Estrus synchronization in cattle using estradiol, melengestrol acetate and PGF

ELSEVIER

ESTRUS SYNCHRONIZATION

IN CATTLE USING ESTRADIOL, MELENGESTROL ACETATE AND PGF

J. P. Kastelic,lB D. H. McCartney,* W. 0. Olson3 A. D. Barth4 A. Garcia,4 and R. J. Mapletofvl ‘Research Centre, P. 0. Box 3000, L&bridge, Alberta, Canada TIJ 4B1 *Research Centre, Melfort, Saskatchewan, Canada SOE 1AO 3Animal Diseases Research Institute, L&bridge, Alberta, Canada TlJ 3Z4 4University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OWO Received for publication: Accepted:

+*i April

7, 11, 16,

1995 1996

ABSTRACT In Experiment 1, all cattle were fed MGA (0.5 mglheadld) for 7 d (designated Days 0 to 6) and given PGF on Day 6. One-half were administered estradiol valerate (EV; 5 mg, im) on Day 0. At Location 1, a higher proportion (P < 0.005) of EV-treated heifers were detected in estrus and bred by AI between Days 7 and 13 than control heifers not receiving EV (27 of 33 versus 15 of 32), hut the number of pregnancies (12 vs 10) was not significantly different. Eighty-three of 104 EV-treated and 89 of 106 control cows were inseminated, resulting in 50 and 45 pregnancies, respectively (not significant). At Location 2, cattle were similarly treated and exposed to bulls on Days 7 to 49. Fall pregnancy rate was higher (P <0.015) for EVtreated than control heifers (44 of 48 vs 33 of 46), but was not significantly different for cows (22 of 26 vs 19 of 23). In Experiment 2, estradiol 178 (E178; 5 mg, im) and progesterone (100 mg, im) were administered on Day 0 (instead of EV). In a third group (designated the PGF group), cattle were bred on Days 0 to 6, and PGF was administered on Day 6 to those not yet bred. For 213 cows, the percentage pregnant to a synchronized estrus was greater in the PGF group (72%) than in either the control group treated with MGA (49%; P=O.O05) or the group receiving MGA and E17P (54%; PcO.025). Fall pregnancy rates were 91, 89, and %% for the 213 cows (not significant) and 89, 93, and 98% for 131 heifers (not significant) in the PGF, MGA and E17P groups, respectively. In cattle without a functional CL, the average diameter of the hugest follicle at Day 6 was I to 2 mm smaller in the E17P +MGA group than in the MGA group (difference significant only in cows at Location 1). Combined for both locations, the synchronized pregnancy rate in heifers without a functional CL on Day 6 was higher (P < 0.05) in the E17B +MGA group than in the MGA group (11 of 2 1, 52% versus 4 of 20, 20%). Estrogen treatment caused regression of ovarian follicles with emergence of a new follicular wave. Including estrogen in an estrus synchronization program utilizing MGA and PGF significantly increased fall pregnancy rate in heifers (at 1 location) and the synchronized pregnancy rate of heifers without a functional CL at the time of PGF treatment (combined for both locations). Key words: estrus synchronization, estradiol, MGA, follicle, cattle Acknowledgments LRC Contribution No. 3879405. Financial support by the Alberta Agricultural Research Institute (FFF # 94-0536); the National Sciences and Engineering Research Council of Canada (NSERC STR0149604); the Canada/Alberta Livestock Research Trust, Inc.; and Agriculture and Agri-Food Canada, and donations of Estntmate (Coopers Agropharm, Inc.) and Lutalyse (The Upjohn Company) are gratefully acknowledged. We thank R. Reid, B. Krampl, T. Wozniak and their staw, F. Braat, B. Cook, T. Entz, L. Falk, G. Wallins, and R. Wilde for their assistance. Portions of these data were presented at the Western Section, American Society of Animal Science, Lethbridge, Alberta, July, 1995. ‘To whom reprint requests should be addressed.

Theriogenology 46: 1295-l 304. 1996 0 1996 by Elsevier Science Inc.

0093-691x/96/$15.00 PII s0093-691x(96)00301-4

1296

Theriogenobgy INTRODUCTION

Melengestrol acetate (MGA) has been used in various regimens for estrus synchronixation (13,14). When cyclic COWSor heifers were fed MGA for 14 to 18 d, most were detected in estrus within 3 to 7 d after the last feeding, but pregnancy rates were usually 40 to 50% lower than in untreated cattle (14). When MGA was fed for 7 d and PGF was given on the last day, pregnancy rates were lower in cattle that were late in the estrous cycle at the start of MGA treatment compared with those that were treated early in the cycle (4). It has been shown that treatment with MGA in the absence of a functional CL resulted in development of a persistent single large follicle and high plasma estrogen concentrations that were associated with decreased fertility (8,9,11,18). The decreased fertility after long-term MGA treatment or after short-term MGA treatment initiated late in the cycle was temporary; fertility at subsequent estrous periods was normal (14). It appeared the low plasma progestagen concentrations in MGA-treated cattle without a CL resulted in increased LH pulse frequencies but insufficient LH release to cause ovulation (11). Following cessation of MGA treatment, ovulation occurred from an aged oocyte from the persistent follicle (9). It has recently been shown that treatment with estradiol valerate (EV) or estradiol 17-p (E17g) caused follicle atresia, followed by synchronous emergence of a new follicular wave 4 to 5 d later (7). Therefore, . . adnum@ation of estrogen at the start of short-term MGA treatment should cause follicle regression resulting in a growing dominant follicle instead of a large, static, estrogen-producing follicle at the time that MGA is wiUtdrawn (particularly in came without a functional CL). Furthermore, this would be expected to improve synchrony and fertility. The overall objective of this study was to evaluate the use of estradiol in an estrus synchronization program tbr beef cattle in which MGA was fed. Two experiments were conducted. Estradiol valerate @v) was used in the first experiment and estradiol 17-p (E17p) was used in the second. In both experiments, the following hypothesis was tested: In cattle fed MGA for 7 d (Days 0 to 6) with PGF given on Day 6, pregnancy rates will be higher when estradiol is given on Day 0. In the second experiment, the following hypothesis also was tested: In cattle treated with MGA for 7 d (Days 0 to 6), treatment with E17P (and progesterone) on Day 0 will significantly reduce the diameter of the hugest follicle on Day 6 in cattle without a functional CL. Progesterone was given to prevent an E17p-induced LH surge in cattle without a functional CL (6).

h$ATERIALS AND METHODS Experiment 1 This experiment was conducted in the summer of 1992 at 2 locations. At Location 1,275 beef cattle, including 65 yearling heifers (average, 420 d of age) and 210 cows (2 yr of age or older), were used. Approximately one-half of these cattle were straightbred Hereford and the remainder were crossbreds (predominantly Angus and Simmental). Most cows had recently calved and were being suckled. No examinations were conducted to determine whether the heifers or cows were cyclic. However, the heifers were ranked according to bcxiy weight and were assigned, by replicate, to treatment groups. Similarly, cows were ranked according to the interval from parturition to the start of the experiment and were assigned, by replicate, to treatments. Barren cows were randomly allocated across treatments.

Theriogenology

1297

Cattle were group-fed MGAa (0.5 mglheadld) in rolled barley (1 kg/head/d) for 7 d (designated Days 0 to 6) given dinoprost tromethamineb (25 mg im) on Day 6, and assigned to either an EV-treatment or a control group. Heifers averaged 337 and 342 kg in the EV-treated and control groups, respectively, and cows averaged 65 d postpartum in both treatment groups. Gn Day 0, EV-treated cattle were administered EVc (5 mg, dissolved in 2 ml of sesame oil) intramuscularly and those in the control group were given a placebo (2 ml of sesame oil). Cattle were observed at least twice daily for signs of estrus on Days 7 to 13 and those detected in standing estrus were inseminated with frozen-thawed semen approximately 12 h later. On Days 19 to 59, all the cows but only the heifers previously artificially inseminated were exposed to bulls (bull to female ratio of 1:30). Bulls used at Location 1 were classified as having satisfactory breeding potential (2). Palpation per rectum was performed on Day 44 to determine the number of cattle that conceived to Al and again on Day 120 to determine the fall pregnancy rate. Abortions occurred in 1 control and 2 EV-treated heifers and in 2 control cows and 1 EV-treated cow. One pregnant EV-treated heifer and 4 pregnant control cows were sold prior to calving. At Location 2, 143 crossbred beef cattle (Hereford, Charolais and Simmental), including 94 yearling heifers (average 430 d of age) and 49 cows (2 yr of age or older) were used. Most cows had recently calved and were beii suckled. The experimental design was similar to that at Location 1 except that the cattle were administered cloprostenold (500 pg im) on Day 6 and were submquently exposed to bulls on Days 7 to 49 (bull to female ratio 1:30). Bulls used were classified as having satisfactory breeding potential (2) and satisfactory serving capacity (3). Heifers averaged 388 and 392 kg, respectively, in the EV-treated and control groups. At Location 2, cows were ranked by both postpartum interval and bcdy weight prior to assignment to treatment groups. Cows averaged 532 kg and 56 d postpartum in the EV-treated group and 541 kg and 57 d post partum in the control group. Pregnancy diagnosis by palpation per rectum was performed on approximately Day 100 (heifers) or Day 150 (cows). Following pregnancy diagnosis, 21 of 33 pregnant control heifers, 32 of 44 pregnant EV-treated heifers and all pregnant cows were retained for calving while the remainder were sold. One EV-treated cow aborted. The proportion of cattle detected in estrus and bred by AI was defined as the estrus rate. The proportion of cattle artificially inseminated that became pregnant was defined as the synchronized conception rate. The number of cattle that became pregnant to AI as a proportion of the total number of cattle treated was defined as the synchronized pregnancy rate. Chi-square analyses were used to detect differences between the 2 treatments in heifers and cows for the e&us rate, synchronized conception rate, synchronized pregnancy rate and fall pregnancy rate. A Student’s t-test was used to determine if there was a difference between the 2 treatments for the interval from administration of dinoprost to AI (Location l), and for the interval from the start of the experiment to calving (Locations 1 and 2). Experiment 2 This experiment was conducted in the summer of 1993 at the same locations as in Experiment 1. At Location 1, 160 crossbred beef cattle, including 36 yearling heifers (average, 410 d of age) and 124 cows (2 yr of age or older) were used. Most cows had recently calved and were being suckled. The cattle were ranked and assigned to treatment groups as described in Experiment 1.

aMGA 100, The Upjohn Company, Orangeville, ON, Canada. bLutalyse, The Upjohn Company, Orangeville, ON, Canada. Cj3-estradiol 17-valerate, Sigma Chemical Company, St. Louis, MO, USA. dEstrumatc, Coopers Agropharm, Inc., Ajax, ON, Canada.

1298

Theriogenology

There were 3 treatment groups. Cattle in the first group, designated the El78 group, were fed MGA (0.5 mg/head/d) in rolled barley (1 wd) for 7 d (designated Days 0 to 6). On Day 0 they were administered 5 mg E17Be and 100 mg progesteronef (in 2 ml of sesame oil) and were subsequently treated with 508 pg, im cloprostenol on Day 6. Cattle in a second group, designated the MGA group, were treated similarly except they received an intramuscular placebo injection of sesame oil on Day 0. Cattle in a third group, designated the PGF group, received rolled barley (1 kg/head/d) with no MGA treatment on Days 0 to 6. Cattle in this group, not detected in estrus and inseminated by Day 6 were administered cloprostenol(500 pg im) on that day. Heifers averaged 344, 346 and 346 kg in the E178, MGA and PGF treatment groups, respectively. Cows averaged, 51, 51 and 53 d postpartum in these 3 treatment groups, respectively. At Location 1, all heifers (n=36) and 80 of 124 cows were designated to be artificially inseminated at the synchronixed e&us. These cattle were exposed to epididectomixed bulls from Days 0 to 17, observed at least twice daily for signs of estrus, inseminated 12 h after observed signs of es&us, and subsequently exposed to intact bulls from Days 18 to 63 (bull to female ratio 125). The remainder of the cows at Location 1 (n=44) were bred by natural service only. Similar treatments were used at Location 2. At this location, 184 crossbred cattle (95 yearling heifers and 89 cows) were bred by natural service only. Heifers averaged 38 1, 378 and 380 kg in the E178, MGA and PGF groups respectively (average age, 425 d). Cows averaged 551,563 and 553 kg in the E178, MGA and PGF groups, respectively. Most cows were being suckled (average postpartum interval, 55 d for the 3 treatment groups). On Days 0 to 6, cattle in the El78 group and in the MGA group were exposed to epididectomixed bulls, while cattle in the PGF group were exposed to intact bulls with chin-ball markers. On Day 6, all cattle in both the E17B and MGA groups and those in the PGF group that had not been marked were administered cloprastenol(50 pg im). All cattle were exposed to intact bulls from Day 7 to Day 59, at a bull to female ratio of 130, and the cows and heifers were distributed in approximately equal numbers from each of the 3 treatment groups. Tramrectal ultrasonography was performed on Day 44 in all cattle at Location 1. Cattle were designated as pregnant to Al or breeding at a synchronized estrus if a conceptus of approximately 30 to 40 d of gestation was detected. Fall pregnancy rates were determined by palpation per rectum of all cattle on Day 120. Transrectal ultrasonography for detection of pregnancy was performed on Day 89 at Location 2. Cattle were designated as pregnant to breeding at a synchronized estrus if a conceptus of approximately 75 to 85 d of gestation was detected. Pregnancy diagnosis by transrectal palpation was subsequently performed on all cattle in the fall (Day 150). On Day 6, both ovaries of all cattle in Experiment 2 were examined by B-mode transrectal ultrasonography using either a 5.0 MHZ (Location 1) or 7.5 MHZ (Location 2) transducer. The diameter of the largest follicle in each ovary and the presence and size of any luteal tissue were recorded. Luteal tissue area (cross-sectional area of the CL minus the croa+sectionsJ area of any central luteal cavity) was determined as described previously (10). A functional CL was defined as one which had a luteal tissue area of at least 175 mm2.

eB-estradiol, Sigma Chemical Company, St. Louis, MO, USA. fProgesterone,

Sigma Chemical Company, St. Louis, MO, USA.

1299

Theriogenology

Chi-square analyses were used to detect differences among the 3 treatments for estrus rates (Location 1) and for synchronixed conception and pregnancy rates. Cbi-square amtlyses were also used to detect differences among the 3 treatments for synchronized pregnancy rates in cattle that had or did not have a functional CL on Day 6. Analysis of variance was used to determine the effect of treatment on the intervals from administration of cloprostenol to AI (Location 1) and from the start of the experiment to the day of calving at both locations. For heifers and cows at each location, 2-way ANOVA were also used to determine the effect of treatment and CL status on Day 6 (functional vs nonfunctional CL) and the treatment by CL status interaction for the diameter of the largest ovarian follicle on Day 6. Differences among treatments were detected using a least significant difference test (15). RESULTS Experiment 1 The e&us rate was higher (P < 0.005) for EV-treated than control heifers at Location 1, but there was no significant difference between the 2 treatments for the synchronized conception rate or for the synchronized pregnancy rate (Table 1). The fall pregnancy rate was higher (P < 0.015) for EV-treated than for control heifers at Location 2. There was no significant difference between the 2 treatments for the synchronized pregnancy rate or the fall pregnancy rate for cows at either location. The intervals from dinoprost to AI (Location 1) and from the start of the experiment to calving (Locations 1 and 2) were not significantly different between treatments. Table 1. Reproductive performance of cattle synchronized with MGA with or without estradiol valerate (EV; Experiment 1) Heifers cows

Location 1 No. treated Estrus rate ( %)a Dinoprost to AI(days) Synchronixed conception rate (%) Synchronized pregnancy rate ( W) Fall pregnancy rate ( %)b Calving n DayC Location 2 No. treated Fall pregnancy rate ( %)d Calving n

Control

EV-treated

Control

EV-treated

32 47 3.0 f 0.2 67 31 41

33 82 2.8 + 0.2 44 36 61

106 84 3.8 f 0.1 51 42 91

104 80 3.9 f 0.1. 60 48 88

90 303.3 f 1.6

91 301.8 f 1.5

12 297.0 f 4.7 46 72

17 300.0 f 4.2 48 92

23 83

26 85

21 32 19 21 307.1 f 2.8 301.9 f 2.3 309.2 f 2.9 316.4 f 2.8 aiikerence between control and EV-treated heifers (P< 0.00s). bGnly heifers detected in estrus and artificially inseminated were subsequently exposed to bulls. CDay 0 was the tirst day of the experiment. dDifference between control and EV-treated heifers (P < 0.015). Y

1300

Theriogenology

Experiment 2 At Location 1, estrus rates were very high, especially for cows (Table 2). In the PGF group, 5 cows (19%) and 3 heifers (25%) were detected in estrus and inseminated by Day 6 and therefore were not treated with cloprostenol. The synchronized conception rate and synchronized pregnancy rate were both higher (PCO.05) in cows in the PGF group than in the MGA group. There were no significant differences among treatments for estrus rates or fall pregnancy rates at Location 1. For the interval from administration of cloprostenol to AI, there was a difference among treatments for cows (P=O.Ol; interval shorter in the PGF group than the other 2 groups) but not for heifers (P > 0.2). In the 44 cows designated to be bred by natural service only at Location 1, three in the PGF group (20%) had been bred by Day 6. In the PGF, MGA, and E17P groups, the proportions of cows pregnant to a synchronized estrus were 10 of 15 (67%), 9 of 14 (64%) and 7 of 15 (47%), while the fall pregnancy rates were 87, 80 and 87%, respectively. At Location 2,6 heifers (19%) and 5 cows (18%) in the PGF group had been mated by Day 6 and were not treated with cloprostenol. The synchronized pregnancy rate was higher (P < 0.01) for the PGF group than for the MGA group (Table 2). There also was a difference (P=O.O2) among treatments for the interval from the start of the experiment to the mean day of calving in cows at Location 2 (cows in the PGF group calved the earliest). Table 2. Reproductive performance of cattle synchronized with PGF alone or with melengestrol acetate or MGA plus estradiol 17p (E17p; Experiment 2) cows Heifers PGF

MGA

E17P

PGF

Location 1 12 26 No. treated 12 12 83 96 Estrus rate ( W) 67 83 Cloprostenol to Al (days) 4.4 f 1.8 3.1 f 1.0 1.9 f 0.2 2.4 f 0.2a Synchronized 84a 80 conception rate (%) 88 60 Synchronized 81a 67 pregnancy rate (%) 58 50 96 100 Fall pregnancy rate (%) 92 100 Calving 23 n 299 f 3.2 DayC Location 2 28 31 No. treated 32 32 Synchronized 64a 39 pregnancy rate (%) 47 25 89 97 Fall pregnancy rate (56) 88 91 Calving 18 25 n 13 16 305f4.3 305*3.1 311f3.8 303f3.1” Y a&thin a row, means without a common superscript are different (P < 0.05). CDay 0 was the first day of the experiment.

MGA

E17P

27 100

27 %

3.0 f 0.2b

3.3 f 0.2b

59b

73ab

59b 93

7oab 93

24 306 f 4.3 31 29b 87 25 313 f 2.gb

24 302 f 3.6 30 43ab 100 30 314 f 3.1b

1301

Therbgenology

Combined for both locations, synchronixed pregnancy rate was greater for the PGF group than for the MGA group (P=O.O05) and for the E17P group (P <0.025; Table 3). In heifers without a functional CL on Day 6, the synchronized pregnancy rate was higher (P
Reproductive performance of cattle combined for Locations 1 and 2 (Experiment 2) Heifers cowsa PGF

MGA

El7P

PGF

MGA

E17P

No. treated

44

44

43

69

72

72

Synchronized pregnancy rate (%) Overall CL presentd CL absent

50 58 4obc

32 42 20b

49 41 52c

72b 74b 67

49c 52c 46

54c 65bc 45

Fall pregnancy rate (W) 89 98 91 89 93 aIncludes 44 cows not designated to be inseminated at Location 1. bcWithin a row, means without a common superscript are different (P <0.05). dcL present indicates the presence of a functional CL at the time of PGF treatment on Day 6. Table 4.

Treatment

96

Least-square means ( f SE) for diameter of the largest follicle on Day 6 in cattle with or without a functional CL (designated + and -, respectively) at the time of PGF treatment on Day 6 (Experiment 2) Location 1 Location 2 CL

Heifers (n=36)

Cows (n= 124)

Heifers (n=95)

Cows (n=89)

PGF 10.5 zt 0.6 12.6 f 0.6b 9.7 f oga 11.4 f 0.8a PGF + 11.0 f 2.0 11.5 * 0.4ab 10.9 f 0.4ab 12.7 f 0.5ab MGA 11.3 f 0.6 11.6 f 0.6b 11.7 f 0.6ab 13.7 f 0.6b MGA + 9.7 f 1.1 10.0 f 0.6a 11.7 + 0.4bc 11.7 f 0.6a El7P 9.1 f 0.6 9.7 f 0.5a 10.4 f 0.6ac 12.6 f 0.6ab El7P 10.5 f 1.4 9.6 f 0.6a 10.4 f 0.4a 11.6 f 0.6a abcWithin a coluk, means without a common superscript are different (P < 0.05). The presence or absence of a fimctional CL on Day 6 is coded as + and -, respectively. There was an interaction of treatment-by-CL status (P
1302

Theriogenology DISCUSSION

Although a higher proportion of EV-treated heifers than control heifers were inseminated in Experiment 1, the synchronized conception rate and synchronized pregnancy rate were not significantly different. However, the fall pregnancy rate was significantly higher for EV-treated heifers than for controls at Location 2 (92 vs 72%) and tended (P=O. 1) to be higher at Location 1 (61 vs 41%). By design, only the artificially inseminated heifers were subsequently exposed to bulls at Location 1, which reduced the fall pregnancy rates in that group of heifers. In recent studies of progestagen-treated heifers, a new follicular wave emerged, on average, 4.3 d after treatment with 5 mg El78 (7). Therefore, treatment with a combination of El78 and progestagen has considerable promise for causing follicle atresia and synchronizing follicular development for both estrus synchronization and superovulation. However, 5 of 6 heifers given only El78 (without exogenous progestagen) 1 d after ovulation had an estrogen-induced LH surge which was not associated with follicle suppression (6). Therefore, exogenous progestagen should be given along with the estrogen (to prevent an LH surge in cattle without a functional CL). Treatment with 5 mg EV also resulted in follicle atresia (S), but the interval from treatment to emergence of the subsequent follicular wave seemed to be more variable than with E178. These ditTerences were considered to be caused by incomplete suppression of the dominant follicle and a prolonged effect of EV (5). Plasma estradiol concentrations were elevated for approximately 5 d after treatment with EV (5) but for only 1.75 d after treatment with El78 (6). Although the 2 different estrogens were used on successive years and were not compared directly in the present experiments, differences in pregnancy rates between EV- and El78-treated cattle were not apparent. Others have given GnRH 6 or 7 d prior to PGF to induce ovulation and/or follicle atresia (16,17). Although this also holds promise for synchronizing follicle development, the effects of GnBH treatment at various stages of follicle growth apparently have not been critically evaluated. In cattle without a functional CL on Day 6 in Experiment 2, synchronized pregnancy rates were significantly higher in the El78 group than in the MGA group for heifers but not for cows. In previous studies, when cattle were synchronized by feeding MGA for several days with PGF given on the last day, pregnancy rates were decreased in cattle that did not have a functional CL at that time (4). Possible reasons for lower pregnancy rates include fertilization of aged ova and/or high plasma estrogen concentrations which may affect uterine contractions, sperm capacitation, transport of spermatozoa or ova, and embryo cleavage (4). When there was a functional CL present on Day 6 in Experiment 2, synchronized pregnancy rates were similar between the El78 group and the MGA group for heifers and cows. Therefore, pregnancy rates were similar or better in the El78 group versus the MGA group, regardless of CL status. These results provide some support for the hypothesis that estradiol treatment on Day 0 improves pregnancy rates in cattle synchronized with MGA for 7 d with PGF administered on the last day. However, this hypothesis was not consistently supported. A large follicle has been reported to develop in cattle with prolonged low plasma progesterone concentrations (1) and in cattle treated with MGA in the absence of a functional CL (8,9,18). In a recent study (9), the dominant follicle was maintained in 10 of 23 cattle treated with MGA for 7 d starting on Day 17 of the estrous cycle. In cattle without a functional CL on Day 6 (Experiment 2), the average diameter of the largest follicle was 1 to 2 mm smaller in the El78 group than in the MGA group (results were significant only for cows at Location 1). Therefore, there was some support for the hypothesis that treatment with El78 at the beginning of MGA treatment would result in a smaller dominant follicle on Day 6 in cattle without a timctional CL. We suggest that El78 treatment of MGA-treated cattle results in follicle atresia and emergence of a new follicular wave 4 or 5 d later, as previously reported (7). As a result, the largest follicle on Day 6 would be the growing dominant follicle of the new wave and this follicle would be expected to ovulate

Theriogenology

1303

approximately 4 d later (i.e. 6 or 7 d after wave emergence). However, in MGA-treated cattle without a functional CL, ovulation would occur from the large follicle which bad been maintained, resulting in reduced fertility. The decreased fertility in cattle after long-term MGA treatment or after short-term MGA treatment initiated late in the estrous cycle has been shown to be temporary; fertility at subsequent estrous periods was normal (14). To avoid this decreased fertility, MGA has been fed for 14 d, with PGF administered 17 d after withdrawal of MGA. In one study utilizing this approach, 76% of heifer8 were detected in e&us and 65% of those inseminated became pregnant (12). This treatment re8uh8 in a well-synchronized, fertile eztrus, but the long interval (approximately 5 wk) from the initiation of treatment to breeding may be impractical for many beef producers. In Experiment 2, cattle in the PGF group had the highest rate of fertility. However, inherent in the design of the experiment, cattle in this group were bred over approximately twice the interval used for the 2 groups receiving MGA, making it less convenient and more labor intensive, In summary, inclusion of estrogen in an eztruz synchronization program utilizing MGA and PGF significantly increased the fall pregnancy rates of heifers (at 1 location), and it synchronized the pregnancy rates of heifer8 lacking a functional CL at the time of PGF treabnent (combined for 2 locations). In cattle without a functional CL on Day 6, the average diameter of the largest follicle at that time was 1 to 2 mm smaller in the El’IP+MGA group than in the MGA group (difference significant only in cows at 1 location). Ufilii El7P (and progesterone) on the first day of a 7-d MGA regimen (with PGF administered on the last day) may improve estrus synchronization. The costs are modest (a single dose of PGF and minimal cozt for MGA and steroids), and most of the cattle were in estrus during a 3-d interval in our study (approximately 10 d after the start of the treatment). The fertility rate was also acceptably high. REFERENCES 1. 2. 3. 4. 5.

6.

7. 8.

9.

10.

Adams GP, Matteri RL Gmther OJ. Effect of progesterone on ovarian follicles, emergence of follicular waves and circulating follicle-stimulating hormone in heifers. J Reprod Fertil 1992;%:627-640. Ball L, Ott RS, Mortimer, RG, Simon8 JC. Manual for breeding soundne88 examination of bulls. J Sot Theriogenology Vol XII, 1983;65. Barth AD. Bull breeding soundness evaluation. Western Canadian Association of Bovine Practitioners 1994;3-5. Beal WE, Chenault JR, Day ML Corah LR. Variation in conception rates following synchronization of estrus with melengesfrol acetate and prostaglandin F,cx. J Anim Sci 1988;66:599-602. Bo GA, Nasser LF, Adams GP, Pierson RA, Maple&t RJ. Effect of estradiol valerate on ovarian follicles, emergence of follicular waves, and circulating gonadotropin8 in heifers. Theriogenology 1993;40:225-239. Bo GA, Adams GP, Pierson RA, Tribulo HE, Caccia M, Mapletoft RJ . Follicular wave dynamics after estradiol-17fi treatment of heifer8 with or without a progestagen implant. Theriogenology 1994;41:1555-1569. Bo GA, Adam8 GP, Pierson RA, Mapletoft RI. Exogenous control of follicular wave emergence in cattle. Theriogenology 1995;43:31-40. Coleman DA, Bartol FF, Riddell MG. Effects of a 2l-day treatment with melengestrol acetate (MGA) with or without subsequent prostaglandin F,a on synchronization of estrus and fertility in beef cattle. J Anim Sci 1990;68:3300-3305. Custer EE, Beal WE, Wilson SJ, Meadows AW, Berardinelli JG, Adair R. Effect of melengestrol acetate (MGA) or progesterone-releasing intravaginal device (PRJD) on follicular development, concentration8 of estradiol-17a and progesterone and luteinizing hormone release during an artificially lengthened bovine estrous cycle. J Anim Sci 1994;72: 1282-1289. Kastelic JP, Bergfelt DR, Gmther OJ. Relationship between ultrasonic assessment of the corpus luteum and plasma progesterone concentration. Tberiogenology 1990;33: 1269-1278.

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77iebgenology Kojhna FN, Chenault JR, Wehrman ME, Bergfeld EG, Cupp AS, Werth LA, Mariscal V, Sanchez T, Kittok RJ, Kinder JE. Melengestrol acetate at greater doses than typically used for estrous synchrony in bovine females does not mimic endogenous progesterone in regulation of secretion of luteinizing hormone and 178Estradiol. Biol Reprod 1995;52:455-463. Mauck HS, Holland MD, King ME, J&Fever DG, Gdde KG. Melengestrol acetate and PGF,a combinations for synchronization in beef heifers. J Anim Sci 1988;66(Supp. 1):2OO. Gdde KG. A review of synchronization of estrus in postpartum cattle. J Anim Sci 1990;68:817-830. Patterson DJ, Kiracofe GH, Stevenson JS, Corah LR. Control of the bovine estrous cycle with melengestrol acetate (MGA): A review. J Anim Sci 1989;67: 18951906. SAS/STAT User’s Guide, Version 6, Volume 1. SAS Institute, Cary NC, 1990. Thatcher WW, Drost M, Savio JD, MacMillan KL, Entwisde KW, Schmitt El, De La Sota RL, Morris GR. New clinical uses of GnRH and its analogues in cattle. Anim Reprod Sci 1993;33:2749. Twagiramunga H, Guilbauh LA, ProuIx J, Villeneuve P, Dufour JJ. Intluence of an agonist of gonadotropin-releasing hormone (buserehn) on e-s&ussynchronization and fertility in beef cows. J Anim Sci 1992;70: 1904-1910. Zimbelman RG, Smith LW. Control of ovulation in cattle with melengestrol acetate. II. Effects on follicular size and activity. J Reprod FertiJ 1966; 11: 193-201.