Effectiveness of Select Synch (Gonadotropin-Releasing Hormone and Prostaglandin F2α) for Synchronizing Estrus in Replacement Beef Heifers1

The Professional Animal Scientist 20 (2004):27–33 Synchronizing Heifers with Select Synch

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of Select Synch Effectiveness (Gonadotropin-Releasing Hormone and Prostaglandin F2α) for Synchronizing Estrus in Replacement Beef Heifers1 G. C. LAMB*,2, PAS, J. A. CARTMILL†, and J. S. STEVENSON† *North Central Research and Outreach Center, University of Minnesota, Grand Rapids 557443396; †Department of Animal Sciences and Industry, Kansas State University, Manhattan 66502-0201

Abstract

tions were performed 10 to 14 h after first detected estrus. Pregnancy was Our objective was to determine if a 7- diagnosed by transrectal ultrasonograd, gonadotropin-releasing hormone phy between 33 and 37 d after insemi(GnRH) + prostaglandin F2α (PGF2α) nation. A greater (P<0.05) percentage of protocol (Select Synch) was effective for heifers on the MGA + PGF2α (83%) synchronizing estrus in replacement beef treatment than on the Select Synch (73%) or 2 × PGF2α (71%) treatment heifers compared with two traditionally were detected in estrus during the target used programs. Heifers (n = 637) breeding week. Average interval to estrus consisting of Angus, Hereford, Simmenfrom the second or only injection of tal, and Hereford × Angus breeding at PGF2α was greater (P<0.001) after MGA three Kansas locations were assigned randomly to each of three treatments: 1) + PGF2α (72.1 ± 2.5 h) and 2 × PGF2α (71.3 ± 2.5 h) treatments than after the melengestrol acetate (MGA; 0.5 mg/d per heifer for 14 d) starting on d –31 Select Synch (54.3 ± 2.4 h) treatment. plus 25 mg of PGF2α administered 17 d Overall conception and pregnancy were later (d 0; MGA + PGF2α; n = 301); 2) not different. There were treatment × two 25-mg injections of PGF2α given 14 d location interactions for conception and apart (d –14 and 0; 2 × PGF2α; n = 176); pregnancy rates. All three programs or 3) injection of 100 mg of GnRH (d –7) synchronized estrus, but overall, because followed by 25 mg of PGF2α (d 0; Select more heifers in the MGA + PGF2α treatment were detected in estrus during Synch; n = 160). Twice daily observathe target breeding week, this protocol tions for estrus began on d –5 and continued until at least d 10. Insemina- tended to be most effective. (Key Words: Heifers, Estrus Synchronization, Artificial Insemination.) 1Contribution

no. 03-243-J, Kansas Agric. Exp. Stn., Manhattan, and Minnesota Agric. Exp. Stn., St. Paul.

2To

Introduction

whom correspondence should be adFeeding melengestrol acetate dressed: [email protected] (MGA) for 14 d and administering

prostaglandin F2α (PGF2α) 17 or 19 d later is the most common method for synchronizing estrus in replacement beef heifers (Patterson et al., 1989; Beal, 1998; Lamb et al., 2000). An advantage of using this protocol beyond controlling the onset of estrus is that MGA delivered in the feed has the ability to induce puberty in some peripubertal heifers (Patterson et al., 1992). One drawback is the duration of time needed to apply this protocol (31 to 33 d). In many cases, during late spring and early summer breeding seasons, MGA must be delivered in a grain carrier when cattle tend to be grazing forage pastures. Thus, a challenge exists to ensure that each heifer receives a sufficient dose of MGA to prevent estrus (minimum of 0.5 mg/d per heifer). Therefore, producers may benefit from an alternative estrussynchronization system that is shorter in duration and does not require the use of MGA. Discovery of protocols to control follicular development and time of ovulation have improved control of the estrous cycle. These alternative systems incorporate gonadotropinreleasing hormone (GnRH), which

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induces preovulatory leutienizing hormone surges in prepubertal heifers (Skaggs et al., 1986) and consistently induces ovulation of large ovarian follicles (>10 mm) present at the time of treatment (Thompson et al., 1999; Wood et al., 2001). The Select Synch estrous synchronization protocol (administration of GnRH followed in 7 d by an injection of PGF2α) effectively synchronized and increased pregnancy rates in postpartum cycling or noncycling beef cows (Stevenson et al., 2000b). Therefore, Select Synch may be a viable option for synchronizing estrus in cycling or noncycling replacement beef heifers and an acceptable alternative for synchronization protocols such as MGA + PGF2α or two injections of PGF2α. Our objective was to compare the performance of two standard estrous synchronization protocols (MGA + PGF2α and two injections of PGF2α) with Select Synch in their ability to 1) induce puberty in noncycling heifers; 2) synchronize and alter the distribution of estrus; and 3) influence conception and pregnancy rates.

Materials and Methods Animals and Treatments. Replacement beef heifers (n = 637) were tested at three Kansas locations [CowCalf Unit (CC), Manhattan, KS; n = 64; Losey Land and Cattle Co. (LL), Agra, KS; n = 434; and Purebred Beef Cattle Unit (PB), Manhattan; KS; n = 139). Heifers consisted of Angus, Hereford, Simmental, and Hereford × Angus breeding. Heifers at both Manhattan locations were maintained in dry lots and were fed ad libitum prairie or alfalfa hay and a concentrate consisting of a protein supplement plus vitamins and minerals to meet or exceed NRC (1996) recommendations for growing heifers. At the LL location, heifers were maintained in dry lots and were fed a total mixed diet consisting of a sorghum-silage base. Heifers were assigned randomly within location, but unequally, to

Lamb et al.

each of three treatments (Figure 1): 1) MGA (0.5 mg/d per heifer for 14 d; Pharmacia Animal Health, Kalamazoo, MI) starting on d –31 plus 25 mg of PGF2α (Lutalyse; Pharmacia Animal Health) administered 17 d later (d 0; MGA + PGF2α; n = 289); 2) two 25-mg injections of PGF2α given 14 d apart (d –14 and 0; 2 × PGF2α; n = 139); or 3) injection of 100 mg of GnRH (Fertagyl; Intervet, Millsboro, NJ) on d –7 followed by 25 mg of PGF2α (d 0; Select Synch; n = 160). For the MGA + PGF2α treatment, MGA was added to the protein supplement (Manhattan locations) or combined into the morning half of a daily fed total mixed diet (LL location). Random but unequal allotment of heifers to treatments at the LL location occurred because this was a privately owned enterprise and not all heifers could be evenly distributed among treatments based on pen size constraints. Twice daily observations for estrus began on d –5 and continued until at least d 10. Artificial inseminations were performed 10 to 14 h after the first detected estrus. Pregnancy was diagnosed by transrectal ultrasonography between 33 and 37 d after AI. Clean-up bulls were not introduced

until at least 10 d after the PGF2α injection (d 0) at the CC location, and AI continued for up to 60 d at the remaining locations. Blood Collection and Radioimmunoassay. At the two Manhattan locations, four blood samples were collected (d –41, –31, –7, and 0) to determine concentrations of progesterone in serum. If either of the first two samples contained progesterone ³1 ng/mL (typical of heifers in the luteal phase of the estrous cycle), heifers were assumed to be cycling before d –31. If both of the first two samples contained serum progesterone <1 ng/mL, but had elevated progesterone on d –7, heifers were assumed to have spontaneously initiated puberty (2 × PGF2α and Select Synch treatments) or were induced to be pubertal (MGA + PGF2α treatment). In the Select Synch protocol, if concentrations in samples were <1 ng/mL on d –7 but then elevated (≥1 ng/mL) on d 0, we assumed that a luteal structure had formed in response to the GnRH administered on d –7 (induced ovulation). If concentrations of progesterone were ≥1 ng/mL on d 0, then we assumed that heifers were cycling and responded to treatment,

Figure 1. Experimental design of treatment protocols for replacement beef heifers. Blood (B) samples were collected for later determination of serum concentrations of progesterone. MGA + PGF2α = MGA (0.5 mg/d per heifer for 14 d; Pharmacia Animal Health, Kalamazoo, MI) starting on d –31 plus 25 mg of PGF2α (Lutalyse; Pharmacia Animal Health) administered 17 d later (d 0), 2 × PGF2α = two 25-mg injections of PGF2α given 14 d apart (d –14 and 0), and Select Synch = injection of 100 mg of GnRH (Fertagyl; Intervet, Millsboro, NJ) on d –7 followed by 25 mg of PGF2α.

Synchronizing Heifers with Select Synch

indicative of heifers expected to respond to PGF2α. Concentrations of progesterone in blood serum were determined by radioimmunoassay (Skaggs et al., 1986). The intra- and interassay CVs of six individual assays were 4.6 and 4.5%, respectively. Statistical Analyses. Procedures GLM and CATMOD (SAS, 1999) were used to analyze all categorical data, and the GLM procedure was used to analyze non-categorical data. Preliminary comparisons of rates of detected estrus, conception, and pregnancy, interval from PGF2α to estrus, and concentrations of progesterone were analyzed in a model consisting of treatment, pubertal status, location, and all interactions with treatment. When no location × treatment interactions were detected among the three locations, data were combined. The proportion of heifers ovulating because of GnRH was analyzed in a model including treatment as a fixed effect, location as a random effect, and treatment × location. Breeds, AI sires, and AI technicians were distributed evenly among treatments at each location and not included in the model because they were confounded with location.

Figure 2. Percentages of heifers that had attained puberty (at least one blood sample with concentrations of progesterone ≥1 ng/mL) by d –31, –7, and 0. Select Synch = injection of 100 mg of GnRH (Fertagyl; Intervet, Millsboro, NJ) on d –7 followed by 25 mg of PGF2α (Lutalyse; Pharmacia Animal Health, Kalamazoo, MI), MGA + PGF2α = MGA (0.5 mg/ d per heifer for 14 d; Pharmacia Animal Health) starting on d –31 plus 25 mg of PGF2α administered 17 d later (d 0), and 2 × PGF2α = two 25-mg injections of PGF2α given 14 d apart (d –14 and 0).

+ PGF2α treatments. By d –7 no differences were detected in the percentage of pubertal heifers, although a treatment × day interaction (P<0.001) occurred. By d 0, a tendency (P=0.07) was detected for a Pubertal and Luteal Characteris- greater percentage of MGA + PGF2α tics. Pubertal status at various stages heifers (89%) to have attained puberty than 2 × PGF2α heifers (77%). relative to the second or only injecPubertal status varied (P<0.01) tion of PGF2α is summarized in Figure among locations. At the two Man2. Because blood samples were only hattan locations on d –31, a greater harvested at the Manhattan loca(P<0.001) percentage of CC heifers tions, pubertal status was not deter(84%) had attained puberty than PB mined for the Agra location. Preheifers (55%), but, by d 0, 98% of CC treatment allocation on d –31 heifers were pubertal compared with resulted in initial differences in the 76% of PB heifers. However, on d 0, percentages of heifers that had 91% of CC heifers had elevated attained puberty among groups of progesterone (≥1 ng/mL) compared heifers assigned to treatments. The with 60% of PB heifers. Select Synch treatment resulted in a The percentage of heifers exhibitgreater (P<0.01) percentage (75%) of ing evidence of luteal function (i.e., pubertal heifers than the MGA + progesterone was ≥1.0 ng/mL) on d 0 PGF2α treatment (52%), whereas the 2 was greater (P<0.01) for the MGA + × PGF2α treatment resulted in an treatment (80%) than for the PGF intermediate (65%) percentage, 2α Select Synch treatment (62%), similar to the Select Synch and MGA

Results and Discussion

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whereas the 2 × PGF2α treatment showed intermediate results (67%). The percentage of heifers with luteal function on d 0 represented those that theoretically were capable of responding to an injection of PGF2α. The MGA + PGF2α protocol has been used successfully in beef heifers to induce puberty in some peripubertal beef heifers (Patterson et al., 1992), whereas the 2 × PGF2α protocol is ineffective in inducing puberty in noncycling heifers (Lauderdale et al., 1974). When small doses of GnRH (Sigma L7134) were either continuously infused (1.25 mg/h) or administered as pulses (2.5 mg pulse every 2 h) for a period of 72 h to prepubertal heifers, preovulatory-like leutienizing hormone pulses or surges were elicited, and subsequent rises in progesterone were observed, indicating ovulation had occurred (Skaggs et al., 1986). Thompson et al. (1999) scanned the ovaries of early postpartum, suckled beef cows

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before, during, and after administration of GnRH and(or) norgestomet and reported that luteal structures were induced in 75% of the noncycling cows treated, resulting in elevated progesterone after 7 d. In contrast, Stevenson et al. (2000b) reported that the rates of induced ovulation for noncycling cows treated with Select Synch were 38 and 49% in two experiments. In prepubertal heifers, GnRH induced the ovulation of dominant follicles ≥10 mm and a subsequent corpus luteum resulting from ovulation, thereby initiating estrous cycles in noncycling heifers (Wood et al., 2001; Dahlen et al., 2003). Although heifers in the present study were assigned randomly to treatments on d –31, a greater percentage of heifers assigned to the Select Synch protocol had attained puberty than those assigned to receive MGA + PGF2α. At d –7, no differences in the percentage of pubertal heifers were evident, but, by d 0, more MGA + PGF2α heifers had attained puberty than 2 × PGF2α heifers; Select Synch heifers were intermediate. Estrual Activity. Overall estrus detection rates (Table 1) were influenced by treatment × location (P<0.05) and location (P<0.05). Rates of detected estrus were greater (P<0.01) at the PB location (84%) than at the LL location (76%); the CC location (81%) showed intermediate results. Distribution of estrus relative to the second or only injection of PGF2α is illustrated in Figure 3. Most of the heifers displayed estrus between 24 and 72 h. The peak period of expressed estrus for Select Synch heifers was between 24 and 48 h after PGF2α, whereas the peak period for MGA + PGF2α and 2 × PGF2α heifers was between 48 and 72 h. There was a tendency (P=0.07) for more heifers (11%) on the Select Synch treatment to display estrus before the target breeding week (before d 0) than for those on the MGA + PGF2α treatment (6%), whereas 8% of 2 × PGF2α heifers displayed estrus during that same period. In addition, a greater

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(P<0.05) proportion (83%) of MGA + PGF2α heifers were detected in estrus during the target breeding week (i.e., between d 0 and 7) compared with Select Synch (74%) and 2 × PGF2α (75%) heifers. Percentages of heifers displaying estrus after the target breeding week (after d 7) were 10, 8, and 13% for Select Synch, MGA + PGF2α, and 2 × PGF2α treatments, respectively; percentages of those not displaying estrus were 6, 3, and 8%, for Select Synch, MGA + PGF2α, and 2 × PGF2α treatments, respectively. As a result of a greater percentage of Select Synch heifers displaying estrus before

d 0 and with the peak estrual activity occurring 12 to 24 h earlier than MGA + PGF2α and 2 × PGF2α treatments, the average interval from the second or only injection of PGF2α was greater (P<0.001) for the MGA + PGF2α (72.1 ± 2.5 h) and 2 × PGF2α (71.3 ± 2.5 h) treatments than for the Select Synch (54.3 ± 2.4 h) treatment. For the two Manhattan locations, estrus detection rates were greater (P<0.01) for heifers that had attained puberty by d –31(n = 131; 88%) than for nonpubertal heifers (n = 72; 75%). Of the 72 non-pubertal heifers, 20 had attained puberty by d

TABLE 1. Fertility of heifers synchronized with gonadotropin-releasing hormone (GnRH), melengestrol acetate (MGA), and prostaglandin F2α α (PGF2α α ). Treatmenta Itemb

Select Synch

MGA + PGF2α

2 × PGF2α

no./no. (%) Estrus detection ratec CCd LL PB Overall Conception ratee CC LL PB Overall Pregnancy ratef CC LL PB Overall

20/21 (95) 72/107 (67) 39/48 (81) 131/176 (74)

15/22 (68) 197/236 (83) 38/43 (88) 250/301 (83)

17/21 (81) 63/91 (69) 40/48 (83) 120/160 (75)

12/20 (60) 47/72 (65) 24/39 (62) 83/131 (63)

8/15 (53) 141/197 (72) 21/38 (55) 170/250 (68)

14/17 (82) 46/63 (73) 16/40 (40) 76/120 (63)

12/21 (57) 47/107 (44) 24/48 (50) 83/176 (47)

8/22 (36) 141/236 (60) 21/43 (49) 170/301 (56)

14/21 (67) 46/91 (51) 16/48 (33) 76/160 (48)

Synch = injection of 100 mg of GnRH (Fertagyl; Intervet, Millsboro, NJ) on d –7 followed by 25 mg of PGF2α (Lutalyse; Pharmacia Animal Health, Kalamazoo, MI), MGA + PGF2α = MGA (0.5 mg/d per heifer for 14 d; Pharmacia Animal Health) starting on d –31 plus 25 mg of PGF2α administered 17 d later (d 0), and 2 × PGF2α = two 25-mg injections of PGF2α given 14 d apart (d –14 and 0). bFor heifers detected in estrus and inseminated during the breeding week (d 0 through 7). cPercentage of heifers exhibiting estrus compared with all heifers treated. dLocations: CC = cow-calf unit in Manhattan, KS; LL = Losey Land and Cattle Company in Agra, KS; and PB = Purebred Beef Cattle Unit in Manhattan, KS. ePercentage of heifers pregnant compared with all heifers inseminated artificially. fPercentage of heifers pregnant compared with all heifers treated. aSelect

Synchronizing Heifers with Select Synch

Figure 3. Percentage distribution of estrus relative to the second or only injection of PGF2α in heifers at the Manhattan locations based on twice daily observations for estrus. Select Synch = injection of 100 mg of GnRH (Fertagyl; Intervet, Millsboro, NJ) on d –7 followed by 25 mg of PGF2α (Lutalyse; Pharmacia Animal Health, Kalamazoo, MI), MGA + PGF2α = MGA (0.5 mg/d per heifer for 14 d; Pharmacia Animal Health) starting on d –31 plus 25 mg of PGF2α administered 17 d later (d 0), and 2 × PGF2α = two 25-mg injections of PGF2α given 14 d apart (d –14 and 0).

–7. Nineteen of the 20 pubertal heifers (95%) exhibited estrus, which was greater (P<0.05) than the detection of estrus rate (67%) for nonpubertal heifers. By d 0, an additional 18 of the 52 nonpubertal heifers had attained puberty. Estrus detection rates were greater (P<0.01) for them (17 of 18; 94%) than for the remaining nonpubertal heifers (18 of 34; 53%). On d 0, 141 of 203 heifers at the Manhattan locations had concentrations of progesterone ≥1 ng/mL, which is typical of heifers in the luteal phase of the estrous cycle that should respond to PGF2α. Estrus detection rates for heifers with concentrations of progesterone ≥1 ng/mL (130 of 141; 92%) were greater (P<0.01) than those for heifers that had concentrations of progesterone <1 ng/mL (39 of 62; 63%). Response of ovaries to GnRH is dependent on the stage of follicular growth at the time GnRH is administered (Geary et al., 2000). A high percentage of cows during the latter stages of their estrous cycle (d 15 to 17) failed to ovulate a follicle after

administration of GnRH and exhibited estrus before an injection of PGF2α (Geary et al., 2000). In addition, Moreira et al. (2000) observed that day of the estrous cycle in which the Ovsynch protocol (injection of GnRH 7 d before and 48 h after an injection of PGF2α followed by timed AI 0 to 24 h later) was initiated in dairy heifers affected dynamics of follicular development, plasma progesterone profiles, and occurrence of premature ovulation. The drawback of the Select Synch protocol is that between 5 and 20% of the females exhibit estrus before or immediately after the injection of PGF2α (Kojima et al., 2000). Although not significant, in the current study, more heifers on the Select Synch treatment were observed in estrus before PGF2α than those on the MGA + PGF2α and 2 × PGF2α treatments. As a consequence, the interval to first estrus after the second or only injection of PGF2α was shorter for Select Synch heifers than for MGA + PGF2α and 2 × PGF2α heifers. More heifers might have been in proestrus, estrus, or early metestrus on d 0,

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which is indicative of why more MGA + PGF2α heifers had luteal function at PGF2α than Select Synch heifers. Fertility. Conception (percentage of heifers pregnant to AI compared with the total number of heifers inseminated) and pregnancy (percentage of heifers pregnant to AI compared with all heifers treated) rates did not differ among treatments. However, there was a treatment × location interaction (P<0.05) detected for conception rates and a tendency (P=0.07) for a treatment × location interaction for pregnancy rates (Table 1). Although no differences existed among treatments for conception and pregnancy rates, significant differences were detected among treatments for pubertal status. Despite a 75% estrus detection rate for nonpubertal heifers at the two Manhattan locations, conception and pregnancy rates were affected by the pubertal status of heifers on d – 31. Conception rates were greater (P<0.05) for pubertal heifers (63%) than for noncycling heifers (43%). Similarly, pregnancy rates were greater (P<0.01) for pubertal heifers (55%) than for noncycling heifers (32%). Of the 20 heifers that attained puberty between d –31 and –7, 13 heifers conceived to AI, resulting in a 68% conception rate and a 65% pregnancy rate, which were greater (P<0.05) than the conception (29%) and pregnancy (19%) rates of the 52 noncycling heifers. By d 0, an additional 18 of the 52 noncycling heifers had attained puberty, but conception rates did not differ between pubertal (6 of 17; 35%) and noncycling (4 of 18; 22%) heifers. However, pregnancy rates were greater (P<0.05) in pubertal heifers (6 of 18; 33%) than in noncycling heifers (4 of 34; 12%). At the Manhattan locations, pregnancy rates for heifers that had concentrations of progesterone ≥1 ng/mL on d 0 (75 of 141; 53%) were greater (P<0.01) than those for heifers with concentrations of progesterone <1 ng/mL (20 of 62; 32%). In con-

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trast, conception rates were similar among heifers that had concentrations of progesterone ≥1 ng/mL (75 of 130; 58%) and heifers with concentrations of progesterone <1 ng/mL (20 of 39; 51%). Dairy heifers treated with Select Synch had pregnancy rates similar to those receiving two injections of PGF2α (Stevenson et al., 2000a). In a recent study (Richardson et al., 2002), conception (59%) and pregnancy (49%) rates for beef heifers treated with Select Synch were similar to those in the present study, whereas conception (48%) and pregnancy (35%) rates for dairy heifers seemed to be less. Heifers that were presynchronized with MGA for 7 d before initiation of the Select Synch protocol had conception rates of 57% and pregnancy rates of 46%, which were similar to the conception (61%) and pregnancy (47%) rates of heifers treated with MGA + PGF2α (Funston et al., 2002). No reports have compared Select Synch to 2 × PGF2α in heifers, but, in peripubertal heifers synchronized by Cosynch (a GnRH injection administered 0 d prior to PGF2α followed by a fixed-time AI and a second injection of GnRH 48 h later) or pre-synchronized with an injection of GnRH 6 d before initiation of Cosynch, pregnancy rates were increased to rates comparable with those of heifers synchronized with 2 × PGF2α (Dahlen et al., 2003). In the present study, the treatment × location interaction for fertility illustrates the challenges associated with selecting the most suitable estrous synchronization protocol. Factors such as pubertal status, breed, age, climate, and management (i.e., facilities, nutrition program, methods of feeding) may all impact the response of heifers to certain estrous synchronization protocols. At the LL location, detection of estrus rates for heifers in the MGA + PGF2α treatment were greater than those for 2 × PGF2α and Select Synch heifers. Although conception rates were similar for all treatments at this location, pregnancy rates were greatest for heifers on the MGA + PGF2α treatment. The

Lamb et al.

LL location was the only location that eliminated heifers prior to the breeding season by performing a prebreeding examination in which heifers were culled for poor average daily BW gain, small pelvic area, poor reproductive tract scores, poor disposition, or structural unsoundness. The prebreeding examination at the LL location likely eliminated heifers less likely to conceive in comparison with the heifers at the CC and PB locations. Overall conception rates were greater for LL heifers than for CC and PB heifers. On d 0, 98% of CC heifers had attained puberty compared with 76% of PB heifers, despite the fact that heifers at both locations were of similar ages at the initiation of the breeding season. However, CC heifers were Angus × Hereford crossbreds compared with purebred Simmental, Hereford, and Angus at the PB location. The greater incidence of puberty in CC heifers partially explains the greater conception rates when compared with PB heifers. In addition, heifers assigned to the 2 × PGF2α treatment at the PB location had the lowest conception rates, potentially because of delayed puberty at this location. In most cases Select Synch is an acceptable alternative for synchronizing estrus in suckled cows. In heifers, the challenge arises when GnRH is administered 7 d before PGF2α. Although GnRH is capable of inducing ovulation in follicles ≥10 mm in diameter (Wood et al., 2001; Dahlen et al., 2003), the maximum diameter of dominant follicles in heifers tends to be less than the maximum diameter of dominant follicles in cows. Therefore, the opportunity for GnRH to induce an ovulation in a group of heifers at random stages of the estrous cycle or in prepubertal heifers may be less likely to occur than in cycling or anestrous cows. In the present study, more MGA + PGF2α heifers were detected in estrus during the target breeding week, whereas more Select Synch heifers tended to exhibit estrus before or immediately following the injection

of PGF2α. In addition, overall pregnancy rates were 47, 56, and 48% for Select Synch, MGA + PGF2α, and 2 × PGF2α heifers, respectively. The primary advantages of utilizing Select Synch to synchronize estrus in heifers are the short duration of treatment (7 to 12 d) and that it may induce puberty after injection of GnRH. The MGA + PGF2α protocol tended to result in more heifers that exhibited estrus during the target breeding week. However, the duration of time needed to apply this protocol (31 to 33 d) is a drawback. Although the 2 × PGF2α protocol achieved similar pregnancy rates to Select Synch, the 2 × PGF2α protocol is unable to initiate estrous cycles in prepubertal heifers. Hormones for the MGA + PGF2α protocol (approximately $3.00 to $4.00) are less costly than those for the 2 × PGF2α (approximately $4.00 to $6.00) and Select Synch (approximately $6.00 to $9.00) protocols. However, when selecting an estrous synchronization protocol, additional considerations include duration of the protocol, labor, facilities adequate for detecting estrus and feeding MGA, and administering hormonal treatments before AI.

Implications Treatment of replacement beef heifers with Select Synch was effective in synchronizing estrus and obtaining fertility rates similar to protocols using MGA + PGF2α or 2 × PGF2α. Distribution of estrus was more variable after Select Synch; more heifers exhibited estrus before and immediately after the injection of PGF2α, thus necessitating more detection of estrus to obtain maximal fertility. Because a treatment × location interaction was detected for conception and pregnancy rates, selecting an estrus synchronization protocol requires consideration of pubertal status of heifers, breed, feeding, and animal handling facilities to ensure that the best protocol is selected to achieve optimal pregnancy rates.

Synchronizing Heifers with Select Synch

Acknowledgments We thank Intervet, Inc. (Millsboro, NJ) for donation of Fertagyl and Pharmacia Animal Health (Kalamazoo, MI) for donation of Lutalyse. We also thank Jeff Heldt, Betty Hensley, Samir El-Zarkouny, and Troy Marple for assistance with data collection.

Literature Cited Beal, W. E. 1998. Current estrus synchronization and artificial insemination programs for cattle. J. Anim. Sci. 76 (Suppl. 3):30. Dahlen, C. R., G. C. Lamb, C. M. Zehnder, L. R. Miller, and A. DiCostanzo. 2003. Fixed-time insemination in peripuberal, light-weight replacement beef heifers after estrus synchronization with PGF2α and GnRH. Theriogenology 59:1827. Funston, R. N., R. P. Ansotegui, R. J. Lipsey, and T. W. Geary. 2002. Synchronization of estrus in beef heifers using either melengestrol acetate (MGA)/prostaglandin or MGA/Select Synch. Theriogenology 57:1485.

Geary, T. W., E. R. Downing, J. E. Bruemmer, and J. C. Whittier. 2000. Ovarian and estrous response of suckled beef cows to Select Synch estrous synchronization protocol. Prof. Anim. Sci. 16:1. Kojima, F. N., B. E. Salfen, J. F. Bader, W. A. Ricke, M. C. Lucy, M. F. Smith, and D. J. Patterson. 2000. Development of an estrus synchronization protocol for beef cattle with short term feeding of melengestrol acetate: 711 synch. J. Anim. Sci. 78:2186. Lamb, G. C., D. W. Nix, J. S. Stevenson, and L. R. Corah. 2000. Increasing the interval to prostaglandin F2α from 17 to 19 days in an MGA-prostaglandin synchronization system for beef heifers. Theriogenology 53:691. Lauderdale, J. W., B. E. Seguin, J. N. Stellflug, J. R. Chenault, W. W. Thatcher, C. K. Vincent, and A. F. Loyancano. 1974. Fertility of cattle following PGF2 alpha injection. J. Anim. Sci. 38:964. Moreira, F., R. L. de la Sota, T. Diaz, and W. W. Thatcher. 2000. Effect of day of the estrous cycle at the initiation of a timed artificial insemination protocol on reproductive responses in dairy heifers. J. Anim. Sci. 78:1568. National Research Council. 1996. Nutrient Requirements of Beef Cattle. (7th Ed.). National Academy Press, Washington, DC. Patterson, D. J., G. H. Kiracofem, J. S. Stevenson, and L. R. Corah. 1989. Control of the bovine estrous cycle with melengestrol acetate (MGA): A review. J. Anim. Sci. 67:1895. Patterson, D. J., R. C. Perry, G. H. Kiracofe, R. A. Bellows, R. B. Staigmiller, and L. R. Corah. 1992. Management considerations in heifers development and puberty. J. Anim. Sci. 70:4018.

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Richardson, A. M., B. A. Hensley, T. J. Marple, S. K. Johnson, and J. S. Stevenson. 2002. Characteristics of estrus before and after first insemination and fertility of heifers after synchronized estrus using GnRH, 2 × PGF2α, and progesterone. J. Anim. Sci. 80:2792. SAS. 1999. SAS/STAT® Users' Guide. (Release 8.2). SAS Inst. Inc., Cary, NC. Skaggs, C. L., B. V. Able, and J. S. Stevenson. 1986. Pulsatile or continuous infusion of luteinizing hormone-releasing hormone and hormonal concentrations in prepubertal beef heifers. J. Anim. Sci. 62:1034. Stevenson, J. S., J. F. Smith, and D. E. Hawkins. 2000a. Reproductive outcomes for dairy heifers treated with combinations of prostaglandin F2α, norgestomet, and gonadotropin-releasing hormone. J. Dairy. Sci. 83:2008. Stevenson, J. S., K. E. Thompson, W. L. Forbes, G. C. Lamb, D. M. Grieger, and L. R. Corah. 2000b. Synchronizing estrus and(or) ovulation in beef cows after combinations of GnRH, norgestomet, and prostaglandin F2α with or without timed insemination. J. Anim. Sci. 78:1747. Thompson, K. E., J. S. Stevenson, G. C. Lamb, D. M. Grieger, and C. A. Löest. 1999. Follicular, hormonal, and pregnancy responses of early postpartum suckled beef cows to GnRH, norgestomet, and PGF2α. J. Anim. Sci. 77:1823. Wood, S. L., M. C. Lucy, M. F. Smith, and D. J. Patterson. 2001. Improved synchrony of estrus and ovulation with the addition of GnRH to a melengestrol acetate-prostaglandin F2α synchronization treatment in beef heifers. J. Anim. Sci. 79:2210.