Effect of estrous cycle stage at Syncro-Mate B treatment on conception and time to estrus in cattle

A five-day progesterone plus eCG-based fixed-time AI protocol improves fertility over spontaneous estrus in high-producing dairy cows under heat stress

This study compared the efficiency of a five-day or standard (nine-day) progesterone-based regimen combined with equine chorionic gonadotrophin (eCG) in a fixed-time AI (FTAI) protocol for dairy cows. The data examined were derived from 3577 inseminations conducted in three dairy herds. Animals with no estrus signs detected over 21 days were randomly assigned to a PRID-9 or PRID-5 group. Cows in each group received a progesterone intravaginal device (PRID) for 9 or 5 days, respectively, PGF(2α) and eCG on PRID removal, and GnRH 48 h later. Fixed-time AI was performed 12 h after the GnRH dose. Cows artificially inseminated following spontaneous estrus during the study period were considered as controls. Based on the odds ratio, the likelihoods of animals in PRID-9 in the warm (conception rate [CR] of 22.3%) and cool (32% CR) periods, and control animals in the warm period (26.6% CR) becoming pregnant were reduced (by factors of 0.6, 0.3 and 0.4, respectively) compared with the control animals in the cool period (CR of 43.7%). The risk of a twin pregnancy was higher (51.4%) for cystic PRID-9 cows (by a factor of 3.6) and lower (9.9%) for cyclic PRID-5 animals (by a factor of 0.4) compared with the PRID-9 cyclic cows. These findings indicate that the proposed protocol achieves similar results during the cool or warm season to those obtained when AI is conducted at spontaneous estrus during the cool season. In addition, PRID-5 reduced twin pregnancy compared with PRID-9.

Short-term progestin treatments prevent estrous induction by a GnRH agonist implant in anestrous bitches

The objective of this study was to test the efficacy and safety of a short-term progestin treatment administered at two different times to prevent estrous induction in response to the administration of an implant releasing the GnRH agonist, deslorelin acetate (DA), in anestrous bitches. Interestrous intervals (IEI) observed prior to and post DA were compared. Forty-two anestrous bitches, with previous IEI history, were randomly allocated to one of the following treatments: PL: placebo sc (n = 12); MA: megestrol acetate 2mg/kg po for 8 days (n = 4); DA: 10mg sc (n = 8); MA&DA-1: MA beginning the day before DA (n = 8); and MA&DA-4: MA beginning 4 days before DA (n = 10). The dose of MA was identical for each treatment. All bitches were examined daily for 1 month and then every 3 months until the next spontaneous post-treatment estrous cycle. Post-GnRH estrous response occurred in 0, 0, 100, 50, and 10% of the PL, MA, DA, MA&DA-1, MA&DA-4, groups, respectively (<0.01). There was an interaction between the treatment and period for the duration of the IEI (< 0.01). Changes in IEI were different among treatments (p<0.01); the three DA-treated groups (147.5% +/- 10.3, 161.3% +/- 14.1, 148.6% +/- 19.2) differed from both the MA (12.9% +/- 17.6) and PL (8.1% +/- 7.8), but not among themselves. It is concluded that an 8 days megestrol protocol and DA on Day 4 was better than DA on Day 1 to prevent estrous response in anestrous bitches and that both protocols significantly increased the IEI.

Synchronization of estrus in postpartum beef cows and virgin heifers using combinations of melengestrol acetate, GnRH, and PGF2α1

The efficacy of various combinations of melengestrol acetate (MGA), GnRH, and PGF2alpha for the synchronization of estrus in Angus-based beef cattle was compared. Hormones were administered as follows: MGA, 0.5 mg x animal(-1) x d(-1) mixed in a grain carrier; GnRH, 100 microg i.m.; PGF2alpha, 25 mg i.m. In Exp. 1, 2, and 3, cows were randomly assigned to treatments by parity and interval postpartum. The detection of estrus and AI were conducted from d -2 until 72 to 96 h after PGF2alpha, at which time cows not detected to be in estrus received GnRH and fixed-time AI (TAI). Data were analyzed separately for primiparous and multiparous cows. In Exp. 1, cows (n = 799) at three locations received GnRH on d -7 and PGF2alpha on d 0 and either no further treatment (GnRH-PGF) or short-term MGA from d -6 through d -1 (STMGA). Among multiparous cows, conception rate at TAI was greater (P < 0.05) for STMGA (41%, 47/115) than for GnRH-PGF treated cows (26%, 24/92). Across herds and parity, synchronized AI pregnancy rate (SPR) was not affected (P > 0.10) by treatment (GnRH-PGF vs. STMGA; 54%, 210/389 vs. 57%, 228/402). In Exp. 2, cows (n = 484) at three locations received either STMGA or long-term MGA from d -32 through d -19, GnRH on d -7, and PGF2alpha on d 0 (LTMGA). Among primiparous cows, SPR was greater (P < 0.01) in LTMGA (65%, 55/85) than STMGA-treated cows (46%, 40/87). Treatment had no effect (P > 0.10) on SPR among multiparous cows (STMGA vs. LTMGA; 59%, 92/155 vs. 64%, 101/157). In Exp. 3, cows (n = 838) at four locations received the LTMGA treatment and either no further treatment or an additional period of MGA exposure from d -6 through d -1 (L&STMGA). Among primiparous cows, SPR tended to be influenced (P < 0.10) by the herd x treatment interaction and was greater (P < 0.01) among L&STMGA (86%, 19/22) than LTMGA-treated cows (56%, 14/25) at a single location. Among multiparous cows, SPR was lower (P < 0.05) in L&STMGA (46%, 165/358) than LTMGA-treated cows (55%, 184/336). In Exp. 4, Angus heifers (n = 155) received either STMGA or 14 d of MGA (d -32 through d -19) and PGF2alpha on d 0 (MGA-PGF). The detection of estrus and AI were conducted from d -2 to d 6. Interval to estrus was greater (P < 0.05) and estrous response was lower (P < 0.05) in STMGA than MGA-PGF-treated heifers. In conclusion, primiparous cows responded more favorably to longer-duration MGA treatments than did multiparous cows. All protocols achieved sufficient SPR to justify their use for improved reproductive management of postpartum beef cows.

Recent advances in bovine reproductive endocrinology and physiology and their impact on drug delivery system design for the control of the estrous cycle in cattle

When methods of drug intervention are being developed to control estrous cycles, a thorough understanding of the endocrine and functional changes together with the reproductive behavior of the animals are essential. This review presents our current knowledge on reproductive endocrinology, physiology and behavior, and the methods of drug intervention to control estrous cycles. It also describes current efforts to develop advanced drug delivery systems that meet the animal scientist's demands to control the estrous cycle in cattle.

Attenuation of premature estrous behavior in postpartum beef cows synchronized to estrus using GnRH and PGF2alpha

The efficacy of GnRH and PGF2alpha (7-day injection interval) for estrus synchronization is diminished by estrous expression before PGF2alpha (premature estrus; PE). Effects of modifications to GnRH-PGF2alpha protocols on the incidence of PE and other indicators of reproductive performance were evaluated. In Experiment 1, Angus-based crossbred cows (n=51) received 25 mg of PGF2alpha i.m. on Day 0. Animals were randomly assigned by parity and interval postpartum to receive GnRH 100 microg i.m. on either Day -7 or Day -6. Estrous detection and AI were conducted from Day -3 to Day 5. Treatment had no effect on the incidence of PE, estrous response, conception rate per AI or synchronized pregnancy rate (6- vs. 7-day interval; 8 vs. 15%; 92 vs. 93%; 77 vs. 76%; 71 vs. 70%, respectively). In Experiment 2, Angus cows (n=150) received GnRH 100 microg i.m. on Day -7 and 25 mg PGF2alpha i.m. on Day 0. Animals were randomly assigned by parity, interval postpartum, and body condition score to receive either no further treatment (Control) or 0.5 mg melengestrol acetate/hd/d from Day -7 to Day -1 (MGA). Estrous detection and AI were conducted from Day -2 to Day 7. Fewer (P < 0.05) MGA-treated cows were detected in PE (0%) compared to controls (7%). Treatment had no effect on estrous response or synchronized pregnancy rates (Control vs. MGA; 78 vs. 84%; 52 vs. 60%, respectively). Conception rate per AI of cows > or = 60 days postpartum were not affected by treatment (Control vs. MGA; 79 vs. 73%) however, control cows < 60 days postpartum tended (P < 0.10) to have lower conception rates per AI (39%) than did their MGA-treated counterparts (69%). In summary, 6- and 7-day GnRH-PGF2alpha injection intervals resulted in similar synchronized reproductive performance. Inclusion of MGA feeding between GnRH and PGF2alpha injections eliminated the occurrence of premature estrus and improved conception rate per AI of late-calving cows.

Induction of ovulation in postpartum suckled beef cows: a review

Prolonged postpartum acyclicity in suckled beef cows reduces the calf crop, and causes economic loss to beef cattle producers. Once anterior pituitary LH stores have been replenished between Days 15 and 30 post partum in suckled beef cows, methods to initiate cyclicity include non-hormonal methods such as weaning of calves (either complete, temporary or partial), or exposure to bulls, and hormonal methods such as administration of GnRH (either single injection, intermittent injections, or continuous infusion), gonadotropins (eCG, FSH, hCG), and steroids (estrogens, anti-estrogens, and progestogens). Weaning is costly, reduces growth rate of weaned calves, and short cycles are common after weaning-induced ovulation. Exposure of cows to bulls is not practical and its effect is not predictable. Repeated injections of GnRH, or a single injection of hCG are not always effective; ovulation is always followed by a short cycle, and usually a return to acyclicity. Estrogens and anti-estrogens do not consistently shorten postpartum anestrus. Exogenous progestogens include intravaginal devices, such as controlled-internal drug release (CIDR) or progesterone-releasing intravaginal device (PRID), norgestomet implants, and the feed-additive melengestrol acetate (MGA). Administration of exogenous progestogens is more practical than, and offers more advantages over, other treatments to shorten postpartum acyclicity in suckled beef cows. Mimicking the short cycle after Week 3 post partum, by maintaining circulating progesterone at subluteal concentrations or circulating progestin at intermediate concentrations, extends the life-span and allows terminal maturation of the postpartum dominant follicle as in cyclic cows, by initiating endogenous GnRH and LH pulses. This is followed by an LH surge, ovulation and normal cycles. The benefit from using exogenous progestogens after Week 3 post partum in suckled beef cows is that ovulation is induced, cyclicity is initiated, the resulting CL has a normal life-span and function, and there is no need to change management, such as weaning of calves. We present a model for the induction of ovulation and initiation of cyclicity using exogenous progestogens after Week 3 post partum in suckled beef cows.

Follicular growth, estrus and pregnancy after fixed-time insemination in beef cows treated with intravaginal progesterone inserts and estradiol benzoate

An experiment was performed to compare the effects of 3 short-term treatments with progesterone and estradiol benzoate (EB) on follicular growth, synchrony of estrus and pregnancy rate after fixed-time insemination in lactating postpartum beef cows. In Treatment 1 (n = 46), each cow received a progesterone-containing intravaginal insert for 7 d with injection of EB (2 mg, i.m.) at the time of device insertion. In Treatment 2 (n = 46), the insert was used for only 5 d with injection of EB (2 mg, i.m.) at the time of insertion. Cows in Treatment 3 (n = 47) received an insert for 5 d with no EB at the time of insertion. Each cow in the 3 groups received PGF2 alpha (25 mg, i.m.) at the time of insert removal, followed by EB (1 mg, i.m.) 30 h later. The cows were then inseminated 28 to 30 h after treatment with EB (58 to 60 h after insert removal). Treatment with 2 mg EB terminated the growth of the largest ovarian follicle (> 5 mm in diameter) at device insertion in 16/16 and 14/15 cows in Treatments 1 and 2, respectively. Estrus was detected within an 8-h target period (48 to 56 h after insert removal) in 93, 87 and 81% of cows in Treatments 1, 2 and 3, respectively (P > 0.05). Pregnancy rates at 39 d post insemination were 60, 50 and 51% for Treatments 1, 2 and 3, respectively (P > 0.05). The pregnancy rates did not differ between cows that were anovulatory or those that had ovulated before the initiation of treatments (54%), or among cows that were 28 to 40, 41 to 60 or > 60 days post partum at insemination (43, 59 and 54%, respectively). Treatment with progesterone inserts for 5 or 7 d, PGF2 alpha at the time of insert removal and 1 mg EB 30 h later induced the high degree of synchrony of estrus and ovulation necessary for fixed-time insemination.

Comparison of oestrous synchronization regimens for lactating dairy cows

The objective of this experiment was to evaluate various programmes for synchronization of oestrus. The focus of the study was to evaluate rates of detection of oestrus, synchrony of oestrus, pregnancy rate, and effect of ovarian status at initiation of the programmes on rates of detection of oestrus and pregnancy rate. Spring-calving, lactating dairy cows (n = 2009) were allocated at random to one of six treatments: (1) A (n = 335), progestogen (controlled intravaginal drug release; CIDR) inserted per vaginum 10 d before breeding season for 8 d, 10 microg of buserelin at CIDR insertion, PGF2alpha treatment on the day prior to CIDR removal, and AI of cows detected in oestrus within 6 d after CIDR withdrawal; (2) B (n = 330), as in A, plus 1 mg of oestradiol benzoate i.m. 10 h post CIDR withdrawal; (3) C (n = 347), as in A, except buserelin was replaced by 10 mg of oestradiol benzoate; (4) D (n = 335), as in A, plus PGF2alpha and oestradiol benzoate at CIDR insertion; (5) E (n = 332), CIDR containing a 10 mg oestradiol benzoate capsule inserted per vaginum for 12 d; or (6) F (n = 330), as in E, plus PGF2alpha on the day prior to CIDR withdrawal. The oestrous detection rate (number of cows detected in oestrus within 6 days of CIDR withdrawal as a proportion of the number of cows submitted for synchronization of oestrus) and oestrous synchrony (oestrous detection rate within 2 d of CIDR withdrawal), respectively, were greater (P<0.05) following B (95.7% of 330, 98.7% of 316) compared with any of the other programmes for synchronization of oestrus (A: 87.5 of 335, 79.4% of 293; C: 86.7% of 347, 80.0% of 301; D: 90.1% of 335, 89.8% of 302; E: 74.4% of 332, 70.4% of 247; F: 76.4% of 330, 78.5% of 252). The oestrous detection rate was reduced (P<0.05) among cows in metoestrus administered E (64.0% of 50) relative to similar cows administered F (82.8% of 64). Pregnancy rate was greater (P<0.05) following B (57.9% of 330) than A (48.9% of 335, P = 0.06), C (43.2% of 347), E (40.0% of 332), and F (35.1% of 330) but not D (59.3% of 302), when based on those cows presented for oestrous synchronization programmes. In conclusion, 1 mg of oestradiol benzoate administered 10 h post CIDR withdrawal (B) resulted in the best overall oestrous detection, oestrous synchrony, and pregnancy rates, which would be beneficial to a fixed-time AI program.

Fertility regulation in cattle

This paper reviews the physiological, endocrinological and pharmaceutical literature pertaining to the design, development and optimisation of subcutaneous and intravaginal progestogen-containing drug delivery systems used in the control of synchrony and ovulation in cattle.

Effects of short-term treatment with progesterone superimposed on 11 or 17 days of norgestomet treatment on the interval to oestrus and fertility in Bos indicus heifers

The aims of this study were to determine: (1) if short-term treatment of Bos indicus heifers with progesterone (P4) while implanted with a s.c. norgestomet implant for 17 days would influence the time interval to oestrus and increase fertility of the synchronised oestrus, and (2) whether the response to treatment with P4 would differ between heifers treated with a norgestomet implant for 17 vs. 11 days when short-term treatment with P4 is applied 3 days prior to implant removal. B. indicus heifers at two separate sites (A and B) were allocated to three groups at each site. Heifers in two groups (NG and NGP4 groups) were given a single s.c. norgestomet implant on the first day of treatment (day 0) while heifers in a third group (NGP4PG group) were implanted on day 6. A single P4 releasing Controlled Internal Drug Release device (CIDR) was inserted on day 14 in heifers in the NGP4 and NGP4PG groups and was removed 23.5 +/- 0.07 h later (day 15). Heifers in the NGP4PG group were administered an analogue of prostaglandin F2 alpha (PGF2 alpha) at the time of CIDR removal to regress corpora lutea. Implants were removed from all heifers on the same day (day 17) and a 400 IU of equine chorionic gonadotrophin (ECG) was administered s.c. Animals were artificially inseminated 11.1 +/- 0.17 h after detection of oestrus, using frozen semen from one bull at site A and one of five bulls at site B. Inseminations were carried out by one of two technicians. Treatment with P4 delayed oestrus and reduced the synchrony of oestrus at site A (hours to oestrus +/- SD: NG group, 39.0 +/- 13.7; NGP4 group, 66.3 +/- 24.4; NGP4PG group, 58.9 +/- 20.5 h; P < 0.05) but not at site B (41.4 +/- 15.2, 42.5 +/- 10.1, 45.4 +/- 10.3 h; P > 0.05). Pregnancy rates 6 weeks after insemination were found to be significantly associated with bull (P < 0.001), treatment group (P = 0.013) and insemination technician (P = 0.033). Pregnancy rates were greater in the heifers in the NGP4 group than heifers in the NG group [50.3% (78/155) vs. 36.4% (60/165); odds ratio = 1.83, 95% CI = 1.14 to 2.96] and similar between heifers in the NGP4 and NGP4PG groups [50.3% (78/155) vs. 51.1% (63/117); odds ratio = 1.06, 95% CI = 0.67 to 1.69]. It was concluded that acute treatment with P4 can improve pregnancy rates in B. indicus heifers treated for 17 days with norgestomet implants. Reducing the duration of norgestomet treatment to 11 days and administration of PGF2 alpha at the time of ending treatment with a CIDR device resulted in no differences in fertility, mean intervals to oestrus or synchrony of oestrus.

Use of short-term progestin treatment to resynchronize the second estrus following synchronized breeding in beef heifers

Two trials were conducted to evaluate the efficacy of short-term progestin administration to resynchronize the second estrus after artificial insemination in yearling beef heifers. In Trial 1 crossbred yearling heifers (n = 208) were synchronized with Syncro-Mate-B (SMB) and artificially inseminated (AI) between 48 and 54 h following implant removal. Implant removal is defined as Day 1. Following AI, the heifers were randomly assigned to 1 of 2 experimental groups. Group 1 heifers were fed melengestrol acetate (MGA) daily from Day 17 to 21 at a rate of 0.5 mg/head, while Group 2 control received no exogenous progestin during this period. Synchrony of estrus was defined as the 3-d period in which the highest number of heifers expressed behavioral estrus in each group. There was no difference (P < 0.05) in the pregnancy rate during the second estrus due to MGA supplementation. More MGA-treated heifers (P < 0.01) expressed estrus in a 3-d period than the controls. In Trial 2, yearling heifers (n = 108) were synchronized with 2 injections of PGF(2alpha) (second PGF(2alpha) injection is designated as Day 1) administered 14 d apart with AI 12 h after the onset of behavioral estrus. The heifers were then randomly assigned to 1 of the following 3 treatment groups after initial AI: 1) MGA fed at 0.5 mg/head daily from Days 17 to 21; 2) norgestomet administered in 6.0-mg implants from Days 17 to 21; 3) untreated control heifers. Blood samples were collected on Day 21 and analyzed for progesterone (P(4)). Elevated P(4) (> 1 ng/ml) on Day 21 indicated pregnancy to the first insemination. Synchrony among the 3 groups of heifers was similar (P > 0.10); however, the second estrus was less (P < 0.05) variable in the MGA and norgestomet treated heifers. During the resynchronized second estrus, conception rates were not affected by progestin treatment (MGA 40%, norgestomet 64%, and control 62%; P > 0.10). However, a proportion of heifers treated MGA 10% 4 36 and norgestomet 3% 1 36 expressed behavioral estrus during second estrus even though they were diagnosed as pregnant from first service by elevated P(4) levels on Day 21. We conclude that short-term use of progestin from Days 17 to 21 following AI causes closer synchrony of estrus; however, inseminating pregnant heifers that exhibit behavioral estrus may cause abortion.

Effect of prostaglandin F2 alpha treatment before norgestomet and estradiol valerate treatment on regression, formation, and function of corpora lutea in beef heifers

Two experiments were conducted to determine if corpus luteum regression, formation, and function were associated with the decreased calving rate observed in beef females administered PGF2 alpha 5 days before Syncro-Mate B (SMB) treatment. Experiment 1 included 31 beef heifers 11 to 13 months old and experiment 2 included 31 beef heifers 19 to 21 months old. Heifers were randomly assigned to 1 of 2 groups (control and PGF2 alpha 5 days before SMB treatment). Heifers were bled 10 days before PGF2 alpha treatment, immediately before PGF2 alpha and SMB treatments, at the time of implant removal, and twice weekly after implant removal. Heifers in experiment 2 were observed twice daily for estrus for 5 days after PGF2 alpha treatment and for 3 days after norgestomet implant removal. Based on the blood samples collected before SMB treatment, 15 heifers in experiment 1 and every heifer in experiment 2 were with estrous cycles. All heifers in experiment 1 had progesterone concentrations < 0.5 ng/ml 2 days after implant removal. However, progesterone concentrations during the luteal phase in control heifers with estrous cycles were higher (P < 0.05) than in PGF2 alpha treated heifers with estrous cycles and in heifers previously without estrous cycles. In experiment 2, based on the occurrence of estrus and progesterone concentrations, heifers were also classified as metestrus or diestrus at the time of SMB treatment. The data were analyzed as a 2 x 2 factorial with treatment (control or PGF2 alpha) and stage of the cycle (metestrus and diestrus) as main effects. More metestrus heifers (40%) had progesterone concentrations > 1.0 ng/ml 2 days after implant removal than diestrus heifers (0%). In addition, progesterone concentrations during the luteal phase in metestrus heifers were lower (P < 0.05) than in diestrus heifers. PGF2 alpha treatment had no effect (P > 0.25) on the number of heifers with > 1.0 ng/ml progesterone 2 days after implant removal and progesterone concentrations during the luteal phase. There was no treatment by stage of the estrous cycle interactions. In summary, the administration of PGF2 alpha 5 days before SMB decreased the calving rate by causing more heifers to be metestrus at SMB treatment. Fewer metestrus heifers (than diestrus heifers) were synchronized (with < 1.0 ng/ml of progesterone 2 days after implant removal) to SMB treatment and those synchronized had lower progesterone concentrations during the luteal phase.

Application of knowledge about corpus luteum function in control of estrus and ovulation in cattle

Understanding corpus luteum (CL) function has led to development of methods of estrus synchronization in cattle that either extend the estrous cycle by administration of exogenous progestins or shorten the cycle by induction of luteolysis. Both methods have limitations, which have been reduced or overcome through sequential or combined treatment with progestin and luteolytic drugs. Future improvements in estrus synchronization methodology are most likely to come from achieving more synchrony between the development of a highly fertile ovulatory follicle and control of luteal function.

Calving rates in a tropical beef herd after treatment with a synthetic progestagen, norgestomet, or a prostaglandin analogue, cloprostenol

Maiden heifers and lactating cows of known ovarian status and of several breeds were treated with a synthetic prostaglandin, cloprostenol, or a synthetic progestagen, norgestomet, at the start of an artificial insemination (AI) program. Animals in the cloprostenol treatment received 2 injections 10 days apart. Over the next 26 days those animals that showed oestrous behaviour were inseminated. Synchronisation rates and calving rates to insemination over the first 7 days were calculated. Those in the norgestomet treatment received an implant of norgestomet plus an injection of norgestomet and oestradiol valerate. The implant was removed 10 days later and the animals were given an injection of pregnant mare serum gonadotrophin (PMSG). They were inseminated at 48 h (maiden heifers) or 56 h (lactating cows) after implant removal. Calving rates to fixed-time insemination were recorded. After completion of the AI program the animals in both treatments were joined with bulls. Overall calving rates (AI plus bulls) were calculated. By day 7 of the program, 82% of the maiden heifers and 76% of the lactating cows in the cloprostenol treatment had been detected in oestrus. By day 21 the respective figures were 99% and 81%. Norgestomet treatment had an immediate and a prolonged effect on ovarian activity in those females classified as having inactive ovaries at the start of the AI program. Calving rates of those females to fixed-time AI and overall were similar to those of the females with active ovaries in both treatments. Their calving rates to fixed-time insemination, and overall calving rates for the lactating females, were significantly higher than the corresponding values of their contemporaries treated with cloprostenol and inseminated on observed oestrus over 7 days. For those females classified as having active ovaries at the start of the AI program, calving rates to first insemination and overall were similar for both treatments. Overall calving rates of lactating cows of each breed were, with one exception, higher in the norgestomet treatment than in the cloprostenol treatment. Although norgestomet treatment was more expensive than cloprostenol treatment, the advantage in calf crop resulted in an overall monetary advantage to the norgestomet treatment.

Evaluation of a melengestrol acetate and prostaglandin F2α system for the synchronization of estrus in beef heifers

This study was conducted to determine the efficacy of feeding melengestrol acetate (MGA) for 14 days and administering prostaglandin F(2)alpha (PGF) 17 days after MGA to synchronize or induce estrus in yearling beef heifers. The study involved 56 Angus (n = 19), Hereford (n = 15) and Simmental (n = 22) heifers that were assigned by breed and pubertal status to either MGA+PGF or to control groups. Heifers in the synchronized group were fed 0.5 mg MGA per head per day for 14 days from a grain carrier and were injected with 25 mg, i.m. PGF 17 days after the last daily feeding of MGA. Control heifers were fed from a grain carrier without MGA and were not treated with PGF. Heifers were classified as pubertal when concentrations of progesterono in the serum exceeded 1 ng/ml in 1 of 2 samples collected prior to the initiation of treatments. Blood samples were collected 7 days before and on the day that treatment with MGA or carrier began and 7 days before and on the day that PGF was administered. Progesterone concentrations in the serum were elevated ( > 1 ng/ml) in 61% (17 28 ) of the MGA+PGF-treated heifers and in 61% (17 28 ) of the control heifers prior to feeding MGA. However, concentrations of progesterone in the serum at the time PGF was administered differed (P<0.05) between MGA+PGF and control groups. Concentrations of progesterone in the serum exceeded 1 ng/ml in 100% (28 28 ) of the MGA+PGF-treated heifers and in 71% (20 28 ) of control heifers at the time PGF was administered (P<0.05). All heifers were inseminated 12 hours after the first detected estrus. Twenty-two of 28 (79%) of the MGA+PGF-treated heifers exhibited estrus within 6 days after PGF compared with 9 of 28 (32%) of control heifers (P<0.05). The conception rate at first service did not differ between MGA+PGF and control groups (64% and 67%, respectively). Synchronized pregnancy rates were higher (P<0.05) for MGA+PGF-treated heifers than for control heifers (14 28 , 50% vs 6 28 , 21%). Increased concentrations of progesterone in serum at the time PGF was administered and higher pregnancy rates during the synchronized period among MGA+PGF-treated heifers demonstrate the efficacy of this treatment for use in estrus synchronization. Moreover, this treatment may have a potential effect on inducing puberty in breeding age heifers.

Pharmacologic manipulation of fertility

The professional application of agents to the manipulation of fertility of cows requires basic and applied knowledge of the physiologic mechanisms that are affected and of the pharmacologic agents that are used. In all areas of the pharmacologic manipulation of fertility, the achievement is less than the ideal, and further research is required to improve the efficiency of treatments. The induction of estrus in acyclic animals can involve a reduction in the depth of anestrus, pretreatment with progestagen to ensure estrous behavior and the formation of a normal corpus luteum, and then treatment with exogenous gonadotropin. Responsiveness to treatment can be variable and reflects the depth of anestrus of the animals. Improved treatment regimens require a knowledge of the basic mechanisms involved with the depth of anestrus, a means of assessing the depth of anestrus, and an understanding of the hormonal requirements of ovarian follicles for development and maturation in animals at different depths of anestrus. The optimal precision in the synchronization of estrus (and ovulation) in cyclic animals requires the synchronization of both follicular waves and the end of progestational phase. The end of progestational phase can be synchronized effectively using prostaglandin F2a (or analogs), or by treatment with progestagens with or without luteolytic agents. Procedures to synchronize follicular waves need to be established. The induction of superovulation can be achieved readily using gonadotropins prior to estrus synchronization using prostaglandin F2a. The responses to standard treatments in terms of ovulation rates and yield of transferable embryos are highly variable. The development of procedures to reduce this variability requires an understanding of the intra-ovarian mechanisms involved in recruitment of follicles for a wave of follicular growth, in the selection of dominant follicles for further development, and in the mechanisms controlling follicular atresia. Cystic ovarian disease can be treated effectively using HCG or GnRH (follicular cysts) or prostaglandin F2a (luteal cysts). The basic mechanisms resulting in failure of estrogen positive feedback on LH secretion (that results in cystic follicles) remain to be determined. Small but significant increases in pregnancy rates can be achieved treating cows with prostaglandin during the post-partum period, with prostaglandin to induce estrus for insemination, with GnRH or HCG at estrus, and with GnRH or progestagen treatment during diestrus. Beneficial effects of treatment have been shown in some trials but not in others.(ABSTRACT TRUNCATED AT 400 WORDS)

Syncro-mate B® induces estrus in ovariectomized cows and heifers

Eleven ovariectomized Hereford x Simmental cows and 10 ovariectomized crossbred heifers (primarily Angus and Hereford) were given the Syncro-Mate B (SMB) estrous synchronization treatment. The SMB treatment consisted of a 2 ml i.m. injection containing 5 mg of estradiol valerate and 3 mg of norgestomet plus a hydron ear implant containing 6 mg of norgestomet. The ear implant was removed 9 d later. Cows and heifers were considered in estrus only if they stood for mounting by a herdmate or a bull. Observations for estrus were made four or six times each day for 3 d after implant removal. The 21 animals were used in eight trials. Each trial involved 9 or 11 cows or 5 or 10 heifers. Four days to three weeks elapsed between implant removal and implant insertion for the next trial. No ovariectomized cow or heifer was observed in estrus for 21 d before treatment with SMB. In the eight trials, 3 of 9, 7 of 9 and 6 of 11 cows exhibited estrus, whereas 5 of 10, 1 of 5, 3 of 5, 3 of 5 and 5 of 5 heifers exhibited estrus after treatment. When data were pooled, 16 of 29 (55.2%) cows and 17 of 30 (56.7%) heifers exhibited estrus after treatment. Our data indicate that the SMB treatment can induce estrus in cows and heifers, independently of the ovaries.