Introduction

The cost of low fertility to the cattle industry was estimated to be around $600 million in 1979 (Gerrits et al., 1979). This is probably an underestimate of the cost of low fertility to the dairy industry today, due to the drastic decline in fertility since 1979. Reproductive failure is second only to mastitis for occurrence of involuntary culling, which stresses the economic importance of getting cows bred in a timely manner.

A “repeat breeder” is typically defined as any cow that has not conceived after three or more services. This syndrome can be one of the more frustrating problems affecting reproductive management of a dairy herd. Commonly, herds with normal conception rates range between 35 and 45% for lactating Holstein cows. This makes the percentage of repeat breeders range from 28 to 17%, respectively (Table 1). As shown in Table 1, the lower the conception rates, the more repeat breeders you have to contend with. As a result, repeat breeders become a significant problem weighting down farm fertility but, more importantly, reducing farm profitability.

Please check this link first if you are interested in organic or specialty dairy production.

Cost of Repeat Breeders

Estimating the cost of repeat breeders varies depending on each farm’s situation. The value of a pregnancy can depend on many factors such as future expected production, age of the cow, current days in milk (DIM), stage of pregnancy, price of milk, cost of average replacement, etc. Two main factors to be considered are milk production and DIM. Generally, the highest percent of repeat breeders will be later in lactation, as one would expect. For example, if the average days to first bred is 65 DIM and the modern dairy cows’ estrous cycles are normally 22 to 24 days in length, the fourth service will occur at approximately 134 days in milk. This, of course, is assuming that you do not miss an estrus, the cow continues with normal interestrus intervals, and the cow is bred off every potential estrus. If dairies are using a full timed artificial insemination program (no heat detection), then this may be even further in lactation, depending on the voluntary waiting period and how quickly they diagnose pregnancy. Most likely, this number is underestimated, and most of the repeat breeders would be much further in lactation. The value of a pregnancy by milk production and DIM is shown in Figure 1. As illustrated, the value of getting a cow pregnant increases the further in lactation a cow becomes, demonstrating the importance of getting repeat breeders bred which are generally later in lactation. However, there is a point where the value will drop drastically (depending on milk production), meaning that at a certain time, it will become more profitable to cull this animal and replace with a heifer.

Figure 1. Value of a new pregnancy during second lactation by days after calving and relative milk yield (low, medium, high) compared with an average lactation curve. Adapted from De Vries et al., 2006.

If the number of repeat breeders on a dairy farm is above 30%, then there may be a significant repeat breeder problem; however, states with hotter climates may have an increased number of repeat breeders during certain times of the year due to heat stress. Diagnosing what the problem is can be difficult because a variety of causes can contribute to a cow being a repeat breeder. Some of these causes can be from a cow/herd problem and (or) management problem.

Problems that can affect both the herd and (or) individual cow, increasing the cow’s chances of becoming a repeat breeder, are as follows: uterine infections (metritis and endometritis); cervical and vaginal infections; infectious diseases due to bacteria, viruses, and protozoa; endocrine and ovulation disorders (cystic ovaries, anovulation, and delayed ovulation); anatomical defects of the reproductive tract and reduced ova quality; and early embryonic or fetal death.

The repeat breeder syndrome can be a problem due to management or a lack thereof. Some possible management factors to consider are as follows: improper timing of insemination (too late, early, already pregnant, etc.), insemination of cows not in estrus, poor compliance with resynchronization programs, inadequate estrus detection, improper semen handling and insemination placement, infertile herd sires, infertile bulls, improper cooling and cow comfort, poor transition and nutrition management, and concurrent disease issues such as mastitis or lameness.

If you were to exclude repeat breeders with anatomical defects, a more normal pregnancy rate could be achieved with a single service. Thus, most repeat breeders are not sterile; rather, they suffer from lowered fertility. Past and even more recent studies have provided possible opportunities to reduce the number of repeat breeders by utilizing reproductive management tools. Below are some possible strategies to reduce the number of repeat breeders.

Possible Solutions to Reduce the Number of Repeat Breeders

Strengthening Your Estrus Detection

Inadequate and inaccurate estrus detection is frequently a cause of cows becoming repeat breeders. Since estrus detection is less than 50% on many dairy farms, there is a substantial need for accurate and efficient heat detection. Utilizing a combination of estrus detection aids to improve both accuracy and number of animals getting inseminated in estrus will improve conception rates (Rorie et al., 2002). For example, some heat detection aids are tail chalk, pedometers, radiotelemetric pressure transducers, records from last heat, palpation for tone of the reproductive tract following some other trigger, and passage of clear vaginal mucus. However, there is still no real cost-effective substitute for the astute observer apart from the bull. There may be a need to “get back to the basics” on heat detection and use visual observations two or three times daily. In addition, the use of PGF2α injections will improve the number of mounts and activity that should improve the number of cows observed in estrus. Using secondary signs is effective in increasing accuracy and number of cows truly in estrus. Research studies have shown that 7 to 20% of the cows bred on detected estrus are not truly in estrus.

Embryo Transfer

The successful transfer of embryos into lactating dairy cattle has shown beneficial effects in improving fertility in dairy cattle, especially during summer heat stress. The transfer of an embryo could bypass certain causes of infertility (i.e., fertilization failure and early embryonic loss). A recent study from Japan investigated the effectiveness of transfer of in vitro produced frozen thawed embryos in establishing pregnancy in repeat breeding Holstein dairy cattle (Dochi et al., 2008). Holstein dairy cattle (n = 532) with at least three previous failed AI attempts were eligible for enrollment. Embryos were transferred 7 or 8 days, after an observed estrus, into one of two groups of animals that were either inseminated or not inseminated with frozen semen. Significantly (P < 0.05) increased pregnancy rates were observed in the embryo transfer group with AI, in comparison to those without AI, in both heifers (49.2 vs. 29.5%) and cows (41.5 vs. 20.4%). In this study, highly acceptable pregnancy rates were obtained in repeat breeder Holstein heifers and cows following the transfer of in vitro produced embryos; therefore, embryo transfer can be used to improve pregnancy rates in repeat breeder dairy cattle.

Administration of GnRH at Insemination

Stevenson et al. (1990) summarized the effects of GnRH given at the time of insemination in repeat breeder cows. A summary of these studies is presented in Table 2, and data acquired from the original publications were subjected to a series of statistical analyses by W. W. Thatcher (1993 Florida Dairy Production Conference Proceedings). Of the studies reported, six had significant beneficial effects of GnRH on conception rates that ranged from 7 to 25%. However, there were no statistically significant effects in seven studies. The difference in conception rates between GnRH and control ranged from -15 to +15%. Across all studies, GnRH increased conception 7.6%; however, a treatment by study interaction was detected, indicating that the effect of GnRH on conception rates was different across studies. Especially during summer months, GnRH given at the time of insemination from an observed estrus improved conception rate compared to cooler months (Ullah et al., 1996; Kaim et al., 2003). Factors contributing to the variability in response to GnRH between studies warrant further investigation but may be a possible strategy to improve fertility in repeat breeders. This could be due to the timing of when GnRH was given in relation to when estrus began, which can pose significant management challenges in timing GnRH injection with estrus and insemination.

Administration of hCG Following AI

Several studies have investigated the effects of human chorionic gonadotropin (hCG) on fertility with little or no effects realized. However, few studies have utilized large number of cows to assess the effectiveness of hCG on conception rates and pregnancy loss of high-yielding dairy cows under field conditions (Eduvie and Seguin, 1982).

Santos et al. (2001) evaluated the effects of hCG administered on day 5 after AI on CL number, plasma progesterone concentration, conception rate, and pregnancy loss in high-producing dairy cows. A total of 406 cows were injected with either hCG or saline on day 5 after AI. Treatment with hCG on day 5 resulted in 86.2% of the cows with more than one CL compared with 23.2% in controls. Plasma progesterone concentrations were increased by 5.0 ng/mL in hCG-treated cows. The increase in the number of CL in the hCG group increased the concentration of progesterone compared to control. Conception rates were also increased (P < 0.01) for hCG-treated cows on day 28 (45.8 vs. 38.7%), day 45 (40.4 vs. 36.3%), and day 90 (38.4 vs. 31.9%) in comparison to control. However, there were no differences between groups for number of pregnancy losses. Therefore, the positive effect of hCG was mediated by reducing early embryonic loss. The benefit of hCG to increase pregnancy rate was most apparent in lactating dairy cows losing BCS between AI and day 28 of pregnancy.

A recent study with embryo transfer recipients detected an increase in pregnancy rate of recipients treated with hCG (Nishigai et al., 2002). Pregnancy rate in cows receiving hCG on day 6 was higher (67.5%) than in control cows (45.0%) or cows receiving hCG on day 1 (42.5%) after estrus. This reinforces that induction of an accessory CL and increased progesterone concentrations reduce early embryonic mortality in cattle. In another study utilizing repeat breeder cows, hCG was given at day 5 post AI, and pregnancy rates and milk progesterone were measured (Kendall et al., 2008). There was a significant (P < 0.05) increase in multiparous (65 vs. 37%) but not primiparous cows (40 vs. 37%). In addition, milk progesterone concentration was increased in hCG versus control cows (34 vs. 11%).

It appears that inducing accessory CL, thereby increasing progesterone, may improve fertility in repeat breeder dairy cows. However, body condition and parity may influence hCG response.

Continued AI vs. Natural Service

A possible management issue that increases the chances of a cow becoming a repeat breeder, or continuing to remain a repeat breeder, may be how extensively a dairy farm utilizes and manages their bull breeding program. Oftentimes, cows are presented to the bull for natural service (NS) at a certain DIM (i.e., 180 DIM) or after a certain number of AI (i.e., > 3 AI). Consequently, many dairy farms across the United States utilize a reproductive program that combines AI and NS (Champagne et al., 2002; NAHMS, 2002; Smith et al., 2004). Many dairymen turn to NS because it bypasses human error such as poor estrous detection accuracy and efficiency, particularly during heat stress situations, and to help inseminate cows during busy times of the year or with heifers. However, by electing to use NS instead of AI, dairymen forgo genetic progress and potential economic gain due to increased milk production. It has been shown that cows sired by proven AI sires produced 1,400 more Kg of herd lifetime actual milk and were $148 more profitable than cows sired by non-AI sires (Cassell et al., 2002). Overton (2005) showed that NS averaged approximately $10 more in cost/cow/year compared to AI. In addition, bulls can impose a risk factor of being dangerous and can carry costly diseases, which can subsequently lower fertility.

Another negative effect of utilizing NS is the fertility problems related to heat stress. Heat stress significantly impairs semen quality when bulls are continually exposed to ambient temperatures of 86ºF for 5 weeks or 100ºF for 2 weeks, despite no apparent effect on libido. Heat stress decreases sperm concentration, lowers sperm motility, and increases percentage of morphologically abnormal sperm in an ejaculate (Ott, 1986). After a period of HS, semen quality does not return to normal for approximately 2 months because of the length of the spermatic cycle, adding to the carry-over effect of HS on reproduction. A possible strategy to bypass the negative effects of NS and reduced estrous expression during the summer is continued timed AI.

If dairy farms use natural service as a part of their reproductive management, they should ensure an intensive bull management program. A recent study in Florida showed that intensively managed bulls can produce similar pregnancy rates to a Presynch/Ovsynch TAI program (Risco et al., 2007).

Resynchronization of Non-Pregnant Cows

Approximately 60% of lactating dairy cows remain not pregnant following first postpartum AI (Chebel et al., 2003; Cerri et al., 2004; Chebel et al., 2006; Galvao et al., 2007). Because heat detection rate is often less than 50%, if cows are not resynchronized following non-pregnancy diagnosis, the interval between inseminations could be at least the length of two estrous cycles, or 44 days, if not longer. Therefore, to avoid extended interval between inseminations and from calving to conception, extended days in milk, and reduced milk yield, it is critical that an efficient protocol for resynchronization and re-insemination of non-pregnant cows is implemented. The most common practice for resynchronization is to initiate the timed AI protocol at non-pregnancy diagnosis; hence, cows diagnosed non-pregnant receive the first GnRH injection of the timed AI protocol immediately at non-pregnant diagnosis and are re-inseminated approximately 10 days later.

Early Resynchronization: Moreira et al. (2000) suggested that injection of GnRH 20 days after AI, 7 days prior to pregnancy diagnosis, could reduce fertility. The study by Moreira et al. (2000), however, was not designed to test the hypothesis that early resynchronization could reduce fertility. Subsequent studies by other researchers (Chebel et al., 2003; Fricke et al., 2003) demonstrated that cows receiving GnRH 7 days prior to pregnancy diagnosis did not have increased pregnancy loss or reduced fertility to the previous AI. Furthermore, following non-pregnancy diagnosis and resynchronized re-insemination, pregnancy per AI (P/AI) was also not affected (Chebel et al., 2003; Fricke et al., 2003). Therefore, treating cows of unknown pregnancy status with GnRH one week before pregnancy diagnosis, to start an early resynchronization protocol, does not affect fertility.

Timing of Resynchronization: Optimal fertility following timed AI protocols is dependent on ovulation in response to the first GnRH injection, which is dependent on the timing of injection in relation to the phase of the estrous cycle. Lactating dairy cows are most likely to ovulate in response to the first GnRH injection when it is given between day 5 and day 9 of the estrous cycle, when a large follicle is present in the ovaries. This assures the recruitment of a new follicular wave and the synchronized ovulation of a fresh oocyte at the end of the timed AI protocol. Lack of ovulation to the first GnRH injection results in prolonged dominance of the ovulatory follicle, ovulation of an aged oocyte at the end of the timed AI protocol, production of poor-quality embryos, and reduced P/AI (Chebel et al., 2006; Cerri et al., 2008).

Fricke et al. (2003) evaluated the fertility of lactating dairy cows of unknown pregnancy status that started the resynchronization protocol at 19 and 26 days after AI, corresponding to day 19 of the initial estrous cycle and day 4 of the new cycle, and of cows diagnosed non-pregnant that started the resynchronization protocol 33 days after AI (day 11 of the new estrous cycle). In this study, cows starting the resynchronization protocol 26 and 33 days after AI had improved P/AI compared with cows starting the resynchronization protocol 19 days after AI.

Considering that the average length of the estrous cycle of lactating dairy cows is approximately 22 days (Sartori et al., 2004), if a cow is inseminated and does not conceive, she is expected to start a new estrous cycle approximately 22 days after AI and, if no re-insemination and conception occur, a new estrous cycle approximately 44 days after AI. Consequently, it is expected that resynchronization protocols beginning at 27 to 31 or 49 to 53 days after initial AI, which correspond to day 5 to day 9 of the second and third estrous cycle following AI, respectively, would result in maximal ovulation in response to the first GnRH injection. However, the true proportion of cows that display estrus 22 days after previous AI is only 15 to 20%, with approximately 45% of cows displaying estrus 20 to 24 days after previous AI, and the remaining cows displaying estrus from 13 to 19 and from 25 to 35 days after previous AI (Chebel et al., 2006). Therefore, it is expected that even when resynchronization is started 27 to 31 or 49 to 53 days after AI, a small proportion of cows would ovulate in response to the first GnRH injection.

Additional Resynchronization Protocols: In order to maximize the proportion of cows that ovulate in response to the first GnRH injection, it is possible to presynchronize non-inseminated cows with prostaglandin (PG) F2α given 11 to 14 days before the initiation of the timed AI protocol (Moreira et al., 2001). It is not possible, however, to treat cows of unknown pregnancy status with PGF2α because of the risk of causing abortion in pregnant cows.

In a recent study, cows diagnosed non-pregnant were presynchronized with PGF2α prior to the start of the resynchronization protocol. In this study, cows were examined for pregnancy at 31 days after AI and, when diagnosed non-pregnant, either started the timed AI protocol at 32 days after AI or were treated with PGF2α on day 34 and started the timed AI 12 days later (46 days after AI). Cows presynchronized with PGF2α before the start of the timed AI protocol had greater P/AI than those starting the resynchronization 32 days after AI (35.2 vs. 25.6%). However, cows presynchronized with PGF2α had an interval between AI 12 days longer, which could counteract the improvement in P/AI.

Recently, our group conducted a research project to evaluate three resynchronization strategies (Dewey et al., 2010). Cows were diagnosed for pregnancy 35 to 43 days after and upon non-pregnancy diagnosis started a timed AI protocol (GnRH injection at non-pregnancy diagnosis, PGF2α 7 days later, and timed AI and GnRH 3 days later). However, one-third of the cows received a GnRH injection one week before pregnancy diagnosis (GGPG), and another one-third of the cows received a CIDR insert from the day of non-pregnancy diagnosis to the day of PGF2α injection. It was hypothesized that by treating cows with GnRH 7 days prior to non-pregnancy diagnosis and start of the timed AI protocol, more cows would ovulate in response to the first GnRH of the timed AI protocol, and that by treating cows with a CIDR insert during the timed AI protocol, better synchrony following the timed AI protocol would be achieved. In this study, cows receiving GnRH 7 days before pregnancy diagnosis were more likely to ovulate in response to the first GnRH injection of the timed AI protocol (number of CL for control = 0.91, GGPG = 1.19, CIDR = 0.93), and cows receiving the GnRH 7 days before pregnancy diagnosis and those receiving a CIDR during the timed AI protocol had greater P/AI (control = 22.1%, GGPG = 31.2%, CIDR = 29.5%).

These studies indicate that presynchronizing non-pregnant cows prior to the start of the resynchronization protocol improves fertility. It is important to take into consideration, however, that there are disadvantages to both strategies. The disadvantage of the presynchronization with PGF2α is that resynchronization is delayed in 12 days, whereas the disadvantage of the presynchronization with GnRH is that some cows may be pregnant and not need the GnRH injection as they will not be resynchronized. Nonetheless, economic analysis of this study demonstrated that by presynchronizing cows with GnRH 7 days before pregnancy diagnosis, greater return per pregnant cow was obtained in 81 different scenarios.

In the United States, CIDR inserts are labeled for use as a resynchronization tool, and cows should receive a CIDR insert from 14 to 21 days after AI. It was hypothesized that during treatment with CIDR inserts, estrus would be inhibited, which would result in greater proportion of cows displaying estrus immediately following CIDR removal. Although this hypothesis was correct and a large proportion of cows treated with CIDR display estrus within 4 days of its removal, in studies in which resynchronization with CIDR was compared with no resynchronization, the overall proportion of untreated cows that displayed estrus from 14 days after AI to pregnancy diagnosis was usually similar to that of CIDR-treated cows (El-Zarkouny and Stevenson, 2004; Chebel et al., 2006; Galvao et al., 2007). Furthermore, some studies reported that the use of CIDR inserts reduced fertility of resynchronized cows, which is probably a consequence of an extended dominance period of ovulatory follicles that may occur in cows with no active corpus luteum (CL) treated with CIDR inserts.

Conclusion

A comprehensive analysis of your reproductive program is key to determining if there is a repeat breeder problem. Strengthening estrus detection programs will reduce the number of repeat breeders; however, due to the challenges of the high-producing dairy cow, further steps should be taken to improve fertility. Implementing reproductive tools can improve fertility in lactating repeat breeder dairy cows.

Author Information

Todd R. Bilby, Ph.D.
Texas A&M AgriLife Extension and Research
trbilby@ag.tamu.edu

References

Bello, N.M., J.P. Steibel, and J.R. Pursley. 2006. Optimizing ovulation to first GnRH improved outcomes to each hormonal injection of Ovsynch in lactating dairy cows. J Dairy Sci. 89:3413-3424.

Cassell, B.G., S.M. Jobst, M.L. McGuillard, and R.E. Person. 2002. Evaluating sire selection practices using lifetime net income functions. J. Dairy Sci. 85:3492-3502.

Cerri, R.L.A., J.E.P. Santos, S.O. Juchem, K.N. Galvão, and R.C. Chebel. 2004. Timed AI with estradiol cypionate or insemination at estrus in lactating dairy cows. J. Dairy Sci. 87:3704-3715.

Cerri, R.L.A. H.M. Rutigliano, R.G.S. Bruno, R.C. Chebel, and J.E.P. Santos. 2008. Effect of artificial insemination (AI) protocol on fertilization and embryo quality in high-producing dairy cows. J. Dairy Sci. 88 (Suppl. 1):73. (Abstr.).

Champagne, J.D., J.H. Kirk, and J.P. Reynolds. 2002. Bull management practices on California dairies: Implications for education and veterinary services. In Proc. 15th Annual Fall Symp. Pp. 15-21.

Chebel, R.C., J.E.P. Santos, S.O. Juchem, R.L.A. Cerri, K.N. Galvao, and W.W. Thatcher. 2003. Effect of resynchronization with GnRH on day 21 after artificial insemination on conception rate and pregnancy loss in lactating dairy cows. Theriogenology 60:1389-1399.

Chebel, R.C., J.E.P. Santos, R.L.A. Cerri, H.M. Rutigliano, and R.G.S. Bruno. 2006. Reproductive performance of lactating dairy cows following progesterone insert based presynchronization and resynchronization protocols. J. Dairy Sci. 89:4205-4219.

De Vries, A. 2006. Economic value of pregnancy in dairy cattle. J. Dairy Sci. 89:3876–3885.

Dochi, O., K. Takahashi, T. Hirai, H. Hayakawa, M. Tanisawa, Y. Yamamoto, and H. Koyama. 2008. The use of embryo transfer to produce pregnancies in repeat-breeding dairy cattle. Theriogenology 69:124-128.

Eduvie, L.O., and B.E. Seguin. 1982. Corpus luteum function and pregnancy rate in lactating dairy cows given human chorionic gonadotropin at the middiestrus. Theriogenology 17:415-422.

El-Zarkouny, S.Z., and J.S. Stevenson. 2004. Resynchronizing estrus with progesterone or progesterone plus estrogen in cows of unknown pregnancy status. J. Dairy Sci. 87:3306-21.

Fricke, P.M. D.Z. Caraviello, K.A. Weigel, and M.L. Welle. 2003. Fertility of dairy cows after resynchronization of ovulation on three intervals following first timed insemination. J. Dairy Sci. 86:3941-3950.

Galvão, K.N., J.E.P. Santos, R.L. Cerri, R.C. Chebel, H.M. Rutigliano, R.G. Bruno, and R.C. Bicalho. 2007. Evaluation of methods of resynchronization for insemination in cows of unknown pregnancy status. J. Dairy Sci. 90:4240-4252.

Kaim, H., A. Bloch, D. Wolfenson, R. Braw-Tal, M. Rosenberg, H. Voet, and Y. Folman. 2003. Effects of GnRH administered to cows at the onset of estrus on timing of ovulation, endocrine responses, and conception. J. Dairy Sci. 86:2012-2021.

Kendall N.R., Flint A.P.F., Mann G.E. 2009. Incidence and treatment of inadequate postovulatory progesterone concentrations in repeat breeder cows. The Veterinary Journal, 181:158-162.

Lucy, M.C. Reproductive loss in high-producing dairy cattle: Where will it end? 2001. J. Dairy Sci. 84:1277-1293.

Melendez, P., G. Gonzalez, E. Aguilar, O. Loera, C. Risco, and L.F. Archbald. 2006. Comparison of two estrus-synchronization protocols and timed artificial insemination in dairy cattle. J. Dairy Sci. 89:4567-4572.

Moreira, F., C. Orlandi, C.A. Risco, R. Mattos, F. Lopes, and W.W. Thatcher. 2001. Effects of presynchronization and bovine somatotropin on pregnancy rates to a timed artificial insemination protocol in lactating dairy cows. J. Dairy Sci. 84:1646-1659.

NAHMS Dairy. 2007. Part I: Reference of dairy cattle health and management practices in the United States, 2007.

NAHMS Dairy. 2002. Part III: Reference of dairy cattle health and health management practices in the United States, 2002.

Nishigai M., Kamomae H., Tanaka T., Kaneda Y. 2002. Improvement of pregnancy rate in Japanese Black cows by administration of hCG to recipients of transferred frozen-thawed embryos. Theriogenology 58:1597-1606.

Overton, M.W. 2005. Cost comparison of natural service sires and artificial insemination for dairy cattle reproductive management. Theriogenology 64:589-602.

Risco, C.A., F. Lima, M.J. Thatcher, and W.W. Thatcher. 2007. Head-to-head comparison of timed AI versus natural service in lactating dairy cows. Proc. Dairy Cattle Reproductive Council. pp. 85.

Roussel, J.D., J.F. Beatty and K. Koonce. 1988. Gonadotropin-releasing hormone therapy in functional infertility of dairy cattle. Theriogenology 30:1115.

Rorie, R. W., T. R. Bilby, and T. D. Lester. 2002. Application of electronic estrus detection technologies to reproductive management of cattle. Theriogenology 57:137-148.

Santos, J.E.P., W.W. Thatcher, L. Pool, and M.W. Overton. 2001. Effect of human chorionic gonadotropin on luteal function and reproductive performance of high-producing lactating Holstein dairy cows. J. Anim. Sci. 79:2881-2894.

Sartori, R., J.M. Haughian, R.D. Shaver, G.J. Rosa, M.C. Wiltbank. 2004. Comparison of ovarian function and circulating steroids in estrous cycles of Holstein heifers and lactating cows. J. Dairy Sci. 87:905-920.

Silva, E., R. Sterry, D. Kolb, M.C. Wiltbank, and P.M. Fricke. 2007. Effect of pretreatment with prostaglandin F2α before initiation of resynchronization of ovulation on fertility of lactating dairy cows. J. Dairy Sci. 90:5509-5517. .

Sirois, J., and J.E. Fortune. 1990. Lengthening the bovine estrous cycle with low levels of exogenous progesterone: A model for studying ovarian follicular dominance. Endocrinology 127:916-925.

Smith, J.W., L.O. Ely, W.D. Gilson, and W.M. Graves. 2004. Effects of artificial insemination versus natural service breeding on production and reproduction parameters in dairy herds. Prof. Anim. Sct. 20:185-190.

Stevenson, J.L., R.C. Chebel, J.C. Dalton, J.E.P. Santos, R. Sartori, and A. Ahmadzadeh. 2008. Effect of synchronization protocols on follicular development and estradiol and progesterone concentrations of dairy heifers. J. Dairy Sci. 91:3045-3056.

Stevenson, J.S., J.R. Pursley, H.A. Garverick, P.M. Fricke, D.J. Kesler, J.S. Ottobre, and M.C. Wiltbank. 2006. Treatment of cycling and noncycling lactating dairy cows with progesterone during Ovsynch. J. Dairy Sci. 89:2567-2578.

Stevenson, J.S., E.P. Call, R.K. Scoby, and A.P. Phatak. 1990. Double insemination and
gonadotropin-releasing hormone treatment of repeat-breeding dairy cattle. J. Dairy Sci. 73:1766.

Thatcher, W.W., and C.A. Risco. 1993. Reproductive management decisions to optimize herd fertility. Proc. Florida Dairy Production Conf. pp. 75-91.

Ullah, G., J.W. Fuquay, T. Keawhoong, B.L. Clark, D.E. Pogue, and E.J. Murphy. 1996. Effect of gonadotrophin-releasing hormone at estrus on subsequent luteal function and fertility in lactating Holstein during heat stress. J. Dairy Sci. 79:1950-1953.