ELSEVIER

SYNCHRONIZATION OF ESTRUS AND FERTILITY IN ZEBU BEEF HEIFERS TREATED WITH THREE ESTRUS SYNCHRONIZATION PROTOCOLS

N.J. Corbet, 1'2 R.G. Miller, 1'3 B.M. Bindon, 1'4 H.M. Burrow,l'2 M.J. D'Occhio, 1'5 K.W.

Entwistle, 1'4 L.A. Fitzpatrick, 1'6 J.F. Wilkins 1'7 and J.E. Kinder 1'8

1Cooperative Research Center for the Cattle and Beef Industry (Meat Quality), Box 5545,

Rockhampton, Australia 4702; 2CSIRO Tropical Agriculture, Rockhampton, Australia;

3Queensland Department Primary Industries, Rockhampton; 4University of New England,

Armidale; 5Central Queensland University, Rockhampton; 6james Cook University, Townsville; 7New South Wales Agriculture, Grafton; 8University of Nebraska, NE, Lincoln, USA.

Received for publication: 2 June 1998 Accepted: 17 August 1998

ABSTRACT

The effects on estrus and fertility of 3 estrus synchronization protocols were studied in Brahman beef heifers. In Treatment 1 (PGF protocol; n=234), heifers received 7.5 mg, im prostianol on Day 0 and were inseminated after observed estrus until Day 5. Treatment 2 (10-d NOR protocol; n=220) consisted of norgestomet (NOR; 3 mg, sc implant and 3 mg, im) and estradiol valerate (5 mg, im) treatment on Day -10, NOR implant removal and 400 IU, im PMSG on Day 0, and AI after observed estrus through to Day 5. Treatment 3 (14-d NOR+PGF protocol; n=168) constituted a NOR implant (3mg, sc) on Day -14, NOR implant removal on Day 0, PGF on Day 16, and AI after observed estrus through to Day 21. All heifers were examined for return to estrus at the next cycle and inseminated after observed estrus. The heifers were then exposed to bulls for at least 21 d. During the period of estrus observation (5 d) after treatment, those heifers treated with the PGF protocol had a lower (P<0.01) rate of estrual response (58%) than heifers treated with the 10-d NOR (87%) or 14-d NOR+PGF (88%) protocol. Heifers treated with the 10-d NOR protocol displayed estrus earlier and had a closer synchrony of estrus than heifers treated with either the PGF or the 14-d NOR+PGF protocol. Heifers treated with the 14-d NOR+PGF protocol had higher (P<0.05) conception and calving rates (51 and 46%) to AI at the induced estrus than heifers treated with the PGF (45 and 27%) or the 10-d NOR (38 and 33%) protocol. Calving rate to 2 rounds of AI was greater (P<0.05) for heifers treated with the 14-d NOR-PGF (50%) protocol than heifers treated with the 10-d NOR (38%) but not the PGF (43%) protocol. Breeding season calving rates were similar among the 3 protocols. The results show that the 14-d NOR+PGF estrus synchronization protocol induced a high incidence of estrus with comparatively high fertility in Brahman heifers. © 1999 by Elsevier Science Inc.

Key words: estrus synchronization, progestagen, PGF, dominant follicle, Bos indicus

Acknowledgments: The authors thank P. Howard, P. Fraser and B. Louden of Nogoa Pastoral Co.; G. Fawcett, W. Sim, I. Gray and D. Geissler for assistance with AI programs; and Drs. S.Newman, R. Shepherd and T. Stewart for advice on data analyses.

Theriogenology 51:647-659, 1999 0093-691X/99/$-see front matter © 1999 by Elsevier Science Inc. PII S0093-691 X(99)00003-5

648 Theriogenology

INTRODUCTION

The use of reproductive technologies, in particular artificial insemination (AI), will become increasingly important to meet the growing demand for accelerated genetic improvement of extensively managed beef cattle herds (13 25). The increased use of AI in extensive environments places new challenges on conventional estrus synchronization protocols, and there is a need to develop protocols which are both practical and which result in close synchrony of estrus with high fertility. An additional factor that requires consideration is that extensive beef cattle production, worldwide, occurs predominantly in tropical and subtropical regions. The major genotypes in these zones are Bo_.As indicus or Bos indicus-derived breeds (18, 21, 34). Conventional estrus synchronization protocols were developed for Bos taurus genotypes (23), and it may be necessary to tailor existing protocols to the reproductive physiology of Bos indicus genotypes (11, 20, 40) in order to optimize fertility.

Estrus synchronization treatments evaluated in Bos indicus breeds have included various combinations of progestagens (7, 8, 16, 19, 27, 32, 35, 36) and prostaglandin (2, 6, 24, 27). Treatment with natural progesterone using intravaginal devices, either the PRID (35, 36) or CIDR (27), for durations of 7 to 14 d consistently induced a relatively close synchrony of estrus compared with prostaglandin treatments. Similar results were obtained with the synthetic progestagen, norgestomet, administered subcutaneously for 10 d (7, 16, 27, 32). Conception rates to AI at the synchronized estrus after progestagen treatments in the above studies were variable, with recorded pregnancy rates often below 40% (7, 27). In a proportion of females, a low conception rate following treatment with progestagen has been attributed to ovulation of a persistent dominant follicle and release of an oocyte with a reduced potential for fertilization and developmental competence (3, 28).

Progestagen-based synchronization treatments are acknowledged as the most efficacious for achieving close synchrony of estrus, their use, however, needs to be mitigated to avoid the negative aspects associated with ovulation of a persistent dominant follicle. One such approach involved treatment with prostaglandin 16 d subsequent to the 14-d treatment with norgestomet (29) or melengesterol acetate (MGA; 5, 31). In the latter protocols? stage of the estrous cycle was synchronized by treatment with progestagen, then treatment with prostaglandin in the subsequent cycle led to ovulation of a follicle with normal life-span, and presumably increased fertility (37, 38). This simplistic approach has particular merit and has not been evaluated in Bos indieus heifers.

The main objective in the present study, therefore, was to evaluate the norgestomet- prostaglandin protocol (29) in Brahman ~ indicus) heifers under extensive management in the subtropics. The protocol was compared with conventional 10-d norgestomet (33) and prostaglandin (24) treatments. The 3 treatments were evaluated for the proportion of heifers that exhibited estrus, degree of synchrony of estrus, and the conception and calving rates to AI at the induced estrus. Calving rates to AI at the subsequent estrus, followed by natural mating, were also studied.

Theriogenology 649

MATERIALS AND METHODS

Experimental Animals

Nulliparous Brahman heifers, 2 to 3 yr of age, were used in 3 estrus synchronization experiments over 2 breeding seasons (Experiments 1 and 2, 1994-95 breeding season; Experiment 3, 1995-96 breeding season; Table 1). Experiment 3 comprised heifers that did not conceive in the 1994-95 breeding season. The experiments were conducted in a relatively dry subtropical environment on the eastern seaboard of northern Australia, and within close proximity to the Tropic of Capricorn (latitude 23.30"S, longitude 148.30 E). Yearly recorded rainfall was 300, 330 and 476 mm for the calendar years 1994, 1995 and 1996 respectively. These yearly totals were less than the long-term yearly average of 620 mm representing the worst recorded drought for the region. Heifers were maintained under extensive management except when required for estrus synchronization studies. During the studies, heifers were kept in 100-hectare paddocks and had access to supplementary general forage (Sorghum son.) hay. Three estrus synchronization treatments were replicated in each experiment.

Table 1. Summary of reproductive and live weight status of Brahman heifers used in.lh~ee 3 estrus synchronization experiments

Experiment No. of Experimental Live weight Weight gain Cyclic heifers heifers mating period (range; kg) (kg/day) a (% of total) b

late spring 358 1 275 to (288 to 500) 0.5 75

early fall mid summer 298

2 187 to (254 to 396) 0.7 72 early fall

early summer 426 3 160 to (308 to 576) na c na c

mid fall

a Live weight gain during the mating period. b Percentage of heifers detected with a corpus luteum and/or follicles > 10 mm in diameter at the

start of the experiment using rectal ultrasonography. e Ovarian status and end of breeding season weights were not obtained in Experiment 3.

Live Weight and Ovarian Status

Heifers were randomly allocated by live weight to 1 of 3 estrus synchronization treatments within each experiment. Ovarian ultrasonography was conducted 16 d before the treatments

650 Theriogenology

began in Experiments 1 and 2, using an Aloka 210 real-time linear-array scanner equipped with a 7.5 MHz transducer (Corometrics Medical Systems, Wallingford, CT, USA). Corpora lutea and follicles > 10 mm in diameter were recorded and the presence of either or both structures was assumed to be indicative of cyclic ovarian activity (4).

Estrus Synchronization Treatments

Treatment 1 (PGF protocol). Heifers were administered PGF2ct (7.5 mg, im prostianol; Prosolvin ®, Intervet, Sydney, NSW, Australia) on Day 0 and were observed for estrous behavior until Day 5. Heifers detected in estrus were inseminated 6 to 14 h later. The latter heifers were monitored for estrus from Day 20 to Day 25, and those heifers that returned to estrus were reinseminated 6 to 14 h after the detected estrus. Heifers not identified in estrus during the first period to Day 5 were given a second injection of prostaglandin on Day 16 and were observed for estrus and inseminated accordingly until Day 21. This hormonal protocol is currently used extensively in Australian beef herds.

Treatment 2 (lO-d NOR protocol). Heifers were treated with norgestomet (NOR, 3mg, sc and 3 rag, im) and estradiol valerate (5 mg, ira; Crestar ®, Intervet) on Day -10. The NOR implant was withdrawn on Day 0 when the heifers were treated with PMSG (400 IU, im; Folligon ®, Intervet). Heifers were observed for estrous behavior to Day 5 and were inseminated 6 to 14 h after the detected estrus. All heifers were monitored for estrus at the ensuing estrous period, from Day 20 to Day 25, and inseminated 6 to 14 h after displaying estrus. The manufacturer of Crestar ® and Folligon ® recommended this hormonal protocol for the treatment of beef heifers.

Treatment 3 (14-d NOR+PGF protocol). Each heifer was administered 1 NOR ear-implant (3 mg, so; Intervet) on Day -14 and had the implant removed on Day 0. Heifers were not inseminated during the period when estrus was synchronized after implant removal, but on Day 16 they received PGF2t~ (7.5 mg, im prostianol; Prosolvin ®, Intervet). All heifers were then inseminated 6 to 14 h after estrus was detected from Day 16 to Day 21. The heifers were observed for estrus at the next estrous period, from Day 37 to Day 42, and inseminated 6 to 14 h after displaying estrus. The hormonal protocol used in this treatment was adapted from a novel regimen developed in the USA (29).

Estrus Detection and Natural Mating

Estrus detection relied primarily on activation of Kamar ® Heat-Mount detectors (Kamar Inc., Steamboat Springs, CO, USA; 7) to identify heifers that were sexually receptive to mounting by their contemporaries. Subsequent to two rounds of AI, heifers were randomly allocated within treatment and placed with bulls of a genotype different to that of the AI sire. The heifers remained with the bulls under extensive management until the end of the mating period (at least 21 d).

Pedigree Determination

Sires of calves either by artificial or natural mating were determined by visual appraisal of calf genotype 3 to 5 mo after birth. If the sire of a calf was not readily apparent by this method,

Theriogenology 651

the pedigree was confirmed by blood DNA type. Heifers inseminated at both the synchronized and the retum estrous periods (8% of heifers calved) received semen from the same sire. Although it was logical to assume that these heifers conceived at the return estrus, a check was made by ensuring that the AI date-to-birth date gestation period was nearest to a mean of 285 d (9). This allowed estimation of conception rate and calving rate to AI at the synchronized estrus (Table 2).

Data Analyses

The effect of estrus synchronization protocol was measured on the variables defined in Table 2. Distribution of observed estrus over time during the period of synchronized estrns following treatment was graphed to study the degree of synchrony of estrus elicited by each protocol.

Table 2. Definition of the dependent variables

Variable Definition

Synchronized estrus

Conception rate

Calving rate to first AI

Calving rate to 2 rounds of AI

Calving rate to breeding season

Number of heifers detected in estrus within 5 d after treatment as a percentage of those treated Number of heifers calved as a consequence of conception to AI within 5 d after treatment as a percentage of heifers artificially inseminated during that synchronized period Number of heifers calved as a consequence of conception to AI within 5 d after treatment as a percentage of heifers treated Number of heifers calved as a consequence of conception during two rounds of AI (synchronized AI plus return AI) as a percentage of heifers treated Number of heifers calved as a consequence of conception during the entire breeding season (synchronized AI plus return AI pins natural mating) as a percentage of heifers treated

Data were analyzed using the general linear model procedures described by SAS (42). Experiment (3 levels), treatment (3 levels) and experiment-by-treatment interaction were fitted as fixed effects to all models. Live weight prior to treatment was fitted as a covariate. Live weight gain during the mating period was also fitted as a covariate in a separate analysis of Experiments 1 and 2. Other effects fitted to the model were ovarian status of the heifers prior to treatment (Experiments 1 and 2) and the effects of morning versus afternoon inseminations (all experiments). First order interactions of the fixed effects were fitted to all models and subsequently removed if found not to be significant (P>0.05). Differences between tabulated variable means were tested for significance using Duncan's Multiple Range Test (12).

652 Theriogenology

RESULTS

Experiment and treatment were significant (P<0.05) sources of variation in conception and calving rates. The experiment-by-treatment interaction, however, did not (P>0.05) affect any of the dependent variables. Live weight at the start of mating (all experiments) and weight gain during the mating season (Experiments 1 and 2), fitted as covariates, did not influence (P>0.05) synchronized estrus, conception rate or calving rate. Insemination time (morning vs afternoon) did not affect (P>0.05) subsequent conception or calving rate. Data for the 3 experiments were therefore pooled for analyses, and the results for estrus synchronization treatment effects are reported for pooled data.

Table 3. Effect of treatment on synchronized estrus, conception rate and calving rate in Brahman heifers

Treatment No. of Synchronized Conception Calving Calving Breeding heifers estrus rate to first rate to first rate to 2 season treated (%) AI AI rounds of calving

(%) (%) AI rate (%) (%)

PGF a 234 58 d 45 e 27 d 43 de 71 d

10-d NOR b 220 87 e 33 a 28 d 38 d 67 d

14-d NOR+PGF ¢ 168 88 e 51 e 46 e 50 e 75 d

a PGF = 7.5 mg, im prostianol at Day 0; AI after observed estrus until Day 5; 7.5 mg, im prostianol again at Day 16 if no estrus observed from Day 0 to 5; return to estrus monitored and inseminated from Day 20 to 25.

b 10-d NOR = 3 mg, sc norgestomet implant, 3mg, im norgestomet, 5 rag, im estradiol valerate at Day -10; implant removal, 400 IU, im PMSG at Day 0; AI after observed estrus until Day 5; return to estrus monitored and inseminated from Day 20 to 25.

c 14-d NOR+PGF = 3 mg, sc norgestomet implant at Day -14; implant removal at Day 0; 7.5 mg, im prostianol at Day 16; AI after observed estrus until Day 21; return to estrus monitored and inseminated from Day 37 to 42.

d, e Values in the same column without a common superscript differ (P<0.05).

Estrus Synchronization

The percentage of heifers that displayed estrus during the synchronized period was affected (P<0.01) by treatment and ovarian status (Tables 3 and 4). Both norgestomet treatment protocols (10-d NOR and 14-d NOR+PGF) induced expression of synchronized estrus in more than 80% of

Theriogenology 653

the heifers detected by ovarian ultrasound to be anestrus before treatments commenced (Table 4). This proportion was not different (P>0.05) from that of estrual heifers displaying synchronized estrus in those treatments. The PGF protocol synchronized estrus in a greater (P<0.05) proportion of estrual than anestrual heifers; however, 43% of heifers classified as anestrous prior to synchrony treatment exhibited estrous behavior after treatment with PGF2ct (Table 4).

Table 4. Effect of ovarian status and treatment on synchronized estrus and calving rate in Brahman heifers in Experiments 1 and 2

Treatment/ovarian No. of Synchronized Calving rate to 2 Breeding season status a heifers estrus rounds of AI calving rate

(n) (%) (%) (%)

PGF b

Estrual 128 63 e 50 eg 78 ef

Anestrual 47 43 f 30 f 75 ef

10-d NOR c

Estrual 127 94 g 38 ef 73 ef

Anestrual 43 86 g 45 efg 63 f

14-d NOR+PGF d

Estrual 86 95 g 60 g 83 e

Anestrual 31 80 eg 51 efg 72 Cf

a The presence of either corpora lutea or follicles > 10 mm in diameter was assumed to be indicative of cyclic ovarian activity.

b PGF = 7.5 mg, im prostianol at Day 0; AI after observed estrus until Day 5; 7.5 mg, im prostianol again at Day 16 if no estrus observed from Day 0 to 5; return to estrus monitored and inseminated from Day 20 to 25.

e 10-d NOR = 3 mg, sc norgestomet implant, 3mg, im norgestomet, 5 mg, im estradiol valerate at Day -10; implant removal, 400 IU, im PMSG at Day 0; AI after observed estrus until Day 5; return to estrus monitored and inseminated from Day 20 to 25.

d 14-d NOR+PGF = 3 mg, sc norgestomet implant at Day -14; implant removal at Day 0; 7.5 mg, im prostianol at Day 16; AI after observed estrus until Day 21; returns to estrus monitored and inseminated from Day 37 to 42 .

e, f, g Values in the same column without a common superscript letter differ (P<0.05).

654 Theriogenology

Treatment with the 10-d NOR protocol induced a closer (P<0.05) synchrony of estrus than either PGF or the 14-d NOR+PGF protocol over the synchronized period (144 h post-treatment). The majority of the heifers (95%) that demonstrated estrus after the 10-d NOR protocol did so within a 48 hour period, while the same proportion of heifers treated with PGF or the 14-d NOR+PGF regimen demonstrated estrus over a 4-d period following treatment (Figure 1).

& e.

0-24 25-48 49-72 73-96 97-120 121-144

Hours post treatment

Figure 1. Distribution of observed estrus over time following treatment with PGF, 10-d NOR or 14-d NOR+PGF protocol.

Conception Rate and Calving Rates

Estrus synchronization treatment affected (P<0.01) both conception and calving rate to AI during the synchronized estrus (Table 3). Conception rate to AI at the synchronized estrus for heifers treated with the 14-d NOR+PGF regimen was greater (P<0.05) than for heifers treated with the 10-d NOR or the PGF protocols (Table 3). The combined conception rate to synchronized and return AI for heifers treated with the 14-d NOR+PGF regimen was greater (P<0.05) than for heifers treated with the 10-d NOR scheme, but did not differ (P>0.05) from heifers treated with PGF (Table 3). Calving rate to the breeding season (AI and natural mating) was similar (P>0.05) for all 3 treatments (Table 3).

DISCUSSION

Brahman heifers treated with the 14-d NOR+PGF estrus synchronization protocol had a greater overall calving rate to AI than heifers treated with either the 10-d NOR or PGF protocols.

Theriogenology 655

Treatment with the 14-d NOR+PGF and 10-d NOR regimens resulted in a relatively high proportion of heifers that displayed estrus within 5 d after treatment (5, 29), but there was a higher conception rate amongst the heifers in the 14-d NOR+PGF treatment. The increased calving rate to AI for heifers treated with the 14-d NOR+PGF protocol was primarily due to a greater proportion of these heifers conceiving to the first AI after synchrony treatment, compared with heifers treated with the 10-d NOR or PGF regimen.

Administration of prostaglandin 16 days after synchronization of estrus with norgestomet in the 14-d NOR+PGF regimen would appear to have resulted in the ovulation of oocytes with a relatively high potential for fertilization and embryo developmental competence. It has been shown that the ovaries of most Bos indicus heifers generate 3 waves of follicular development, although some have only 2 waves during the estrous cycle, and that the dominant follicle of the final wave normally ovulates subsequent to spontaneous regression of the corpus luteum (14, 41, 43). The first wave of follicular development is never the ovulatory wave, but the dominant follicle of the first follicular wave has previously been shown to ovulate in response to prostaglandin-induced luteolysis (17, 26) and to produce similar numbers of embryos in response to superstimulatory treatment as the dominant follicle in the second follicular wave (1). The 3 waves of follicular development begin at approximately Day 0, Day 10 and Day 16 after ovulation (1, 41). Since the majority of heifers ovulated (Day 0 of estrous cycle) around Day 2 after treatment (Figure 1), it can be inferred that most heifers in the 14-d NOR+PGF treatment would have had the corpus luteum induced to regress by exogenous prostaglandin administration on approximately Day 14 of the estrous cycle (treatment Day 16). In the 14-d NOR+PGF protocol, the stage of the second follicular wave at the time of prostaglandin administration would have been synchronized among heifers because of prior treatment with NOR. The latter explains the relatively high proportion of heifers that showed estrus within 5 d of prostaglandin treatment in the 14-d NOR+PGF protocol.

Treatment with 10-d NOR regimen was associated with a closer synchrony of estrus immediately after treatment than with 14-d NOR+PGF and PGF treatments, which was consistent with the findings of previous studies (5, 29, 32, 39). However, conception rate to AI at the synchronized estrus was low for heifers treated with the 10-d NOR protocol compared with heifers treated with the PGF or 14-d NOR+PGF protocol. This may partly be due to false estrous behavior evoked in heifers by the estradiol valerate (30) used in the 10-d NOR treatment. The reduced fertility of heifers treated with 10-d NOR protocol was also likely due, in part, to ovulation of a persistent dominant follicle and the release of an ovum with reduced fertilizing potential and low developmental competence (28). Use of the 10-d NOR protocol was therefore dependent on the relative importance of close synchrony of estrus and the desired conception rate.

Heifers treated with the PGF protocol had the lowest proportion of animals showing an estrual response within 5 d after synchrony treatment. This was anticipated since, at the time of prostaglandin treatment, heifers that had initiated estrous cycles would have been at random stages of the cycle, and hence at random phases of follicular waves. Theoretically, a response to prostaglandin is dependent on the presence of a corpus luteum susceptible to the luteolytic actions of prostaglandin. The combination of these factors resulted in a lesser synchrony of estrus with a single prostaglandin treatment than with the other treatments. The apparent relatively low

656 Theriogenology

conception rate to AI at the synchronized estrus for heifers treated with the PGF protocol was, however, unexpected and cannot be explained by the data presented in this study. There is evidence, however, that treatment of anestrous cows, as determined by progesterone assay, with prostaglandin has resulted in estrous behavior in a portion of animals treated (10). There is also evidence that social interaction in large groups of heifers treated to synchronize estrus modified the behavior of anestrual heifers such that they too displayed signs of estrus (15). The animals in the latter two categories are likely to show false or subfertile estrus, thus contributing to lower herd conception rates.

Ovarian ultrasonography provides the opportunity to gauge the general reproductive status of a group of heifers destined for use in an estrus synchronization and AI program. The outcome of ovarian ultrasonography can be an important factor in determining the estrus synchronization protocol that is utilized. For example, if the majority of heifers are undergoing regular estrous cycles (presence of a corpus luteum), then either a progestagen-based or prostaglandin-based estrus synchronization treatment could be used. If only a relatively small proportion of heifers are undergoing regular estrous cycles, then a progestagen-based protocol would be indicated (8, 22). In our present study, the 10-d NOR and 14-d NOR+PGF protocols induced estrous behavior in approximately 80% of heifers that did not have a detectable corpus luteum and the PGF protocol induced estrus in 40% of heifers not detected with a corpus luteum. The behavior of some anestrual heifers may have been modified by the effects of treatment (10, 30) and social interaction (15). It is also likely that a proportion of heifers would have had follicles smaller than 10 mm in diameter but had either started (at Day 0 to Day 5 of the estrous cycle) or were about to start cycling at the time of ovarian examination. It is noted, therefore, that while the absence of ovarian structures (>10 mm) is not an absolute indicator of anestrus, a single ovarian scan could, however, be viewed as a management option for evaluating the reproductive status of a population of heifers, rather than a definitive characterization of individual heifers.

An alternate approach to assessing the general reproductive status of a group of heifers under extensive management is to apply an estrus detection device to each heifer 4 to 6 wk prior to the anticipated start of an estrus synchronization and AI program. Estrus detection beacons used in this and other studies (7) have proved to be particularly useful in herds of Bos indicus heifers, which typically have a relatively short duration of estrus, which often occurs nocturnally and is of low intensity (20, 39).

In summary, the 14-d NOR+PGF estrus synchronization protocol has been shown to result in a combination of close synchrony of estrus and relatively high conception rate to AI in Brahman heifers. A desirable attribute of the 14-d NOR+PGF protocol is that it appears to induce estrous activity in a high proportion of anestrous heifers. These heifers are not bred to the NOR induced estrus, which is likely to be associated with low fertility, but are bred to the estrus subsequent to PGF2e~ which should be of relatively high fertility. The spread of estrous activity over 5 d after treatment with the 14-d NOR+PGF protocol, compared with the closer synchrony for the 10-d NOR regimen, could be an additional desirable feature of the former protocol in extensive environments, since a lower number of heifers would require insemination on each day of the synchronized period.

Theriogenology 657

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