Synchronization of estrus in yearling beef heifers with the melengestrol acetate/prostaglandin F(2alpha) system: efficiency of timed insemination 72 hours after prostaglandin treatment

Two experiments were conducted to determine the conception rates of heifers time-inseminated following melengestrol acetate/prostaglandin F(2alpha) (MGA/PG) estrous synchronization treatment. In Experiment 1, timed insemination of heifers at 72 h after the PG injection, without regard for behavioral estrus, tended to improve (P < 0.15) the percentage of heifers pregnant to artificial insemination (AI) compared with that of synchronized heifers bred 12 h after they were first detected in estrus. In the timed-insemination treatment, heifers exhibiting behavioral estrus 48 to 72 h after PG tended to have an increased (P < 0.15) conception rate to AI compared with heifers exhibiting estrus within 48 h of PG administration. In Experiment 2, the number of heifers conceiving to AI following the MGA/PG estrous synchronization regimen was increased by mass insemination of all heifers not exhibiting estrus by 72 h after PG. The pregnancy rate to AI was higher in heifers with serum progesterone (P(4)) concentrations higher than 1 ng/ml compared with that of heifers with concentrations lower than 1 ng/ml. Of heifers with serum P(4) greater than 1 ng/ml, the pregnancy rate to AI tended to be higher when concentrations exceeded 2 ng/ml than when concentrations were 1 to 2 ng/ml. In cyclic heifers, timed insemination can increase the percentage of heifers pregnant after being synchronized with MGA/PG.     

Effect of prostaglandin F2alpha dosage and stage of estrous cycle on the estrous response and corpus luteum function in beef heifers

Ninety-five normal cyclic crossbred beef heifers were used to determine if the proportions of heifers showing estrus, intervals to estrus and corpus luteum (CL) function were influenced by PGF(2alpha) dosage and (or) the stage of luteal phase when PGF(2alpha) was administered. Heifers were assigned randomly to treatments in a 4 x 3 factorial arrangement. Treatments were 5, 10, 25 or 30 mg PGF(2alpha) injected either in early (5 to 9 d), mid (10 to 14 d) or late (15 to 19 d) stages of the luteal phase. Jugular samples were taken at 0 h and at 8 h-intervals for 48 h and again at 60 h after PGF(2alpha) treatment for progesterone assay. Heifers were observed for estrus continuously for 120 h PGF(2alpha) treatment. The proportion of heifers showing estrus was dependent upon (P<0.05) both dosage of PGF(2alpha) and stage of luteal phase. Heifers given 5 mg of PGF(2alpha) showed estrus only if treated during the late stage, while those given 10 mg of PGF(2alpha) showed a progressive increase of heifers in estrus as stage of luteal phase advanced. The proportion of heifers showing estrus after 25 and 30 mg of PGF(2alpha) increased from 56% for the early stage to 100% for the mid and late stages. Interval to estrus in heifers showing estrus within 120 h after PGF(2alpha) treatment did not differ (P>0.05) among dosages but tended (P=0.10) to be longer in heifers treated during the mid luteal stage (67 h) than in heifers treated in the two other stages (56 h). A greater proportion of heifers (P<0.05) showed estrus by 60 h after PGF(2alpha) when treated during the early and late luteal stages (75.5%) than for heifers treated during the mid luteal stage (30.4%). Patterns of progesterone concentrations were influenced (P=0.08) by the three way interaction of dosage, stage and time. In heifers that showed estrus, rate of decline in progesterone tended (P=0.07) to be slower during the mid luteal stage than during the early and late stages. Progesterone did not drop below 1 ng/ml until 32 h in heifers treated during the mid luteal stage; whereas progesterone dropped below 1 ng/ml by 24 h in heifers treated during the early and late stages. These data may be useful in designing more efficient systems for using PGF(2alpha) or its analogues in estrus synchronization of beef cattle.     

Luteolytic activity of a synthetic prostaglandin and PGF2alpha in heifers.

Two experiments involving 44 cycling heifers were conducted to evaluate the luteolytic activity of a synthetic prostaglandin, AY 24366, and PGF2alpha. Activity was assessed by the decline in progesterone level of peripheral blood and occurrence of estrus. Progesterone concentrations of jugular blood plasma were quantified by radioimmunoassay. In the first experiment, 36 heifers were treated during diestrus with AY 24366 (A-10mg intrauterine, B-30mg intramuscular and C-60mg im) or with PGF2alpha (D-5mg iu, E-15mg im and F-30mg im). Mean progesterone 0, 24 and 48 hours after treatment were A-6.33, 5.55 and 5.06; B-6.35, 2.79 and 3.92; C-5.23, 2.69 and 3.91; D-5.19, 1.50 and 1.51; E-4.69, 0.85 and 0.61; F-6.66, 0.80 and 0.48 ng/ml. Standing estrus was observed in 1, 1, 1, 4, 5 and 6 females in groups A, B, C, D, E and F respectively within 72 hours of treatment. PGF2alpha resulted in significantly (P less than 0.01) lower progesterone at 24 and 48 hours than AY 24366. However, in administration of the latter did significantly (P less than 0.05) lower progesterone at 24 hours. In the second trial six heifers were treated with either 120 or 180mg of AY 24366 im on day 12 of the cycle. Mean progesterone declined from 3.84 to 2.12 ng/ml (P less than 0.01) by 6 hours and to 1.59 ng/ml by 12 hours. Thereafter the decline was gradual and reached a level of 0.65 ng/ml at 72 hours. All six heifers showed standing estrus at 78 +/-2 hours and were inseminated. Two in each group conceived. Doses of 15mg PGF2alpha and 120mg AY 24366 were effective in causing luteal regression, however, the latter caused respiratory discomfort for 5 to 10 minutes post treatment.     

Effect of timing of artificial insemination on gender ratio in beef cattle

It was recently reported that cows inseminated at approximately 10 or 20 h before an expected ovulation deliver predominately a bull or heifer calf, respectively. The objective of this study was to further investigate the effect of timing of insemination on the gender of offspring in cattle. Angus heifers (n = 41) and cows (n = 98) were used in the study. Heifers were synchronized with a 16-d treatment of melengestrol acetate followed 17 d later with an injection of PGF2alpha. Cows were synchronized with GnRH followed 7 d later with PGF2alpha. A HeatWatch electronic estrus detection system was used to determine the onset of estrus. Based on previous studies, it was assumed that ovulation occurs approximately 32 h after the onset of estrus. Therefore, animals were artificially inseminated at either 8 to 10 h (early; > or = 20 h before expected ovulation) or 20 to 25 h (late; < or = 10 h before expected ovulation) after the onset of estrus. Sixty to 80 d after insemination, ultrasonography was used to confirm pregnancy status and to determine the gender of fetuses. Gender of calves was subsequently confirmed at calving. Data were analyzed for effects of time of insemination and sire or semen batch on gender ratio, as well as any effect of length and/or intensity of estrus on conception rate and gender ratio. Twenty-nine of 41 heifers and 69 of 98 cows were detected in estrus after synchronization and were inseminated; 20 of 29 heifers and 48 of 69 cows were subsequently confirmed pregnant. Neither the length of estrus nor its intensity (number of mounts) had an effect on pregnancy rate or gender ratio (P > or = 0.418). Timing of insemination (early versus late) had no effect on gender ratio (P = 0.887). Semen from 13 sires representing 17 lots was used to inseminate the cows and heifers. No differences (P = 0.494) were detected in the gender ratios resulting from different sires or semen batches. In contrast to previous findings, our results indicate that inseminating beef cattle at approximately 20 or 10 h before an expected ovulation does not alter the gender ratio of the resultant calves.     

Management of reproduction in dairy heifers based on the synchronization of estrous cycles

Israeli-Holstein breed dairy heifers (n = 571), 13 to 15 mo old, were utilized in two experiments. In Experiment 1, the reproductive performance of synchronized heifers was compared with that of untreated controls. The heifers in both groups were inseminated following the detection of estrus. In Experiment 2, all heifers were synchronized and inseminated following the detection of estrus. Half of the animals in this experiment also received one or two fixed-time inseminations 72 and 96 h after the last synchronization treatment. Synchronization of estrous cycles was performed by two prostaglandin F(2alpha) (PG) injections given 12 d apart. In the control group of Experiment 1, observation of estrous behavior and insemination of heifers detected in estrus were carried out daily throughout the experiment. In the synchronized groups of Experiments 1 and in 2, the management of reproduction consisted of estrus detection followed by the insemination of heifers in estrus carried out only during 6 d of every 3 wk. Five days following the second PG injection, 86% of the heifers were detected in estrus, 71% of them at 49 to 96 h after treatment. In Experiment 1, age at first insemination, age at conception, and conception rate were, respectively, 425 d, 446 d and 54% in the control group vs 432 d (P<0.02), 449 d and 62% in the PG-treated group. In Experiment 2, the respective figures were 436 d, 462 d and 59% in the group inseminated following the detection of estrus vs 427 d (P<0.002), 464 d and 51% (P<0.05) in the group in which heifers were inseminated at estrus and also received one or two fixed-time inseminations.     

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

Two trials involving 85 heifers and 67 cows were conducted to determine the effect of estrous cycle stage at the time of Syncro-Mate-B((R)) (SMB) treatment on interval to estrus following implant removal and on conception rate at the synchronized estrus. In Trial 1, 57 beef and 28 dairy heifers were treated with SMB on each representative day of a 22-d estrous cycle (estrus = Day 0). Beef heifers were artificially inseminated approximately 48 h after implant removal, whereas dairy heifers were inseminated 0 to 12 h after detection of estrus. Inseminations were scored by the inseminator according to their difficulty. Interval to the onset of estrus was not different between heifers treated early ( Day 11) in the cycle (35.2 +/- 7.2 h). Conception rate at the synchronized estrus was slightly higher in early-cycle heifers (22 47 = 47% ) compared to late-cycle heifers (14 38 = 37% , P = 0.2). Heifers that were difficult to inseminate had lower (P < 0.01) conception rates (2 11 = 18% ) at the synchronized estrus than heifers considered normal (21 51 = 41% ) or easier than normal to inseminate (13 23 = 57% ). In Trial 2, of the 131 beef cows synchronized, 67 that were estimated to be either early or late in the estrous cycle by progesterone analysis were utilized. Cows were treated with SMB and inseminated without regard to estrus 48-h after implant removal. Inseminations were scored as in Trial 1. Calves were separated from cows from the time of implant removal to insemination. Conception rate was higher (P < 0.05) in cows treated with SMB early ( Day 11, 16 35 = 46% ). Cows that were difficult to inseminate had a lower (P < 0.01) conception rate (0 8 = 0% ) than cows that were normal (43 94 = 46% ) or easier than normal to inseminate (13 29 = 45% ).     

Comparison of timed insemination with insemination at estrus following synchronization of estrus with a MGA-prostaglandin system in beef heifers

Three trials utilizing 231 beef heifers were conducted in 1993 to determine if a timed insemination would result in similar synchronized pregnancy rates as insemination by estrus following synchronization of estrus using the 14-d MGA-prostaglandin system. All heifers were fed 0.5 mg MGA/h/d fof 14 d and given a 25 mg injection of PGF(2)alpha im 17 d after the final day of MGA feeding. Heifers in Group 1 (timed AI treatment) were inseminated at 72 h after the prostaglandin injection independent of whether or not they were observed in estrus. Heifers in Group 2 (AI by estrus) were inseminated 12 to 18 h after the onset of estrus. Since the trial was a significant source of variation for synchronized pregnancy rate, the effect of treatment on pregnancy rate was analyzed for each trial. Synchronized pregnancy rates in Trials 2 and 3 were similar in both treatment groups; 37 vs 35% and 61 vs 58% for the timed AI vs AI by estrus (Groups 1 and 2) in Trials 2 and 3, respectively. In both of these trials the degree of estrous synchrony was high. In Trial 1, the synchronized pregnancy rate in heifers that were time-inseminated was significantly lower than that of heifers that were inseminated by estrus (29 vs 57%). The lower synchronized pregnancy rate of Group 1 (timed AI) heifers in Trial 1 appeared to be due to the low degree of estrous synchrony in this trial. Our results indicate that using timed insemination with the 14-d MGA-prostaglandin system will give similar synchronized pregnancy rates as inseminating by estrus in groups of beef heifers where the degree of synchrony is high. However, in heifers where the degree of estrous synchrony is low, a timed insemination reduces synchronized pregnancy rates.     

Effect of stage of the estrous cycle on interval to estrus after PGF(2alpha) in beef cattle

Effect of stage of the estrous cycle at the time of prostaglandin F(2alpha) (PGF(2alpha)) injection on subsequent reproductive events in beef females was studied in four trials involving 194 animals. Cycling animals were given two injections of 25 mg PGF(2alpha) 11 days apart or, in some cases, the interval was altered to allow the second injection to fall on a specific day of the cycle. Day of estrous cycle at time of the second injection was determined by estrous detection. Interval from the second PGF(2alpha) injection to the onset of estrus (interval to estrus) was shorter (P<.01) in heifers than in cows. Both cows and heifers injected on days 5 to 9 (early cycle) had a shorter (P<.01) interval to estrus (estrus = day 0) than did those injected on days 10 to 15 (late cycle). Conception rate was lower (P<.05) for early-cycle heifers than for late-cycle heifers inseminated by appointment at 80 hours. There was no significant difference in conception rate of early-or late-cycle heifers or cows inseminated according to estrous detection or early- or late-cycle cows inseminated at 80 hours. Progesterone concentrations in blood samples collected in heifers at 4-hour intervals after the second PGF(2alpha) injection on either day 7 or day 14 declined linearly (P<.05) through 36 hours. Day of the estrous cycle at PGF(2alpha) injection had no effect on rate of progesterone decline, even though heifers injected on day 7 had a shorter (P<.05) interval to estrus. All animals whose cycle length was not affected by the second PGF(2alpha) injection were treated on days 5 through 8 of the cycle, indicating that PGF(2alpha) was less effective in regressing the corpus luteum between days 4 and 9 of the cycle than later in the cycle.     

Single appointment insemination for heifers after prostaglandin or progestin synchronization of estrus

Pregnancy rates to a single appointment insemination were compared in seven groups of beef or dairy heifers following estrus synchronization with the prostaglandin F(2a) product, cloprostenol, versus the progestín product, Syncro-Mate-B. For cloprostenol synchronization, two injections of cloprostenol were given 11 d apart, with insemination occurring at 61+/-1 h after the second injection. The Syncro-Mate-B treatment consisted of a norgestomet/estradiol injection and a norgestomet implant on Day 0, followed by insemination at 49+/-1 h after implant removal on Day 9. Treatments were coordinated so all heifers in each group were housed together at the time of and for 48 h before a common insemination time so that the treatment received by individual heifers was not known. Overall pregnancy rates did not differ between treatment groups: 42% for 111 heifers treated with cloprostenol and 38% for 108 heifers treated with Syncro-Mate-B. Four blood samples per heifer taken during each replicate showed that two groups (n = 67) had many noncyclic heifers, while five groups (n = 152) had very few. Differences in pregnancy rates between these categories existed for both estrus synchronization methods, 18 versus 53% for cloprostenol and 21 versus 45% for Syncro-Mate-B. Thus the two methods were equally effective for cyclic heifers and equally ineffective for noncyclic heifers.     

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.     

Effects of administering progesterone at selected intervals after insemination of synchronized heifers on pregnancy rates and resynchronization of returns to service

In 3 separate trials at 2 locations, dairy heifers (n = 396) were treated with a Controlled Internal Drug Release (CIDR) progesterone device for 9 d. On Day 7 of CIDR treatment, all heifers were injected with PGF(2alpha). Synchronized estruses were detected using a tailpaint and chalk (TPC) scoring system. An animal's tailhead was painted at device insertion, and this strip was covered with a contrasting color of chalk at device removal. Over all trials, 85.1% of the heifers were detected in estrus and were inseminated at 48 or 72 hours after CIDR removal. These synchronized and inseminated heifers were divided into the following treatment groups: 1) untreated controls, receiving no further treatment (n = 138); 2) post-insemination progesterone supplementation with a new (n = 59) or used (n = 29) CIDR device for Days 1 to 8 or 2 to 9, respectively, following insemination; or 3) resynchronization of return to service with a used CIDR device for Days 17 to 22 after insemination (n = 112). The pregnancy rate to first insemination in the control and resynchronized groups (Groups 1 and 3) was 46.4%, but decreased to 18.2% with the post-insemination progesterone supplementation. Resynchronization of returns to service (estrus detected 1 to 4 d following removal of second CIDR) occurred in 58.9% of all nonpregnant heifers in Group 3. In summary, CIDR devices used in conjunction with PGF(2alpha) effectively synchronize estrus in dairy heifers. Progesterone supplementation within 2 d of first insemination for 7 d suppressed fertility. Used CIDR devices inserted for Days 17 to 22 after first insemination resynchronized heifers not pregnant to first insemination.     

The effect of estradiol benzoate administration on estrous response and synchronized pregnancy rate in dairy heifers after treatment with exogenous progesterone

The objectives of this trial were to determine the effects of 0.5 mg estradiol benzoate administered intramuscularly 24 h after the removal of progesterone-containing intravaginal devices on the occurrence and timing of estrus, synchronized pregnancy rate and synchronized conception rate in dairy heifers. A clinical trial was conducted involving 750 dairy heifers in 13 herds. Within each herd heifers were randomly allocated to 1 of 2 estrus synchronization treatments. All heifers received a CIDR-B progesterone-containing intravaginal device containing a 10 mg estradiol benzoate capsule for 12 d. Twenty-four hours after CIDR-B removal one group received 0.5 mg, im estradiol benzoate while the other group received an intramuscular injection of a placebo. Estrus detection was performed at 48 and 72 h after intravaginal device removal, and heifers detected in estrus at those times were inseminated. Administration of estradiol benzoate 24 h after removal of CIDR-B devices significantly increased the number of heifers exhibiting estrus within the observation period (96.1 vs 90.5%; P < 0.01). It also altered the onset of estrus so that significantly more heifers were in estrus (86.6 vs 72.3%; P < 0.01) and conceived (47.1 vs 37.5%; P < 0.05) by 48 h after CIDR-B device removal. The synchronized conception rate was unaffected by treatment. The distribution of estrus was such that fixed-time insemination after estrus synchronization with this treatment program may be recommended.     

Luteolytic activity of a synthetic prostaglandin and PGF2α in heifers

Two experiments involving 44 cycling heifers were conducted to evaluate the luteolytic activity of a synthetic prostaglandin, AY 24366, and PGF_2α. Activity was assessed by the decline in progesterone level of peripheral blood and occurrence of estrus. Progesterone concentrations of jugular blood plasma were quantified by radioimmunoassay. In the first experiment, 36 heifers were treated during diestrus with AY 24366 (A - 10mg intrauterine, B - 30mg intramuscular and C - 60mg im) or with PGF_2α (D - 5mg, iu, E - 15mg im and F - 30mg im). Mean progesterone 0, 24 and 48 hours after treatment were A - 6.33, 5.55 and 5.06; B - 6.35, 2.79 and 3.92; C - 5.23, 2.69 and 3.91; D - 5.19, 1.50 and 1.51; E - 4.69, 0.85 and 0.61; F - 6.66, 0.80 and 0.48 ng/ml. Standing estrus was observed in 1, 1, 1, 4, 5 and 6 females in groups A, B, C, D, E and F respectively within 72 hours of treatment. PGF_2α resulted in significantly (P<0.01) lower progesterone at 24 and 48 hours than AY 24366. However, im administration of the latter did significantly (P<0.05) lower progesterone at 24 hours. In the second trial six heifers were treated with either 120 or 180mg of AY 24366 im on day 12 of the cycle. Mean progesterone declined from 3.84 to 2.12 ng/ml (P<0.01) by 6 hours and to 1.59 ng/ml by 12 hours. Thereafter the decline was gradual and reached a level of 0.65 ng/ml at 72 hours. All six heifers showed standing estrus at 78±2 hours and were inseminated. Two in each group conceived. Doses of 15mg PGF_2α and 120mg AY 24366 were effective in causing luteal regression, however, the latter caused respiratory discomfort for 5 to 10 minutes post treatment.     

Failure of hCG to support luteal function in bitches after estrus induction using deslorelin implants

Induction of estrus with deslorelin implants was followed by abortions in bitches that conceived during the induced estrus. Lowering the deslorelin dose and choosing a better implantation site prevented the abortions. This study investigated the hypothesis that induction of estrus with deslorelin is followed by reduced serum progesterone concentrations (SPC) during the ensuing diestrus. Assuming that reduced luteal function resulted from reduced LH secretion due to hypophyseal down-regulation of GnRH receptors, the effect of human chorionic gonadotropin (hCG) treatment on the SPC of diestrous bitches was also investigated. In Experiment 1, 10 spontaneously cycling bitches served as controls, whereas estrus was induced with deslorelin implants in 24 others. In Experiment 2, six diestrous bitches were treated with a single dose of hCG between Days 39 and 45 of diestrus. The SPC was lower in deslorelin-induced bitches from Days 35 to 56 of diestrus and hCG increased SPC during the first 24 h after treatment, followed by a dramatic decline thereafter. Although SPC recovered in pregnant bitches, it remained much lower (< or = 1 ng/mL) than in untreated, non-pregnant bitches. The suppression of progesterone secretion after hCG treatment suggested that decreased luteal activity in deslorelin-induced bitches may not be a simple consequence of down-regulation of hypophyseal GnRH receptors.     

Twenty-four-hour secretory patterns of cortisol,progesterone and estradiol in heifers during the follicular and luteal phases of the ovarian cycle

Daily variations of plasma cortisol, progesterone and estradiol concentrations were measured by radioimmunoassay in six different normally cycling heifers during estrus (day 1 of the cycle) and diestrus (days 12–15 of the cycle). Each animal was fitted with an indwelling jugular catheter, and blood was withdrawn at 30-min intervals over a 24-h period. Statistical evaluation of the hormonal profiles using time series analysis revealed that all three steroids are secreted episodically with secretory episodes varying in number, magnitude and timing among different heifers. After dividing the 24-h into three 8-h time periods (I, 09.00–17.00 h; II. 17.00–01.00h; III, 01.00–09.00 h) a prominent circadian rhythm was found for cortisol during estrus and diestrus. Diurnal periodicity similar to that of cortisol was noticed for plasma progesterone during estrus but not diestrus when a functional corpus luteum was present. Estradiol secretion during the follicular and luteal phase of the estrous cycle was characterized by intermittent sustained elevations lasting about 9–15 h and marked by a graded rise and fall of hormone levels unrelated to photoperiod.From our results obtained in cycling heifers we conclude the following: (1) Plasma cortisol exhibits a distinct circadian rhythm during estrus and diestrus which is highly correlated with the light–dark cycle. (2) Plasma progesterone during estrus demonstrates a diurnal pattern which is absent in diestrous heifers bearing a corpus luteum. (3) Plasma estradiol lacks circadian rhythmicity but shows a distinct pattern different from that of progesterone, indicating that both steroids are secreted independently and not controlled by a circadian pacemaker.     

Response of prepubertal Bos taurus and Bos indicus x Bos taurus 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%).     

The economic effect of estrus synchronization in beef heifers on average weaning weight of calves

Prostaglandin was used to induce estrus in 129 Hereford x Holstein heifers for artificial insemination. At the start of the breeding season heifers were observed twice daily and inseminated 12 hours after first being observed in estrus. On the morning of the sixth day of the breeding season all heifers not yet observed in estrus or inseminated were treated with prostaglandin (25 mg Lutalyse(R)). Observation was continued and heifers were bred 12 hours after first observed in estrus. Of the 129 heifers 33 were inseminated before prostaglandin treatment, the remainder of the heifers were bred within 4 days of treatment. Two heifers developed cysts and did not express estrus. First service pregnancy rates were 70% before treatment and 79% after treatment. All heifers were inseminated the first time within the first 10 days of the breeding season and 100 pregnancies occurred in the first 10 days of the season. As a result median days to pregnancy was 8. Calves were all weaned and weighed at approximately 150 days of age. These results were then used to model the economic impact of estrus synchronization of beef heifers on the increased weaning weights of their calves. Prostaglandin treatment altered the distribution of pregnancies and resulted in a cluster of conceptions from Day 7 to 10 of the breeding season with no conceptions from Day 11 to 19. In order to study the influence of estrus synchronization on weaning weights a simulation model was constructed. All heifers that conceived on the first service after prostaglandin were assigned an alternate date of conception from Day 7 to 21 of the breeding season with a random number generator. In this way a uniform random distribution of pregnancies over the first 21 days of the breeding season was constructed. An alternate calving date was then estimated using the known gestation length and this date was used to calculate the weaning weight of the calf using the known weight per day of age. The average weaning weight of the uniform random distribution was then compared to the synchronized distribution with Student's t -test. The simulation model was repeated 10 times and each time weaning weights of the uniform random distribution were significantly less than with the synchronization program (p<.01). At a value of $4.00 for prostaglandin and $1.76 for each kilogram of calf the average increase of 3.8 kgs in weaning weight per calf resulted in a return on investment of $1.92 for every dollar invested in prostaglandin.     

Effect of dexcloprostenol on endogenous prostaglandin F2alpha release in dairy heifers

Cloprostenol was previously believed to be unable to release endogenous prostaglandin F2alpha (PGF2alpha) when administered during early bovine diestrus. A prostaglandin release is, however, seen in late diestrus. The aim of this study is to find out whether dexcloprostoenol (containing the only biologically active isomer, d-isomer, of cloprostenol) induces endogenous PGF2alpha release during early and late diestrus. Twelve heifers of the Finnish Ayrshire breed were allocated into two equal groups. Their estrous cycles were synchronized with dexcloprostenol. A further luteolysis was induced with 0.15 mg of dexcloprostenol either on Day 7 (group D7 or early diestrus) or on Day 14 (group D14 or late diestrus) after ovulation. Blood for progesterone and the PGF2alpha metabolite 15-ketodihydro-PGF2alpha determinations was collected immediately before dexcloprostenol treatment and thereafter every second hour for 48 h. Five of the six heifers in both groups showed significantly increased blood levels of 15-ketodihydro-PGF2alpha at some time during the 48-h experimental period. The intervals from treatment to the first significant increases of the PGF2alpha metabolite were 32.8+/-2.3 h (min. 30 h, max. 36 h) and 20.0+/-4.2 h (min. 14 h, max. 24 h) in groups D7 and D14, respectively (P < 0.01). We have concluded that dexcloprostenol induced endogenous PGF2alpha release in most cases, regardless the time of its administration (early or late diestrus). This release, however, differs from that observed during spontaneous luteolysis.     

A two injection schedule with prostaglandin F2alpha for the regulation of the ovulatory cycle of cattle

Thirty randomly cycling Holstein heifers were given two injections of prostaglandin F_2α 10 days apart, to determine whether this treatment would cause most of the animals to show estrus shortly after the second injection. Twenty-five of the 30 heifers (83%) were in estrus and inseminated 2 to 4 days following the second injection and 10 of the 25 (40%) were pregnant at 60 days post insemination. Thirteen of 15 control heifers (87%) were detected in estrus over a 3 week period and seven of the 13 (54%) were pregnant at 60 days. Chi square analysis indicated that the proportion of animals showing estrus and subsequent pregnancy in the treated and in the control groups were not significantly different. The two injection system may have commercial application for the regulation of the ovulatory cycle of cattle.     

Factors influencing estrus and conception in dairy heifers after prostaglandin F2-alpha

The influence of interval between insemination (AI) and estrus on subsequent fertility of PGF(2alpha)-treated (two injections of 25 mg, 11 days apart) heifers was assessed in two experiments. In Experiment I, 240 heifers were allotted to Control (AI 8 to 16 hr after estrus detection), PGF(2alpha)-E (AI 8 to 16 hr after estrus within five days of second PGF(2alpha)) or PGF(2alpha)-T (AI 80 hr after second PGF(2alpha)). In Experiment II, 130 heifers were assigned to control (AI as before) or PGF(2alpha) (AI 72 or 80 hr after second PGF(2alpha)) with half the PGF(2alpha) heifers receiving 100 mug GnRH 72 hr after first PGF(2alpha). Heifers of both experiments that were bred at a predetermined time were arrayed by interval from AI to estrus. Conception rates of heifers detected in estrus from 32 hr before AI to 24 hr after AI did not differ (x(2)=3.35, df=5, P>0.5). The percentage of GnRH-treated heifers in estrus within five days (81.8%) was not (P>0.75) greater than those not receiving GnRH (77.3%) but they had higher (P<0.05) serum progesterone (P(4)) concentration at second PGF(2alpha) (3.17 vs 2.41 ng/ml). When P(4) values were arrayed for both groups at 1 ng intervals, the percentage of heifers exhibiting estrus increased with increasing P(4) level (P<0.05).