Enclomiphene does not alter the postpartum interval of suckled beef cows.

The objective of this study was to determine whether an antiestrogen (enclomiphene) would shorten the interval to first estrus and conception in postpartum beef cows. Sixty postpartum Angus beef cows were stratified by age, body condition, and calving date and were randomly assigned to one of two treatment groups. Group 1 cows (n = 24) received three silastic implants, each containing 150 mg of enclomiphene, on d 20 postpartum. Implants were removed on d 30 postpartum. Group 2 cows (n = 28), received empty implants and served as controls. Cows were artificially inseminated at first detected estrus. Estrus detection and ovulation were further verified by increased serum progesterone. Concentrations and pulse frequencies of LH were determined from blood samples collected at 15-min intervals for 6 h on d 20, 25, 30, and 40 postpartum. Hypothalami and pituitaries were collected from four cows in each treatment group on d 30 postpartum and analyzed for concentrations of estradiol receptors. Concentrations of total and unoccupied hypothalamic and pituitary estradiol receptors were reduced by enclomiphene. Neither concentrations nor pulse frequencies of LH differed significantly between treatment groups on any of the 4 d. Days to first estrus did not differ (P greater than .05) between enclomiphene-treated (57 +/- 6; n = 24) and control (56 +/- 4; n = 28) cows. Days to conception did not differ between treated (81 +/- 9) and control (79 +/- 8) cows. The dose of enclomiphene used in this study reduced hypothalamic and pituitary estrogen receptors but did not alter secretion of LH or days to first estrus in the postpartum beef cow.     

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.     

Serum concentrations of IGF-I in postpartum beef cows

Four experiments assessed changes in serum IGF-I under various physiologic conditions in postpartum cows. In Exp. 1, anestrous suckled cows (n = 25) were infused for 6 d with either saline or glucose at two different infusion rates. In Exp. 2, anestrous cows (n = 29) received either a saline (weaned and suckled controls) or 3 g/d phlorizin (weaned phlorizin) infusion for 3 d. Calves from the weaned groups were removed from 15 h before and throughout infusions. In Exp. 3, cycling suckled cows (n = 20) received prostaglandin F2 alpha (PGF2 alpha) when the 5-d saline or phlorizin infusion began. In Exp. 4, suckled cows (n = 20) had ad libitum access to feed or received 50% of control feed consumption from 30 to 40 d postpartum. Increasing glucose availability (Exp. 1) increased (P less than .05) serum IGF-I by 30 to 35%. IGF-I remained stable after weaning (Exp. 2) in phlorizin-infused cows (128.8 +/- 12.7 ng/ml), but increased (P less than .05) by 3 d after calf removal in weaned control cows (152.2 +/- 7.5 ng/ml). IGF-I also remained stable in phlorizin-infused cows following PGF2 alpha injection (Exp. 3), but increased in control cows by 2 d after PGF2 alpha (156.8 +/- 18.3 on d 2 vs. 133.7 +/- 9.8 ng/ml pre-injection; P less than .05) and remained elevated (P less than .05) during the periovulatory period. In cows receiving restricted feed intake (Exp. 4), IGF-I decreased by approximately 50% within 4 d of feed restriction (71.3 +/- 9.4 vs 137.4 +/- 16.6 ng/ml; P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationship of pre- and post-ovulatory gonadotropin concentrations to subnormal luteal function in postpartum beef cattle

Early weaning of calves from anestrous cows results in formation of short-lived corpora lutea (CL) unless the animals are pretreated with a progestagen (norgestomet). This study was conducted to investigate the relationship between pre- and post-ovulatory gonadotropin secretion and luteal lifespan. Postpartum beef cows were assigned randomly into two groups, control (n = 5) and norgestomet (implant given at weaning for 9 d; n = 7). Calves from all cows were weaned 30 to 33 d postpartum. Coccygeal artery cannulas were placed into cows in the control group 1 d prior to weaning and 2 d before implant removal in cows in the norgestomet group. Plasma for determination of luteinizing hormone (LH), follicle stimulating hormone (FSH), estradiol-17 beta (E) and progesterone (P) was collected daily at 10-min intervals for 6 h from weaning (control) or the day prior to implant removal (norgestomet) to estrus (d 0) and on d 2, 4 and 6 following estrus. Average interval (X +/- SE; P less than .05) from weaning to estrus or implant removal was 4.2 +/- .8 and 2.3 +/- .2 d for the control and norgestomet groups, respectively. Estrous cycle length for the control group was 12.4 +/- 1.8 d compared with 20.4 +/- .3 d for the norgestomet group (P less than .05). Four of five control cows had an estrous cycle length of 7 to 14 d; all cows in the norgestomet group and the remaining control cow had an estrous cycle of normal length (16 to 21 d).2+ estrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of biostimulation on reproduction in postpartum beef cows.

Three experiments were conducted to evaluate the effects of biostimulation (exposure to bulls or androgenized females) on various reproductive variables in suckled beef cows. Bulls or testosterone-treated cows (TTC) were introduced to cows, randomly allotted to one of four groups, within 72 h postpartum. In Exp. 1, Groups 1 and 3 were exposed to bulls and Groups 2 and 4 were exposed to TTC. In Exp. 2, Groups 1 and 3 were exposed to bulls and Groups 2 and 4 served as controls (isolated from biostimulation). In Exp. 3, Groups 1 and 3 were exposed to TTC and Groups 2 and 4 served as controls. Mean postpartum intervals to estrus were not different between cows exposed to either bulls or TTC in Exp. 1 (P greater than .10). However, in Exp. 2 and 3, cows exposed to either bulls or TTC had reduced postpartum intervals to estrus (44 and 41 d, respectively) compared with control cows (52 d; P less than .05). Fewer control cows were in estrus at either 40 d (P less than .05; Exp. 2 and 3) or 60 d (P less than .05; Exp. 3) postpartum than were cows exposed to bulls or TTC. No differences were observed between groups in any experiment for postpartum intervals to pregnancy (P greater than .10). These data indicate that cows exposed to biostimulation from either bulls or TTC immediately after calving return to estrus earlier than do cows isolated from biostimulation.     

Hypoglycemia alters pulsatile luteinizing hormone secretion in the postpartum beef cow

This study tested the hypothesis that the increased glucose requirement of lactation had effects that were independent of the suckling-dependent inhibition of postpartum endocrine function in beef cows. Mature Hereford cows were either suckled ad libitum and infused with saline iv (n = 9) from d 2 through 4 (d 0 = jugular catherization on d 32 +/- 3 postpartum); were nonsuckled and infused with saline from d 2 through 4 (n = 10); or were nonsuckled and infused with phlorizin (3 g/d) from d 2 through 4 (n = 10). Nonsuckled cows infused with phlorizin had lower (P less than .05) plasma concentrations of glucose and amino acid nitrogen (AAN) on d 2 compared with pre-infusion levels (d 1), but their metabolic profile returned to levels similar to the suckled cows by d 3 and 4. Nonsuckled cows infused with saline had elevated glucose and insulin and lower AAN and free fatty acids (FFA) on d 3 and 4 compared with pre-weaning (d 1) levels (P less than .05). Nonsuckled cows infused with phlorizin did not show this weaning-induced elevation in glucose and insulin. The number of luteinizing hormone (LH) pulses was not affected by treatment. However, in contrast to the large LH pulses observed in the nonsuckled cows infused with saline, both the suckled cows and the nonsuckled cows treated with phlorizin had more small and fewer large amplitude pulses (P less than .01). Treatment did not affect serum concentrations of follicle stimulating hormone, gonadotropin release in response to gonadotropin releasing hormone (25 micrograms) or the number of cows ovulating by 55 d after calving. We conclude that the increased glucose clearance caused by phlorizin infusion or lactation results in depression of LH pulse amplitude in suckled postpartum beef cows.     

Influence of the ovary and suckling on luteinizing hormone response to naloxone in postpartum beef cows

The influence of the suckling stimulus and ovarian secretions on LH response to naloxone was studied in 16 postpartum anestrous beef cows that were assigned randomly to one of four groups (n = 4/group): intact suckled (IS), intact nonsuckled (IN), ovariectomized suckled (OS) or ovariectomized nonsuckled (ON). Ovariectomy (OS + ON) and calf removal (IN + ON) were performed on d 2, 3 or 4 after parturition. Jugular venous blood was collected at 15-min intervals for 4 h before and 4 h after administration of naloxone (1 mg/kg BW, i.v.) on d 14 and d 28 after parturition. Gonadotropin-releasing hormone (5 micrograms, i.v.) was given 3 h after naloxone. Both IN and OS increased (P less than .05) mean pretreatment LH above IS values (mean +/- SE, ng/ml; IS 1.6 +/- .1 vs IN 2.5 +/- .3 and OS 2.7 +/- .4; P less than .01), whereas ON increased (P less than .01) LH (3.7 +/- .3 ng/ml) even further. Mean LH increased (P less than .05) after naloxone administration in all treatment groups. However, magnitude of this response was variable and dependent on ovarian status. Amplitude of the naloxone-induced LH response was greater (P less than .05) for ovariectomized (5.9 +/- 1.1 ng/ml) than for intact groups (2.7 +/- .5 ng/ml). Gonadotropin-releasing hormone increased mean LH concentrations in all groups. We suggest that ovarian secretions and the suckling stimulus contribute to endogenous opioid inhibition of LH during the postpartum interval.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of naloxone on serum luteinizing hormone, cortisol and prolactin concentrations in anestrous beef cows

Two experiments were conducted with the opioid antagonist naloxone to determine the effect of opioid receptor blockade on hormone secretion in postpartum beef cows. In Exp. 1, nine anestrous postpartum beef cows were used to measure the effect of naloxone on serum luteinizing hormone (LH), cortisol and prolactin concentrations. Cows received either saline (n = 4) or 200 mg naloxone in saline (n = 5) iv. Blood samples were collected at 15-min intervals for 2 h before and after naloxone administration. Serum LH concentrations increased (P less than .01) in naloxone-treated cows from 1.8 +/- .04 ng/ml before treatment to 3.9 +/- .7 ng/ml and 4.2 +/- .5 ng/ml at 15 and 30 min, respectively, after naloxone administration. In contrast, LH remained unchanged in saline-treated cows (1.6 +/- .3 ng/ml). Serum cortisol and prolactin concentrations were not different between groups. In Exp. 2, 12 anestrous postpartum beef cows were used to examine the influence of days postpartum on the serum LH response to naloxone. Four cows each at 14 +/- 1.2, 28 +/- .3 and 42 +/- 1.5 d postpartum received 200 mg of naloxone in saline iv. Blood samples were taken as in the previous experiment. A second dose of naloxone was administered 2 h after the first, and blood samples were collected for a further 2 h. Serum LH concentrations increased (P less than .01) only in cows at 42 d postpartum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endogenous opioid modulation of luteinizing hormone and prolactin secretion in postpartum ewes and cows

A possible role for endogenous opioid peptides (EOP) in the control of luteinizing hormone (LH) and prolactin (PRL) secretion was studied by injecting the opioid antagonist, naloxone (NAL), into postpartum ewes and cows. Twelve ewes that lambed during the fall breeding season and nursed their lambs were injected iv with NAL (1.0 mg/kg) on d 10, 14, 18, 22 and 26 postpartum. Blood samples were collected at 15-min intervals from 2 h before to 2 h after NAL, and serum concentrations of LH and PRL were quantified. Following treatment on d 10, suckling lambs were removed from 6 of the 12 ewes, creating non-suckled (NS) and suckled (S) treatment groups for subsequent study on d 14 through 26. On d 10, NAL treatment increased LH (P less than .01) but concentrations of PRL were not affected. When averaged across d 14 to 26, post-NAL concentrations of LH were greater (P less than .001) than pre-NAL concentrations (6.5 +/- .7 vs 1.9 +/- .4 ng/ml). In contrast, concentrations of PRL in the post-NAL period were lower (P less than .001) than pre-NAL concentrations (129 +/- 15 vs 89 +/- 10 ng/ml). Compared with S ewes over d 14 to 26, those in the NS group had similar pre-NAL concentrations of LH, tendencies for higher (P less than .10) post-NAL concentrations of LH, lower (P less than .001) mean serum concentrations of PRL (pre- and post-NAL) and similar pre-NAL vs post-NAL differences in serum PRL. Six suckled beef cows on d 24 to 35 were injected iv with either saline or NAL (.5 mg/kg) in a replicated crossover design. Injections of NAL increased serum concentrations of LH (P less than .05), when averaged over all 12 injections in the six cows, but serum PRL was not changed. However, three of six cows did not respond to NAL with increases in serum LH. These non-responding cows were similar to the responding cows in their pre-injection concentrations of LH and PRL, but they tended (P = .10) to have higher serum concentrations of cortisol than responding cows.     

Effect of the uterus on subnormal luteal function in anestrous beef cows

The effect of the uterus on luteal lifespan and pattern of secretion of progesterone following early weaning of calves from anestrous beef cows was studied. Calves were weaned from 15 anestrous beef cows 23 to 33 d postpartum, and cows were allotted to a control (sham surgery, n = 8) or a hysterectomy (n = 7) group, with surgery performed at weaning. Cows in the hysterectomy group were injected (im) with 25 mg prostaglandin F2 alpha (PGF2 alpha) approximately 20 d after first estrus (d 0). The interval from weaning to estrus was longer (P less than .05) for the hysterectomy group (10.4 +/- 1.6 d) than the control group (6.2 +/- .5 d). In the control group, the first estrous cycle (8.8 +/- .3 d) was shorter (P less than .01) than the second estrous cycle (20.2 +/- .5 d). Following first estrus in the hysterectomy group, cows were not detected in estrus until after injection of PGF2 alpha and did not return to estrus. From d 0 to 5, mean concentrations of plasma progesterone were similar (P greater than .05) between groups for both estrous cycles; after d 5 of estrous cycle 1, concentrations of plasma progesterone decreased in the control group. Within the hysterectomy group, the pattern of secretion of progesterone from d 0 to 16 was similar after the first and second estrus. Furthermore, there was no difference in the pattern of secretion of progesterone from d 0 to 16 between hysterectomy (first or second estrous cycles) and control (second estrous cycle) groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endocrine profiles associated with life span of induced corpora lutea in postpartum beef cows

Two experiments were designed to examine whether hormonal profiles were related to luteal life span in pluriparous postpartum anestrous beef cows. Cows (Exp. 1, n = 34; Exp. 2, n = 23) received norgestomet (N) for 9 d or served as controls (C). Each cow received 1,000 IU human chorionic gonadotropin (hCG) 48 h after removal of N (d 0). Blood samples collected every 15 min for 8 h on d -5, 3 and 5 (Exp. 1) or on d -10 and -1 (Exp. 2) were assayed for luteinizing hormone (LH) and follicle stimulating hormone (FSH). Cortisol was determined in hourly samples collected on d -5 and in samples collected every 2 min during suckling on the same day (Exp. 1). Concentrations of 15-keto-13,14-dihydro-PGF2 alpha (PGFM) were determined in samples collected at 15-min intervals for 2 h on d -5, 3, 5 and 10 (Exp. 1). Estradiol-17 beta was measured in samples collected on d -5 (Exp. 1) or on d -10 and -1 (Exp. 2). Life span of induced corpora lutea was longer (P less than .05) in N than C cows. Percentages of N cows in which corpora lutea, formed in response to hCG, exhibited a normal life span were 83% on farm 1 and 25% on farm 2 (Exp. 1), and 90% (Exp. 2), compared with 0% in C cows. Concentrations of FSH were not affected by N but were lower (P less than .05) on d -5 in cows on farm 2 (.6 +/- .1 ng/ml) than in cows on farm 1 (.8 +/- .1 ng/ml). On d -5, a treatment X farm interaction (P less than .05) for mean LH was observed and frequency of pulses of LH was higher (P less than .01) in N than C cows (2.7 +/- .4 vs. .8 +/- .8 pulses/8 h). Neither cortisol nor PGFM was affected by N. Estradiol was increased in d -1 (6.1 +/- .5 vs 2.6 +/- .8 pg/ml; P less than .01) by N. It is suggested that pre-treatment with N enhanced life span of induced corpora lutea, in part, by influencing secretion of LH and development of follicles, but a threshold concentration of FSH was required for N to exert this effect.     

Effect of dietary energy restriction on metabolic and endocrine responses during the estrous cycle of the suckled beef cow

A replicated trial was conducted with suckled Angus and Polled Hereford cows (110 d postcalving) to determine metabolic and endocrine responses to an energy-restricted diet after cows had re-established postpartum estrous cyclicity. Cows were individually fed 26.5 Mcal ME (H) or 15.2 Mcal ME (L) for a 30-d preliminary period and fitted with an indwelling jugular cannula at synchronized estrus. Average daily weight change during the estrous cycle was .60 +/- .25 and -1.37 +/- .30 kg/d for H and L, respectively (P less than .05). Blood concentrations of cortisol, progesterone and LH during the estrous cycle were not affected by diet, nor did diet affect frequency or amplitude of LH pulses (P greater than .05). No dietary differences were observed for daily concentrations of total protein, glucose, nonesterified fatty acids or acetate. Mean blood concentrations of propionate and butyrate were not different between diets; however, L cows had lower concentrations of propionate and butyrate on d 11 of the cycle (P less than .05). Cows fed L had higher concentrations of blood urea nitrogen (P less than .05), but they had lower concentrations of cholesterol (P less than .05) on d 4, 11, 18 and subsequent estrus (E). Insulin was not different on d 4 and 11; however, cows fed L had lower insulin concentrations on d 18 and d E (P less than .05). Dietary energy restriction in these cyclic cows caused no change in endocrine responses. Of metabolic responses measured, only blood urea nitrogen, cholesterol and insulin showed consistent changes.     

Postpartum reproduction in protein restricted beef cows: effect on the hypothalamic-pituitary-ovarian axis

The influence of dietary CP on circulating LH and anterior pituitary and hypothalamic function was examined. In Exp. 1, 28 cows were randomly assigned to four treatment groups: adequate CP (ADQ; .96 kg/d) or deficient CP (DEF; .32 kg/d) beginning at 90, 60 and 30 d before parturition and continued at a 33% increase in feed consumption after parturition. Cows were bled at 15-min intervals for 8 h on d 20, 40 and 60 after parturition. Pituitaries were collected on d 62 to analyze GnRH receptor numbers and gonadotropin content. Frequency of pulsatile LH release increased (P less than .05) from 20 to 60 d in ADQ cows. Basal and mean LH were not affected (P greater than .10) by CP restriction or by days after parturition. Crude protein did not affect pituitary GnRH receptors (P greater than .10), but it did affect pituitary LH content, FSH content and FSH concentration (P less than .05). In Exp. 2, 28 cows were assigned to treatment groups as in Exp. 1. All cows were challenged with GnRH (.22 micrograms/kg BW) at 20, 40 and 60 d after parturition and were bled every 30 min for 6 h. Responsiveness to GnRH increased with increased time after parturition (P less than .07). Deficient CP decreased GnRH-induced LH release (P less than .05). In Exp. 3, 12 cows were randomly assigned to ADQ or DEF CP beginning 120 d before parturition. All cows received 1 mg estradiol-17 beta (E2) on d 19, 39 and 59 after parturition and were bled every 30 min for 14 h beginning 14 h following E2. Response to E2 was unaffected by CP restriction (P greater than .10), whereas time to E2-induced LH peak decreased as time after parturition increased in ADQ cows (P less than .05). Results suggest that delayed return to estrus in CP-deficient postpartum beef cows might be due to reduced gonadotropin release from the anterior pituitary and decreased anterior pituitary responsiveness to GnRH.     

Postpartum interval to estrus and patterns of LH and progesterone in first-calf suckled beef cows exposed to mature bulls

Two trials were conducted in which Angus x Hereford first-calf cows were assigned randomly at calving to one of two treatments: exposure to mature penile-blocked bulls (BE) or isolation from bulls (NE). In Trial 1 (BE, n = 38; NE, n = 37), cow to bull ratio increased from 12:1 to 19:1 over a 14-d period; in Trial 2 (BE, n = 25; NE, n = 24), this ratio was maintained at 13:1. In both trials, blood samples were collected weekly for progesterone and ovaries and uteri of cows were examined rectally. Cows were observed for estrus twice daily (am:pm) beginning 10 d after calving. In Trial 2, intensive blood sampling for LH began 10 d after calving (eight cows per treatment) and continued at weekly intervals until estrus or the end of the trial. Postpartum weight change, condition score change and time to uterine involution did not differ (P greater than .10) between treatments in either trial. Interval to estrus was shorter (P less than .05) for BE cows than for NE cows in both trials. A greater proportion (P less than .05) of BE cows exhibited estrus by 60 and 90 d after calving and showed an increase in progesterone before first estrus. Mean and baseline LH concentrations and amplitude, frequency and duration of LH pulses were not altered (P greater than .10) by bull exposure. In conclusion, exposing first-calf suckled beef cows to bulls after calving hastened resumption of estrous cycles. Bull exposure did not alter patterns of LH concentrations but did increase proportions of cows that showed increased progesterone before first estrus.     

Effect of GnRH pretreatment on reproductive performance of postpartum suckled beef cows following synchronization of estrus using GnRH and PGF2alpha

The effect of GnRH pretreatment on estrus detection rate, precision of estrus, and reproductive performance of postpartum beef cows synchronized to estrus using GnRH and PGF2alpha was evaluated. In Exp. 1, Angus cows (n = 87) were randomly assigned by parity, postpartum interval, and body condition score (BCS) to receive either 1) GnRH on d -7 and PGF2alpha on d 0 (GP) or 2) the GP treatment and an additional injection of GnRH on d -16 (GGP). Estrus detection and AI were conducted twice daily from d -3 to d 3. At 72 h after PGF2alpha, all animals not previously detected in estrus were bred by AI and received a concurrent injection of GnRH (TAI). Synchronized pregnancy rates were numerically increased (P = 0.15) in cows treated with GGP (55%) compared with those on the GP treatment (44%). In Exp. 2, 1,276 spring-calving, suckled beef cows in nine herds were randomized to treatments as described for Exp. 1, except that the initial GnRH injection for the GGP treatment was administered on d -14. Herd affected all indicators of reproductive performance (P < 0.05). The percentage of animals detected in estrus prematurely (d -3 to d 0; 7%) was not affected by treatment. Estrus response rate was influenced by postpartum interval (< 60 vs > or = 60; 61 vs 73%; P < 0.01) and a three-way interaction of parity, BCS, and treatment (P < 0.01). Within animals with a BCS > or = 5.5, the GGP treatment tended to increase the detection of estrus in primiparous cows (GP vs GGP; 76 vs 91%; P = 0.11) and decrease detection in multiparous cows (GP vs GGP; 78 vs 72%; P < 0.10). However, because conception rate to TAI in animals with a BCS > or = 5.5 was greater (P < 0.05) in the GGP than in the GP group (28 vs 8%, respectively), this interaction was interpreted to represent a shift in interval to estrus induced by the GGP treatment, rather than a reduction in the synchronization of ovarian function. Conception rates of animals inseminated to an observed estrus did not differ among treatments (P = 0.15). Synchronized pregnancy rate tended (P = 0.06) to be greater in GGP- (53%) than in GP-treated animals (47%). In conclusion, pretreatment with GnRH tended to increase pregnancy rates during a 6-d synchronization period, primarily through enhanced conception rates of cows bred by TAI. In contrast to our hypothesis, GnRH pretreatment did not increase the percentage of animals detected in estrus or the precision of estrus expression.     

Development of an estrus synchronization protocol for beef cattle with short-term feeding of melengestrol acetate: 7-11 synch

An estrus synchronization protocol (7-11 Synch) was developed to synchronize the first follicular wave and timing of ovulation in postpartum beef cows. In Exp. 1, follicular development and timing of ovulation in response to the following protocol were evaluated. Beef heifers (n = 12) and cows (n = 6), at random stages of the estrous cycle, were fed melengestrol acetate (MGA; .5 mg x animal(-1) x d(-1)) for 7 d and injected with PGF2alpha (PG; 25 mg) on the last day of MGA. A second injection of PG was administered 11 d after cessation of MGA. After the second injection of PG, estrus was synchronized in 6/12 heifers and 3/6 cows. The interval to estrus in heifers and cows was 54 and 64 h, respectively (P > .10). All animals exhibiting estrus ovulated first-wave follicles. Animals that failed to respond to the second injection of PG were in estrus later than 6 d after cessation of MGA and had corpora lutea that were unresponsive to the injection of PG. Based on the variation in interval to estrus following the first PG injection on the last day of MGA feeding in Exp. 1, an injection of GnRH (100 microg) was added to the protocol 4 d after the cessation of MGA to ensure ovulation or luteinization of dominant follicles and synchronization of first-wave follicular development. This revised protocol was termed "7-11 Synch." In Exp. 2, two estrus synchronization protocols were compared. Multiparous beef cows were stratified by breed and postpartum interval and randomly assigned to the 7-11 Synch (n = 44) or Select Synch protocols (GnRH injection followed by PG injection 7 d later; n = 45). Timing of estrus after the last PG injection (0 h) ranged from 42 to 102 h in the 7-11 Synch group and -30 to 114 h in the Select Synch group. Eight cows (18%) in the Select Synch group exhibited estrus 30 h before to 18 h after PG. Synchronized estrus peaked between 42 and 66 h after the last PG injection, and a maximum number of cows were in estrus at 54 h for both treatment groups. Synchrony of estrus from 42 to 66 h was greater (P < .05) in 7-11 Synch (91%: 41/44) than in Select Synch cows (69%: 31/45). Artificial insemination pregnancy rate from 42 to 66 h was greater (P < .05) in the 7-11 Synch group (66%: 29/44) than in the Select Synch group (40%: 18/45). In summary, the 7-11 Synch protocol improved synchrony of estrus without reducing fertility. This protocol has potential future application for fixed-time AI in beef cattle production systems.     

The strategic use of estradiol to enhance fertility and submission rates of progestin-based estrus synchronization programs in dairy herds

Two experiments were performed to evaluate the efficacy of a progestin-based estrus synchronization program that incorporated the use of estradiol at the initiation of progestin treatment and at 48 h after progestin withdrawal (Exp. 2). In Exp. 1, cyclic, lactating dairy cows (n = 112) were assigned to receive either 1 (1mg) or 2 (2mg) mg of estradiol benzoate via an i.m. injection on d -9 (d 0 = initiation of the breeding season). All cows received an intravaginal progesterone-releasing insert (IPI; CIDR-B) on d -9. On d -2, the IPI was withdrawn and all cows were administered 500 microg of cloprostenol sodium. Beginning on d 0, cows were bred by AI upon detection of estrus. Estrus was observed in similar proportions of cows in each treatment during the first 6 d of AI (90% across treatments), but the interval to estrus was shorter (P < .05) in 1mg (1.26 +/- .18 d) than in 2mg (1.77 +/- .18 d). Conception and pregnancy rates did not vary among treatments; however, cows in estrus on d 0 tended to be less fertile (P = .11) than those in estrus on d 1. In Exp. 2, 408 cyclic cows from three herds were assigned to receive either no synchrony treatment (Control, n = 214) or the treatments described in Exp. 1 (1mg, n = 100; 2mg, n = 94). Anestrous cows from all herds received an IPI from d -9 to -2 (n = 143; Anestrus). All cows in the 1mg, 2mg, and Anestrus groups, with the exception of those detected in estrus between d -1 and 0, also received 1 mg of estradiol benzoate on d 0. Greater than 90% of cows that received an IPI were in estrus between d -1 and 3, and 92.1% of cows in the Control group were in estrus by d 21. Conception rate to first service in 2mg (61.7%) was similar to Control (57.0%), tended to be higher (P = .06) than 1mg (49.0%), and was greater (P < .05) than Anestrus (39.9%). The mean day of conception was earlier (P < .05) in the 2mg (d 13.1 +/- 2.0) than the Control (d 23.2 +/- 1.6) and Anestrus (d 22.4 +/- 1.9) groups. Conception occurred earlier in 1mg (d 17.4 +/- 2.1) than in Control. The proportion of cows that were pregnant at the end of the breeding season tended (P = .09) to be greater in the 2mg and Anestrus groups. This regimen of estrus synchronization improved reproductive competence in cyclic cows and resulted in similar reproductive performance in anestrous cows and untreated cyclic cows inseminated at a spontaneous estrus.     

Cortisol and luteinizing hormone after adrenocorticotropic hormone administration to postpartum beef cows

Cortisol and luteinizing hormone (LH) were measured in serum after the administration of adrenocorticotropic hormone (ACTH) to suckled (S) and nonsuckled (NS) beef cows. Blood was sampled on 2 consecutive days every 2 weeks for four bleeding periods starting 14 days after calving. Cows were injected with 200 IU ACTH or saline in a 2-day switchback design. Serum was collected before ACTH or saline injection and at 30-min intervals thereafter for 8 hours. Average cortisol concentrations in serum were similar in S and NS cows (6.4 +/- .6 and 6.1 +/- .8 ng/ml, respectively) after saline. Average cortisol concentrations in serum collected during an 8-hr period after ACTH on days 14, 28, 42 and 56 postpartum were 24.7 +/- 2.4, 31.8 +/- 3.5, 36.4 +/- 4.2 and 40.7 +/- .5 ng/ml, respectively, for S cows, and 31.1 +/- 2.9, 44.7 +/- 5.2, 45.0 +/- 5.7 and 46.0 +/- 5.4 ng/ml, respectively, for NS cows. Cortisol response to ACTH, measured as area under the response curve, was greater (P less than .05) in NS than in S cows. Amount of cortisol released by 200 IU ACTH was maximal by days 28 to 29 postpartum in NS cows, but the response increased gradually between days 14 to 15 and days 56 to 57 in S cows. overall, LH in serum averaged .55 +/- .08 ng/ml for S cows and .92 +/- .06 ng/ml for NS cows after saline, and .49 +/- .07 ng/ml for S cows and .94 +/- .06 ng/ml for NS cows after ACth. Although mean and peak serum LH concentrations did not differ between cows given ACTH and those given saline, the number of LH peaks and the number of cows having LH after saline. Mean serum LH concentrations were lower (P less than. 05) in S than in NS cows at 28 days postpartum. The number of LH peaks was lower (P less than .05) and the magnitude of the largest LH peak tended to be lower (P less than .06) in S cows at all sampling periods.     

Efficacy of an intravaginal progesterone insert and an injection of PGF2alpha for synchronizing estrus and shortening the interval to pregnancy in postpartum beef cows, peripubertal beef heifers, and dairy heifers

The objective was to test the efficacy of an intravaginal progesterone insert and injection of PGF2alpha for synchronizing estrus and shortening the interval to pregnancy in cattle. Cattle were assigned to one of three treatments before a 31-d breeding period that employed artificial insemination. Control cattle were not treated, and treated cattle were administered PGF2alpha or an intravaginal progesterone-releasing insert (CIDR) for 7 d and treated with PGF2alpha on d 6. The treatments were applied in one of three experiments that involved postpartum beef cows (Exp. 1; n = 851; 56+/-0.6 d postpartum), beef heifers (Exp. 2; n = 724; 442.5+/-2.8 d of age), and dairy heifers (Exp. 3; n = 260; 443.2+/-4.5 d of age). Luteal activity before treatment was determined for individual cattle based on blood progesterone concentrations. In Exp. 1, there was a greater incidence of estrus during the first 3 d of the breeding period in CIDR+PGF2alpha-treated cows compared with PGF2alpha-treated or control cows (15, 33, and 59% for control, PGF2alpha, and CIDR+PGF2alpha, respectively; P < 0.001). The improved estrous response led to an increase in pregnancy rate during the 3-d period (7, 22, and 36% for control, PGF2alpha, and CIDR+PGF2alpha, respectively; P < 0.001) and tended to improve pregnancy rate for the 31-d breeding period for cows treated with CIDR+PGF2alpha, (50, 55, and 58% for control, PGF2alpha, and CIDR+PGF2alpha, respectively, P = 0.10). Improvements in rates of estrus and pregnancy after CIDR+PGF2alpha, were also observed in beef heifers. Presence of luteal activity before the treatment period affected synchronization and pregnancy rates because anestrous cows (Exp. 1) or prepubertal heifers (Exp. 2) had lesser synchronization rates and pregnancy rates during the first 3 d of the breeding period as well as during the entire 31-d breeding period. The PGF2alpha, and CIDR+PGF2alpha but not the control treatments were evaluated in dairy heifers (Exp. 3). The CIDR+PGF2alpha-treated heifers had a greater incidence of estrus (84%) during the first 3 d of the breeding period compared with the PGF2alpha-treated heifers (57%), but pregnancy rates during the first 3 d or during the 31-d breeding period were not improved for CIDR+PGF2alpha compared with PGF2alpha-treated heifers. In summary, the concurrent treatment of CIDR and PGF2alpha improved synchronization rates relative to PGF2alpha alone or control. Improved estrus synchrony led to greater pregnancy rates for beef cows and beef heifers but failed to improve pregnancy rates for dairy heifers.     

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.