INTRODUCTION Estrous synchronization has become a powerful tool in managing breeding seasons to compliment niche markets. Synchronization of estrus, along with the use of AI, has become a popular technology that can introduce new sire genetics along with control of the breeding and calving season. Cattle producers have long searched for methods to efficiently and effectively synchronize females for artificial insemination without compromising conception or pregnancy rates versus conventional natural service breeding. Over the past 40 years, research scientists have developed and tested many synchronization protocols to synchronize estrus and ovulation in beef and dairy cattle with a goal to consistently produce acceptable pregnancy rates. Success in meeting these goals has been limited. Current approaches to synchronization have included the use of progestins (MGA & CIDR-B), prostaglandins (PGF2_), and gonadotropin-releasing hormones (GnRH; Table 1). PROSTAGLANDINS, PROGESTINS, AND GONADOTROPINS Over the years, scientist and pharmaceutical companies have worked together to evaluate different products that would induce an animal to come into heat and ovulate. Researchers have taken these products and developed an extensive program for timing and use of these products capable of being producer friendly. Prostaglandin F2_ (PG) and analogues were developed to induce luteolysis, demise of the corpus luteum, for synchronizing estrus in cattle. Acceptable pregnancy results have been reported, yet the use of PG alone could not cause luteolysis in cattle during certain stages of the estrous cycle or cause non-cyclic animals to become cyclic (Table 2 & Table 4). Progestins became popular, in collaboration with PG s, to alter the estrous cycle and induce cyclicity in non-cycling cattle in order to have more control over synchronizing standing estrus. Progestins, such as MGA, have been used to suppress estrus in cycling cattle to allow for a narrow window for AI (Table 3). Gonadotropins (GnRH) were later introduced to control the effects of PG on the estrous cycle during any stage and create a tighter synchrony for ovulation (Table 2 & Table 3). The use of timed-inseminations increased with incorporation of GnRH into synchronization protocols (Table 2), yet early heats were not controlled. Some of the more recent synchronization protocols combine PG's, GnRH, and progestins to reduce the amount of time needed for detecting estrus and eliminate early heats (Table 3). Although the use of MGA in many synchronization protocols has dramatically improved estrous synchrony and pregnancy rates, a smaller variance in ovulation time within a group of animals has been reported by incorporating a controlled intravaginal drug release (CIDR-B). One of the first studies using CIDR inserts for initiation of FDA approval was done by Lucy et al (2001). CIDR inserts were left in for 7 days with PG given on day 6. They reported an increase in estrous synchrony within the first 3 days after CIDR removal for beef cows [CIDR (59 %) vs PG (45 %) vs control (15 %)] and beef heifers [CIDR (65 %) vs PG (27 %) vs control (13 %)]. It appeared that using CIDR inserts improved estrous synchrony, creating a tighter window of standing heats. This improved synchrony may provide opportunities to incorporate timed-insemination programs into herds to try and benefit from the advantages offered from synchronization and artificial insemination. The use of CIDR inserts was approved by the Food and Drug Administration for beef cows and beef & dairy heifers in the summer of 2002, and dairy cows in July of 2003. Richardson et al., (2002) reported a higher and tighter estrous response and higher conception rate, in dairy and beef heifers combined, when GnRH was given or not given at day 1 of a 7 day CIDR insertion protocol with PG on day 6 (ER = 84.1 & 87.1 %; CR = 58.2 & 58.6 %) vs a Select Synch protocol (ER = 77.7 %; CR = 53 %). By moving the PG injection from day 6 to day 7 (at CIDR removal), Lamb et al., (2001) showed that pregnancy rates were higher in suckled beef cows that received a CIDR + CO-Synch protocol (58 %) vs a Co-Synch protocol (48 %) alone. Estrous response and pregnancy rates in beef heifers were higher in a modified CIDR + Co-Synch protocol (using GnRH with PG given at day 7; ER = 65.0 % and PR = 65.0 %) vs a 6 d MGA feeding on top of a Co-Synch protocol using GnRH (ER = 35.6 % and PR = 52.5 %) with peak estrous response for both MGA and CIDR groups ranging from 36 to 48 h post CIDR removal (Martinez et al., 2002). If estrous response is highest within 36 48 h post CIDR removal, possibly due to incorporation of GnRH at CIDR insertion and PG can be given at CIDR removal, then delaying timedinsemination may also result in higher pregnancy rates. COLORADO STATE UNIVERSITY CIDR STUDY Heifer Study: A study conducted by Colorado State University involved timing ovulation for delayed fixed-time AI in beef heifers using a modified Co-Synch + CIDR protocol (Figure 1) with two research herds in Colorado and Wyoming and one cooperator herd in South Dakota. A total of 375 nulliparous crossbred beef heifers were synchronized and blocked by BCS, weight, and AI technician and randomly assigned to one of two treatment groups. Treatment 1 heifers were timed-inseminated at 54 h post CIDR removal and treatment 2 heifers were timed-inseminated at 54 h post CIDR removal with a second injection of GnRH at breeding. All heifers were diagnosed for pregnancy to AI via transrectal ultrasonography 45 d post insemination. Results from the modified Co-Synch + CIDR protocol with delayed fixed-time AI are depicted in Table 5. Calf Removal Study: A second study at Colorado State University involved the effects of calf removal on estrous response and pregnancy to AI in suckling multiparous beef cows at two research herds in Colorado and Wyoming. Angus and Red Angus crossbred beef cows (n = 583) were all synchronized for estrus using a modified Hybrid Synch + CIDR protocol (Figure 2) and blocked by BCS, weight, and cyclicity status and randomly assigned to one of two treatments groups. Treatment 1 consisted of no calf removal and treatment 2 consisted of a 54 h calf removal beginning at PG injection. Calves remained separated until their dams were inseminated. Cows were visually observed for estrus 1 hour at dawn, noon, and dusk for signs of standing heat beginning at time of PG injection and continuing for 36 h. Cows that were detected in standing estrus were artificially inseminated 12 h later, and cows not detected for standing estrus were timed-inseminated 54 h post PG injection. All cows were diagnosed for pregnancy to AI via transrectal ultrasonography 45 d post insemination. Results from the modified Hybrid Synch + CIDR protocol with calf removal are depicted in Table 6. DISCUSSION As the diversity of genetics broaden and the interest in estrous synchronization and use of artificial insemination remains constant within beef herds across the US, producers will continue to practice and preach low cost production as a management strategy. Due to the increasing number of synchronization protocols available for use today, understanding cost of producing a pregnancy while using some of these different breeding systems and estimating an expected outcome can become very valuable. Years ago, natural service was the only means of synchronizing a herd of cattle, but with today's technology, one can manipulate the cycle of an animal and control what and when that animal produces. Because there are so many synchronization protocols available today, understanding what system can be implemented correctly and efficiently within a given production environment, when considering AI, and which system would fit your low cost management strategy can be very important. Listed below in Table 7 is projected cost for some commonly used breeding systems today and reported responses in pregnancy rates to these systems. This table will give you an idea of the cost per head on each one of these breeding systems, but take into account that the number of animals in each of these reports is not the same (some with small numbers and some with large numbers). These costs only account for cost of the drugs used in those systems and not extra expenses such as semen, AI supplies, labor, time, and clean up bulls. These system costs vary depending upon the system you use, drugs involved and the status of your herd. Some of these protocols, for different reasons, work better on heifers vs cows and some work well on cows vs heifers. Understanding endpoints from your management strategies and evaluating the condition your animals and the conditions around you, producers can implement some of these breeding systems to accomplish their management goals. ** The authors would like to express their special appreciation to Pharmacia & Upjohn (Kalamazoo, MI) for their generous donation of Lutalyse and CIDR inserts, Intervet Inc. (Millsboro) for their generous donation of Fertagyl, and Select Sires for their generous donation of semen. The authors would also like to express their special appreciation to Quinn Cattle Company in Chadron, Nebraska, the Beef Improvement Center in Saratoga, Wyoming, and the staff at San Juan Basin Research Center in Hesperus, Colorado for their cooperation, respectively, and CSU faculty and graduate students for their assistance in the data collection. REFERENCES Food and Drug Administration. 2002. Certain other dosage form new animal drugs; progesterone intravaginal inserts. Federal Register 67:41,823. Geary, T. W., J. C. Whittier, E. R. Downing, D. G. LeFever, M. D. Holland, T. M. Nett, and G. D. Niswender. 1998. Pregnancy rates of postpartum beef cows that were synchronized with the Syncro-Mate B or Ovsynch protocol. J. Anim. Sci. 76:1523. Geary, T. W., and J. C. Whittier. 1998. Effects of a timed insemination following synchronization of ovulation using the Ovsynch or Co-Synch protocol in beef cows. Prof. Anim. Sci. 14:217-220. Grieger, D. M., G. C. Lamb, T. G. Rozell, K. E. 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