal of the initial increase in LH pulse frequency was observed following calf return. A significant linear regression (y = 1.9 +/- .64x; P less than .03) described the increase in LH pulse frequency that occurred in cows which ovulated following weaning. Nonovulators were sensitive to calf return and responded by exhibiting a linear decline (y = 2.87 - .43x; P less than .04) in LH pulse frequency following this event. The amplitude of LH pulses increased (P less than .02) during the period after calf return in ovulators, but did not change in nonovulators. Percentage ovulating by day 10 increased (P less than .05) with increased weaning duration past 72 hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of repeated low doses of gonadotrop in releasing hormone on postpartum interval and serum luteinizing hormone in Brahman cross cows

Two experiments were conducted to examine the effects of repeated low-dose injections of gonadotropin releasing hormone (GnRH) 30 to 40 d post partum on reproductive characteristics in multiparous suckled Brahman cross cows. In Experiment I, 39 cows were injected (i.v.) with GnRH (5 mug/injection) at 2-h intervals for either 0 (control), 6, 12, or 24 h at 30 to 37 d post partum. GnRH injections for short periods (6h) increased the number of cows exibiting estrus within 45 d of treatment, but cows injected for 24 h failed to exhibit estrus during this period. The period from treatment to first estrus was shorter in the 6-h GnRH group compared to the control group. Injections for 6h significantly (P < 0.05) increased in serum luteinizing hormone (LH) concentrations 1 d after GnRH treatment. In Experiment II we examined the effect of i.v. GnRH injections (5 mug/injection at 2-h intervals) for 6h in a larger group of cows (n = 70). The days from treatment to first estrus were reduced (P < 0.05) in GnRH-treated cows; however, first-service conception rates were significantly lower (P < 0.01) in treated compared to control cows (46.4 and 80.0%, respectively). The results led us to believe that GnRH injections for short periods reduce postpartum interval to first estrus, but fertility at first estrus is lowered.

Effects of progesterone and weaning on LH and FSH responses to naloxone in postpartum beef cows

Two experiments were conducted to evaluate the effects of naloxone, an endogenous opioid receptor antagonist, on LH and FSH secretion in postpartum beef cows. In Experiment 1, 24 cows were divided into three equal groups. On day 15 postpartum, all cows were bled for 8 hr at 10 min intervals to evaluate LH secretory parameters. On day 18 postpartum, three treatments were administered: (a) saline at 0730 and 1130 hr; (b) 275 mg naloxone at 0730 and 1130 hr; (c) naloxone as in (b) above, plus this group was also treated with 50 mg progesterone (P4) twice daily from day 16 to day 19. In each treatment, jugular vein samples were collected at 10 min intervals from 0800 to 1600 hr. On day 19 the same treatments were administered at the same times, however, all cows were given 25 micrograms GnRH at 1200 hr to evaluate the LH secretory response. Naloxone increased mean LH concentration (P less than .05) and tended to increase pulse amplitude and frequency compared to controls. However, the most dramatic difference was due to P4 treatment which suppressed mean LH, pulse amplitude and frequency. Treatments had no effect on LH secretion in response to a 25 micrograms dose of GnRH. In Experiment 2, the effects of suckling on the naloxone response were examined in 16 postpartum cows. On day 21 postpartum, blood was collected at 10 min intervals for 8 hr and then calves were removed from half the cows. After 3 days of calf removal, all cows were sampled at 10 min intervals for 4 hr; then naloxone was injected after each 10 min sample at a dose rate of 200 mg/hr (33 mg per injection). Naloxone treatment and sampling continued for an additional 8 hr. Calf removal alone had very little effect on LH pulsatility. However, naloxone resulted in increased pulse frequency and mean LH compared to the control period. We conclude that LH release in the early postpartum cow is partially regulated by endogenous opioid peptides. We were unable to detect any effects on FSH secretion nor on pituitary sensitivity to exogenous GnRH.