Estrus In Beef Cows Research Articles

Does GnRH treatment at TAI regardless of estrus occurrence increase pregnancy rate in crossbred Bos taurus suckled cows?

The impact of GnRH treatment on the day of TAI in beef cows has received limited investigation, especially concerning its association with estrus expression. Consequently, two experiments were conducted to assess the potential of GnRH treatment on the day of TAI to enhance fertility according to the expression or not of estrus in beef cows. Experiment 1 aimed to determine ovulation rate and luteal function, while Experiment 2 aimed to determine the effect of the two GnRH treatment approaches on pregnancy rate. In Experiment 1, multiparous Brangus suckling cows (n = 17) were submitted to an 8-day TAI protocol. Estrus occurrence was evaluated based on chalk removal on D10 (TAI) and cows were assigned to receive GnRH (25µg lecirelin; im) according to the group: GnRH (n = 7), regardless of estrus expression; or selectGnRH (n = 10), only cows not detected in estrus. Ovulation rate occurring until 77h after IVD removal did not differ (p = 0.17) between GnRH (85.7%; 6/7) and selectGnRH (100%; 10/10). Also, corpus luteum size and serum progesterone concentration were not affected (p>0.05) by treatments. In Experiment 2, crossbred taurine suckled cows (n = 384) were submitted to the same protocol as described in Experiment 1 and were randomly allocated to GnRH or selectGnRH groups. There was no difference in P/AI between groups (selectGnRH = 55.6%; GnRH = 54.3%; p = 0.7) 30 days after TAI. As expected, there was a pronounced effect (p<0.0001) of estrus expression on P/AI (Estrus = 61.5%; No estrus = 33.0%), regardless of group. In summary, ovulation timing and rate and luteal function did not differ between groups. Also, GnRH administration only in cows that do not show estrus is recommended, considering hormone savings and similar conception rate.

Application of low dose of equine chorionic gonadotropin at acupoint Hou Hai for fixed-time artificial insemination in beef cows

This study evaluated the efficiency application of low dose of equine chorionic gonadotropin (eCG) using Hou Hai acupoint for fixed-term artificial insemination (FTAI) in beef cows. Seventy cows received intravaginal devices with progesterone, and 3 mg of estradiol benzoate on day zero (D0) of FTAI. On D9, the devices were removed, and 150 ?g of prostaglandin F2? was applied; the animals were then randomly distributed to three treatment groups: T1 (n = 23), 300 IU of eCG intramuscularly (IM); T2 (n = 23), 90 IU of eCG at acupoint Hou Hai; and T3 (n = 25), 90 IU of eCG at false acupoint IM. On D10, the animals received 1 mg of gonadotropin-releasing hormone IM, and FTAI was performed, 52 h after the devices were removed. The quantitative variables with normal distribution were assessed using analysis of variance at 5% probability. For variables that did not show normal distribution, Kruskal-Wallis test at 5% probability was used. No significant difference was noted among the groups (P &gt; 0.05) for the interval between device withdrawal at ovulation (58.2 ± 12.2 h), diameter of the largest follicle on D9 (9.9 ± 2.2 mm), diameters of the ovulatory follicle (12.2 ± 3.0 mm) and the second largest follicle (6.7 ± 2.1 mm), follicular growth rate (0.8 ± 0.3 mm/d), ovulation rate (82%), corpus luteum size (2.32 ± 0.35 cm2), and pregnancy rate (58.67%). The protocol cost per animal was US$ 10.67 (T1) and US$ 8.50 (T2 and T3). The use of 90 IU of eCG applied at Hou Hai acupoint or at false acupoint caused satisfactory synchronization of estrus in beef cows. In addition, this procedure was cost-effective.

Comparison of Long-term CIDR-based Estrous Synchronization Protocols on Estrous Response and Pregnancy Rates in Postpartum Beef Cows

Estrous synchronization and artificial insemination allow beef producers to maximize genetic improvement in their operations. However, many beef producers have been reluctant to incorporate this technology into their program due to the management requirements it entails. Therefore, any improvements to decrease time and/or labor inputs may increase the use of such technology. The CIDR-Select protocol is an effective method of synchronizing estrus in beef cows. This protocol involves 14 days treatment with a CIDR progesterone insert, GnRH treatment 12 days after CIDR removal, and treatment with prostaglandin F2alpha seven days after GnRH. The entire protocol requires over 33 days, from initial CIDR insertion to detection of estrus and insemination. Reducing the interval from CIDR removal to GnRH treatment would shorten the protocol and potentially make it more attractive for use by producers, providing effectiveness was maintained. The objective of this experiment was to characterize the estrous response and subsequent fertility of postpartum beef cows following synchronization utilizing the CIDR-Select protocol, in which the interval from CIDR removal to GnRH treatment was reduced from 12 to two days.

Estrus and Luteal Activity of Postpartum Beef Cows After Treatment with Estradiol

Multiparous Hereford × Angus postpartum cows were used to determine the effects of treatment with estradiol cypionate (ECP) on estrus and luteal activity (LA) of thin ( 1 ng/mL for 1 d) within 10 d after treatment (P = 0.52). Days to normal LA were greater (P = 0.02) for thin and moderate cows treated with ECP at 25 d after calving (74 ± 5 d) compared with control cows (61 ± 5 d), and ECP treatment tended to increase (P = 0.07) days to LA for thin cows treated at 50 d after calving. Although ECP induced estrus in beef cows during the first 50 d after calving, normal luteal function was not initiated.

Nutritional controls of beef cow reproduction

Abstract The livestock industry and animal scientists have long recognized the importance of proper nutrition for cattle to achieve reproductive success. Timely resumption of estrus following parturition is a major milestone that a cow must reach for optimal reproduction. Dynamic interplay among all strata of the hypothalamo-hypophyseal-ovarian axis occurs during the cow's transition from postpartum anestrus to reproductive competence. The reproductive axis integrates a milieu of nutritionally related signals that directly or indirectly affect reproduction. Directing nutritional inputs toward anabolic processes is critical to stimulating key events that promote reproductive success. Although prepartum and postpartum energy balance are the most important factors affecting duration of the postpartum interval to first estrus in beef cows, other nutritional inputs likely impinge on the hypothalamo-hypophyseal-ovarian axis to influence reproduction. For example, feeding fat to beef cows for approximately 60 d before calving may improve pregnancy rates in the upcoming breeding season. Supplementing postpartum diets with lipids high in linoleic acid can impede reproductive performance of beef cows. Precise mechanisms through which nutritional inputs mediate reproduction have not yet been fully elucidated. Scientists investigating nutritional mediators of reproduction, or how nutritional inputs affect reproduction, must be cognizant of the interactions among nutrients and nutritional cues responsible for mediating reproduction.

208 FOLLICULAR DYNAMICS, ESTRUS, AND OVULATION IN COWS SYNCHRONIZED TO OVULATE FIRST OR SECOND WAVE DOMINANT FOLLICLES USING SHORT- OR LONG-TERM MELENGESTROL ACETATE- OR EAZI-BREED CIDR INSERT-BASED PROTOCOLS

The objective of this experiment was to determine the feasibility in substituting EAZI-BREED CIDR inserts (CIDR; Pfizer Animal Health, Groton, CT, USA) for melengestrol acetate (MGA) in progestin-based protocols to synchronize estrus in beef cows. Follicular dynamics, timing of estrus, and ovulation were compared in beef cows synchronized to ovulate first or second wave dominant follicles using short- or long-term MGA- or CIDR-based protocols. The study was conducted with 48 nonsuckled, estrous cycling, crossbred beef cows assigned to one of four treatments (T1 to T4; n = 12/T) by age and body condition. Cows were synchronized to ovulate first wave (T1 and T2) or second wave (T3 and T4) dominant follicles based on assignment to treatment. Cows in T1 were fed MGA (0.5 mg h−1 d−1) for 7 days, and were injected with PGF2α (PG; 25 mg Lutalyse; Pharmacia Animal Health, Kalamazoo, MI, USA) on Day 7, GnRH (100 μg Cystorelin; Merial, Athens, GA, USA) on Day 11, and PG on Day 18. Cows in T2 had CIDR (1.38 g progesterone) inserted for 7 days, and were injected with PG on Day 7, GnRH on Day 9, and PG on Day 16. Cows in T3 were fed MGA for 14 days, and were injected with GnRH on Day 26, and PG on Day 33. Cows in T4 had CIDR inserted for 14 days, and were injected the GnRH on Day 23, and PG on Day 30. Transrectal ultrasonography was performed daily to monitor follicular dynamics from GnRH to estrus after PG; and every 4 h from 20 h after the onset of estrus until ovulation. Estrus detection was performed continuously using the HeatWatch® estrus detection system (DDx, Denver, CO, USA). Blood samples for progesterone (P4) were collected daily beginning one day prior to the initiation of treatment and continuing through ovulation following PG. Data were analyzed using the General Linear Models procedure of SAS (SAS Institute, Inc., Cary, NC, USA) and are summarized in the following table. Animals that responded to treatment and were included in the analysis were those that initiated a new follicular wave after administration of GnRH and that displayed estrus within 144 h after PG. Although estrous response was similar among treatments, there were differences in follicular dynamics, steroid secretion patterns, and timing of events that culminated in differences in timing and synchrony of estrus and ovulation among the short- and long-term groups. These differences may be important in relation to fixed-time AI programs. These data suggest that in situations that are not conducive to feeding MGA, substituting CIDR inserts into MGA-based protocols to synchronize estrus may be feasible. This work was supported by USDA-NRI grant 2000-02163; Pfizer Animal Health, New York, NY; and Merial, Athens, GA, USA.

Follicular dynamics and steroid profiles in cows during and after treatment with progestin-based protocols for synchronization of estrus.

Two progestin-based protocols for the synchronization of estrus in beef cows were compared. Cyclic, nonlactating, crossbred, beef cows were assigned by age and body condition score to one of two treatments. Cows assigned to the MGA Select protocol were fed melengestrol acetate (MGA; 0.5 mg x cow(-1) x (-1)) for 14 d, GnRH was administered (100 microg i.m. of Cystorelin) 12 d after MGA withdrawal, and PGF2alpha (25 mg of i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol were fed MGA for 7 d and were injected with PG on d 7 of MGA, GnRH on d 11, and PG on d 18. Transrectal ultrasonography was performed daily to monitor follicular dynamics from the beginning of MGA feeding through ovulation after the synchronized estrus. All cows exhibited estrus in response to PG. Mean interval to estrus was shorter (P < 0.01) for 7-11 Synch-treated cows (56 +/- 1.5 h) than for cows assigned to the MGA Select protocol (73 +/- 4.7 h). Mean interval from estrus to ovulation did not differ between treatments (P > 0.10). Variances for interval to estrus differed (P < 0.01) between treatments. Mean follicular diameter at GnRH injection, PG injection, and estrus did not differ (P > 0.10) between treatments. Relative to MGA Select, serum estradiol-17beta concentrations were higher (P < 0.01) for 7-11 Synch 2 d and 1 d before, on the day of GnRH injection, in addition to 4 d after GnRH, and 24 h after PG. Mean progesterone concentrations were greater (P < 0.01) for MGA Select cows from 4 d before to 7 d after GnRH. Forty-four percent of the variation in interval to estrus between treatments was explained by differences in estradiol-17beta concentrations 24 h after PG. This study suggests that follicular competence is likely related to steroidogenic capacity of the follicle and the endocrine environment under which growth and subsequent ovulation of the dominant follicle occurs.

Follicular dynamics and steroid profiles in cows during and after treatment with progestin-based protocols for synchronization of estrus12

Two progestin-based protocols for the synchronization of estrus in beef cows were compared. Cyclic, nonlactating, crossbred, beef cows were assigned by age and body condition score to one of two treatments. Cows assigned to the MGA Select protocol were fed melengestrol acetate (MGA; 0.5mg•cow−1•−1) for 14 d, GnRH was administered (100 μg i.m. of Cystorelin) 12 d after MGA withdrawal, and PGF2α (25 mg of i.m. Lutalyse) was administered 7 d after GnRH. Cows assigned to the 7-11 Synch protocol were fed MGA for 7 d and were injected with PG on d 7 of MGA, GnRH on d 11, and PG on d 18. Transrectal ultrasonography was performed daily to monitor follicular dynamics from the beginning of MGA feeding through ovulation after the synchronized estrus. All cows exhibited estrus in response to PG. Mean interval to estrus was shorter (P < 0.01) for 7-11 Synch-treated cows (56 ± 1.5 h) than for cows assigned to the MGA Select protocol (73 ± 4.7 h). Mean interval from estrus to ovulation did not differ between treatments (P > 0.10). Variances for interval to estrus differed (P < 0.01) between treatments. Mean follicular diameter at GnRH injection, PG injection, and estrus did not differ (P > 0.10) between treatments. Relative to MGA Select, serum estradiol-17β concentrations were higher (P < 0.01) for 7-11 Synch 2 d and 1 d before, on the day of GnRH injection, in addition to 4 d after GnRH, and 24 h after PG. Mean progesterone concentrations were greater (P < 0.01) for MGA Select cows from 4 d before to 7 d after GnRH. Forty-four percent of the variation in interval to estrus between treatments was explained by differences in estradiol-17β concentrations 24 h after PG. This study suggests that follicular competence is likely related to steroidogenic capacity of the follicle and the endocrine environment under which growth and subsequent ovulation of the dominant follicle occurs.

Synchronization of Estrus in Beef Cows and Beef and Dairy Heifers with Intravaginal Progesterone Inserts and Prostaglandin F2α with or Without Gonadotropin-Releasing Hormone

Intravaginal progesterone (CIDR) inserts consist of a layer of silicone containing 10% progesterone (P4) by weight (1.38 g) molded over a T-shaped nylon spine. In the absence of a corpus luteum, the CIDR insert releases sufficient P4 to maintain concentration >2 ng/ mL in blood. A synchronization of estrus program utilizing a 7-d administration of CIDR inserts with the administration of 25mg of prostaglandin F2α(PGF2α) on d 6 (CIDR + PGF2α) was compared with a single injection of PGF2αor no treatment (control) in beef cows (six locations), beef heifers (five locations), and dairy heifers (four locations). These studies indicated that estrus was synchronized more precisely over a 3- to 4-d interval with the CIDR + PGF2αtreatment regimen than with a single injection of PGF2α. Furthermore, administration of the CIDR insert induced estrous cyclicity in some of the anestrous cows and prepubertal heifers. Cows and heifers inseminated over the 3-d period following removal of inserts had conception rates (CR) comparable with the CR of cows and heifers administered PGF2αalone and comparable with those of untreated controls. Pregnancy rates (PR) were significantly greater in the CIDR + PGF2αtreatment group than in the PGF2αonly group after the first 3 d or after 7 d of AI in beef cows and beef heifers, but not in dairy heifers. Estrus after CIDR insert removal was distributed over a 3- to 4-d interval; therefore, this treatment regimen may not be acceptable for use with fixed-time artificial insemination (TAI). The COSynch synchronization of ovulation program [100 μg of gonadotrophinreleasing hormone (GnRH), PGF2α7 d later, and a second injection of 100 μg of GnRH and AI at 48h after PGF2α] has been developed to allow TAI of beef cows. Two studies compared PR to TAI in suckled beef cows following the COSynch program with PR of beef cows on the CO-Synch program with administration of a CIDR insert for the 7 d between the first injection of GnRH and the injection of PGF2α(CO-Synch + CIDR). Mean PR were greater in beef cows receiving the CO-Synch + CIDR treatment than in cows receiving the CO-Synch treatment in one but not both studies. Among cows that were anestrous prior to treatment administration, a greater PR was observed in cows in the CO-Synch + CIDR group than in cows in the COSynch group.

Environmental, genetic and social factors affecting the expression of estrus in beef cows

Genetic, social and environmental factors affecting behavioral estrus were evaluated in Angus ( n=10), Brahman ( n=10) and Senepol ( n=10) cows during a PGF 2α synchronized estrus and subsequent spontaneous estrus. Cows were equally stratified by breed to two groups of 15. Both groups were pre-synchronized with a modified two-injection PGF 2α protocol. At the start of the experiment, cows were treated with 25 mg PGF 2α followed by a second and third administration of 12.5 mg PGF 2α, 11 and 12 days later to induce synchronized estrus. The subsequent estrus was designated as spontaneous estrus. Behavioral estrus data including the onset and end of estrus, estrous duration and the total number of mounts received for the synchronized and spontaneous estruses were collected using HeatWatch ®. Interval from the third PGF 2α treatment to the onset of a HeatWatch ® estrus occurred earlier ( P<0.05) in Angus (31±5 h) than Brahman (53±7 h) or Senepol (53±4 h) cows, with dominant Senepol and Brahman cows taking longer to exhibit estrus after PGF 2α than subordinate cows. The duration of the synchronized estrus tended to be shorter ( P<0.06) in Senepol (12±3 h) than in Angus (19±2 h) or Brahman (17±2 h) cows. Behavioral estrus data between the two periods were confounded by greater temperature–humidity index (THI) values during spontaneous estrus. The THI during spontaneous estrus appeared ( P=0.09) to affect the duration of estrus (9±1 h versus 16±1 h) and did affect ( P<0.0001) the total number of mounts received (8±4 mounts versus 34±4 mounts) during spontaneous estrus compared to synchronized estrus. Breed had no effect ( P>0.10) on the duration and total number of mounts received during synchronized and spontaneous estruses. In conclusion, type of estrus (synchronized or spontaneous), THI, social dominance and breed exerted significant effects on characteristics associated with behavioral estrus in beef cattle in subtropical environments.

The effect of progesterone alone or in combination with estradiol on follicular dynamics, gonadotropin profiles, and estrus in beef cows following calf isolation and restricted suckling.

The effects of calf isolation and restricted suckling on LH pulse characteristics and interval to first ovulation (postpartum interval) were studied in 52 multiparous beef cows, with or without exogenous progesterone. At 30 d postpartum, cows were randomly allocated to one of four treatments (n = 13/treatment): 1) Ad lib, ad libitum access of cows to calves; 2) CI/RS, calf isolation/restricted suckling, where suckling was restricted to once daily; 3) CI/RS+P4, same as CI/RS but cows received an intravaginal progesterone-releasing device at calf isolation for 6 d; or 4) CI/RS+P4+E2, as CI/RS+P4 but the intravaginal progesterone-releasing device had a 10-mg estradiol capsule attached. Daily ovarian scanning and twice-daily blood sampling were performed from d 25 postpartum until the day of second ovulation. A random sample of cows from each treatment (n = 31 in total) were blood-sampled at 15-min intervals for 10 h on d 29, 32, 35, and 38. Ovulatory response to treatment was regarded as ovulation of either the dominant follicle growing at d 30 or the subsequent DF. There was a treatment x day effect (P = .09) on LH pulse frequency, but neither progesterone (CI/ RS+P4) nor progesterone and estradiol (CI/RS+P4+E2) treatment suppressed the calf isolation/restricted suckling-induced increase in LH pulse frequency. The estradiol capsule (CI/RS+P4+E2) delivered sufficient estradiol to delay new follicle wave emergence (treatment x stage; P < .001) and the associated preemergence increase in concentrations of FSH (treatment, P < .05) in cows treated at the postselection stage of follicle wave development, prolonging dominance of the dominant follicle present at treatment initiation (P < .001). The number of cows that ovulated in response to treatment was greater (P < .001) in cows with calf isolation/restricted suckling than in cows suckled ad libitum. Hence, cows assigned to the Ad lib treatment had a longer postpartum interval (P < .001) than cows of the other treatments. Exogenous progesterone treatment increased the frequency of cows exhibiting clinical signs of estrus at first ovulation (P < .001) and reduced the frequency of short estrous cycles (P < .001). We conclude that, in beef cows with calves, a 6-d progesterone treatment does not suppress the calf isolation/restricted suckling-induced increase in LH pulse frequency. Hence, on progesterone withdrawal, the LH pulse frequency is sufficient to stimulate first ovulation, accompanied by overt estrous expression and elimination of a short estrous cycle in most cows.

Factors influencing the initiation of estrous cycles and expression of estrus in beef cows (2000)

The factor most limiting early impregnation of suckled cows is the proportion of cows that are not cycling (anestrous) at the beginning of the breeding season. Continual presence of a suckling calf prolongs anestrus and delays the reinitiation of estrous cycles. Although insufficient energy and protein intake and insufficient body condition at calving are also limiting factors, temporary or permanent weaning of the calf usually initiates estrus within a few days. Younger cows nursing calves generally have more prolonged anestrus because of their additional growth requirement.

Measurement of vaginal temperature by radiotelemetry for the prediction of estrus in beef cows

Peaks in vaginal temperature were assessed as predictors of estrus in 22 suckled beef cows during 2 breeding seasons. A flexible plastic anchor was used to retain a temperature radiotransmitter within the cow vagina and vaginal temperature was monitored at 4-min intervals. Blood was collected twice weekly for analysis of serum progesterone to confirm the occurrence of estrus and ovarian status was checked weekly using transrectal ultrasonography. Visual observations of estrous behavior were made for 20 min/h between 0400 and 0800 h and casually from 0800 to 1600 h (4 to 6 times). Values for vaginal temperature were available for 47 estrous periods. The prediction of estrus based on vaginal temperature was excellent when an estral peak in vaginal temperature was defined as an increase of at least 0.4°C for 3 or more consecutive hours using the corresponding hourly means of a 2 or 3-d baseline. Combining the results of 2 separate years and using a 3-d baseline, a peak in vaginal temperature was found for 42 of 47 confirmed estrus periods (detection sensitivity of 89.4% and prediction power positive of 85.7%). Corresponding detection sensitivity and prediction power positive for visual observations of standing estrus were 53.2 and 96.2%, respectively. The mean maximal increase in vaginal temperature at estrus was 0.9 ± 0.3°C and the mean duration of the estrual peak in vaginal temperature was 6.5 ± 2.7 h. In addition, vaginal temperature was found to be significantly depressed for 3 d prior to estrus and significantly elevated at mid-cycle.

Synchronization of estrus in beef cows with Norgestomet-Alfaprostol or Syncro-Mate B

In the spring of 1986, 506 beef cows were used to evaluate the effectiveness of two estrus synchronization systems. Cows were synchronized with either a 6-mg Norgestomet implant placed in the ear for 14 d followed by a 6-mg Alfaprostol injection given 16 d after implant removal (Norgestomet-Alfaprostol) or with Syncro-Mate B (6-mg Norgestomet implant for 9 d with an injection containing 5 mg estradiol valerate and 3 mg Norgestomet at the time of implantation). The Alfaprostol injection in the Norgestomet-Alfaprostol group was given the same day as implant removal in the Syncro-Mate B group. These treatment groups were compared to a group of untreated controls. Cows were allotted to treatments by days postpartum, age and breed. Syncro-Mate B cows had a higher estrous response within 5 d after treatment (78.6 vs 64.0%) and a shorter interval to estrus (39.2 vs 66.7 h) than did Norgestomet-Alfaprostol cows (P < 0.05). Controls had a significantly lower estrous response compared to either of the synchronized groups (27.1%). The degree of estrus synchrony was identical in both synchronization systems (72.7%). Synchronized conception rate tended to be higher (P = 0.06) in the Norgestomet-Alfaprostol cows than in the Syncro-Mate B cows (74.5 vs 62.5%). Synchronized, 21-d, 25-d and breeding season pregnancy rates were 51.2, 70.8, 76.8 and 92.9% for Norgestomet-Alfaprostol cows; 48.5, 63.0, 73.2 and 87.8% for Syncro-Mate B cows; and 15.6, 56.3, 61.3 and 86.9% for control cows. The four pregnancy rates were not different between the two synchronization treatments (P > 0.10). Controls had lower synchronized and 25-d pregnancy rates when compared to either of the synchronized groups (P < 0.05). Days postpartum had no effect on the reproductive performance of cows synchronized with Norgestomet-Alfaprostol. Our results indicate that the Norgestomet-Alfaprostol system is as effective as Syncro-Mate B in synchronizing estrus in beef cows.

Evidence for hypothalamic dysfunction in suckled beef cows

Multiparous Hereford × Angus postpartum cows were used to determine the effects of treatment with estradiol cypionate (ECP) on estrus and luteal activity (LA) of thin (<5 BCS) cows at 25 or 50 d after calving, and moderate (≥5 BCS) cows at 25 d after calving. Ovaries were examined by ultrasonography at treatment, and plasma progesterone concentrations were used to verify the absence of corpora lutea. Treatment of cows with ECP at 25 d after calving increased (P < 0.01) the incidence of estrus within 4 d after treatment in moderate (55 vs. 11%) and thin (50 vs. 0%) cows compared with control cows. Treatment of thin cows with ECP at 50 d after calving increased (P < 0.01) estrus within 4 d (50 vs. 0%). Treatment with ECP did not influence LA (progesterone >1 ng/mL for 1 d) within 10 d after treatment (P = 0.52). Days to normal LA were greater (P = 0.02) for thin and moderate cows treated with ECP at 25 d after calving (74 ± 5 d) compared with control cows (61 ± 5 d), and ECP treatment tended to increase (P = 0.07) days to LA for thin cows treated at 50 d after calving. Although ECP induced estrus in beef cows during the first 50 d after calving, normal luteal function was not initiated.

Synchronization of estrus in beef cows

We used 79 cows to evaluate Syncromate B (G.D. Searle, Co.) as an estrus-synchronizing agent. Management of five groups of cows was: (1) nonsynchronized, bred naturally; (2) nonsynchronized, bred artificially; (3) synchronized, bred naturally; (4) synchronized, bred artificially at estrus; and (5) synchronized, bred artificially 60 hours after implant was removed. Conception rates were not affected by artificial breeding or synchronization. However, not all cows synchronized showed estrus. Percentages of cows bred during the first 25 days of the breeding season were 72.2, 71.4, 84.6, 68.7, 83.8 for groups 1 through 5, respectively.