Chapter 78 G. Cliff Lamb and Vitor R.G. Mercadante North Florida Research and Education Center, University of Florida, Marianna, Florida, USA The primary use of embryo transfer in cattle has been to amplify reproductive rates of valuable females. Ideally, embryo transfer can be used to enhance genetic improvement and to increase marketing opportunities with purebred cattle. Embryo transfer is especially useful with cattle because of their relatively low reproductive rate and long generation interval.1 Once transferable embryos are collected from a donor cow, a decision is made as to which of the available recipients should receive embryos to achieve the greatest number of pregnancies.2 The success of embryo transfer depends on factors associated with the embryo, the recipient, and the embryo transfer technician, or an interaction among these factors. Suitability of recipients is dependent on numerous management, nutritional, and estrous cycle control factors to ensure the presence of a functional corpus luteum (CL) at transfer. Although studies have focused on these factors, differences in techniques, sample sizes, and other elements have limited the applicability of the results of these studies. In many ways, management of the recipient is more critical to the success of an embryo transfer program than the donor, since the recipient is expected to conceive to the transferred embryo, maintain the pregnancy until full term, calve without assistance, and raise a calf of high genetic merit. This chapter focuses on recipient-related factors responsible for the success of an embryo transfer program. Insufficient intake of energy, protein, vitamins, and microminerals and macrominerals has been associated with suboptimal reproductive performance. Of these nutritional effects on reproduction, energy balance is probably the single most important nutritional factor related to poor reproductive function in cattle. The metabolic use of available energy in ruminants for each physiological state is ranked, in order of importance, as follows: (i) basal metabolism; (ii) activity; (iii) growth; (iv) energy reserves; (v) pregnancy; (vi) lactation; (vii) additional energy reserves; (viii) estrous cycles and initiation of pregnancy; and (ix) excess energy reserves.3 Based on this list of metabolic priorities, reproductive function is compromised because available energy is directed toward meeting minimum energy reserves and milk production. Generally, beef cows do not experience a period of negative energy balance because they fail to produce the quantity of milk that dairy cows produce; however, beef cows need to be in sufficient body condition to resume estrous cycles after parturition and overcome anestrus, short estrous cycles, and uterine involution just to become pregnant every year. Body condition score (BCS) is a reliable method for assessing the nutritional status of recipients. A visual BCS system developed for beef cattle uses a scale from 1 to 9, with 1 representing emaciated and 9 obese.4 A linear relationship exists between body weight change and BCS, where an approximate 40-kg weight change is associated with each unit change in BCS. Managers of recipients should understand when cows can be maintained on a decreasing plane of nutrition, when they should be maintained on an increasing plane of nutrition, or when they can be kept on a maintenance diet. Understanding the production cycle of the cow and how to manipulate the diet will improve the ability of the recipients to conceive to the transferred embryo.5,6 BCS at calving has been shown to be a more predictable indicator of the duration of postpartum anestrus than prepartum change in either weight or BCS.4,7 When cows were thin at calving or had a BCS of 4 or less, increased postpartum level of energy increased the percentage of females exhibiting estrus during the breeding season. BCS at parturition and breeding are the dominant factors influencing pregnancy success, although body weight changes during late gestation modulate this effect. However, altering poor body condition after parturition may reduce the negative impact on reproduction, but seldom overcomes or eliminates those negative effects. A recent study by Stevenson et al.8 using blood samples at initiation of the breeding season to determine estrous cycling status demonstrated that only 47.2% of the cows were cycling at the onset of the breeding season. However, as BCS increased, the percentage of cows that were cycling also increased. It is important to note that when cows had a body condition score of less than 4 at the beginning of the breeding season, only 33.9% had resumed their estrous cycles. The general belief is that cows maintained on an increasing plane of nutrition prior to parturition usually have a shorter interval to their first ovulation than cows on a decreasing plane of nutrition. Energy restriction during the prepartum period results in a low BCS at calving, prolonged postpartum anestrus, and a decrease in the percentage of cows exhibiting estrus during the breeding season.9 Pregnancy rates and intervals from parturition to pregnancy are also affected by level of prepartum energy.9 Conversely, when prepartum nutrient restriction was followed by increased postpartum nutrient intake, the negative effect of prepartum nutrient restriction was partially overcome; however, the effectiveness of elevated postpartum nutrient intake depended on the severity of prepartum nutrient restriction.7,9 The effect of BCS prior to calving also has implications for calf birth and weaning weights. When cows were fed to achieve a BCS of either 4 or 6 prior to calving, body weights were greater and calf birth and weaning weights (with similar genetics) were also greater for those cows at a BCS of 6.10 Despite the greater birthweights, there was no difference in calving difficulty, demonstrating the added advantage for recipients to wean calves with greater weaning weights. In addition, there tended to be an increased number of cows calving with a medium BCS that were cycling at the beginning of the breeding season and after a 60-day breeding season than cows in poor condition, resulting in a greater proportion of cycling cows at various stages of the breeding season.10 Numerous studies document that increasing nutritional levels following parturition increases conception and pregnancy rates in beef cows.4,11 Increasing the postpartum dietary energy density increased body weight and BCS and decreased the interval to first estrus.7 However, suckled beef cows in relatively poor body condition gaining in excess of 1 kg/day while consuming an 85% concentrate diet did not resume cyclic ovarian activity before 70 days postpartum.7 Therefore, although an enhanced plane of nutrition after calving may partially overcome the negative effects of poor prepartum nutrition, the added stress and negative impact of suckling and lactation must also be considered. A major impact on postpartum fertility is the length of the breeding season. Having a restricted breeding season has many advantages, such as a more uniform, older calf crop, but most importantly a breeding season of 60 days or less increases the percentage of females cycling during the next breeding season. If the breeding season is shortened, then all cows have a higher probability for pregnancy during the next breeding season. Strategic feeding to obtain ideal BCS can be achieved by understanding the production cycle of the cow. The period of greatest nutritional need occurs shortly after calving; a cow is required to produce milk for a growing calf, regain weight lost shortly before and after parturition, and repair her reproductive tract to become pregnant within 3 months after calving. During this stage, a cow is usually consuming as much feed as she can and adjusting BCS at this time often is futile. Cows are usually grazing and tend to consume their full protein, vitamin, and mineral requirements; however, the grass is often lush with a high percentage of moisture, which occasionally can cause a deficiency in energy.12 Factors such as nutrition, management, and inefficient detection of estrus affect the widespread use of embryo transfer in commercial beef cattle operations. The most useful alternative for increasing the number of animals receiving embryos is to utilize protocols that allow for embryo transfer without the need for estrus detection, usually called fixed-time embryo transfer (FTET) protocols. However, much of the research related to the systems currently used in embryo transfer programs was for fixed-time artificial insemination (TAI) rather than FTET. Transfer of embryos into estrus synchronized cows has been most effective when embryos were transferred 6–8 days after detected estrus or gonadotropin-releasing hormone (GnRH) injection.13 Early estrus synchronization systems focused on altering the estrous cycle by inducing luteolysis with an injection of prostaglandin (PG)F2α followed by estrus detection. Once systems involving a single PGF2α treatment became successful, researchers focused on multiple injections of PGF2α to further reduce days required for estrus detection.14,15 The next generation of estrus synchronization systems involved the use of exogenous progestins, such as an intravaginal progesterone release insert (CIDR) or melengestrol acetate (MGA), which were used to delay the time of estrus following natural or induced luteolysis and extend the length of the estrous cycle.16,17 Not until the discovery that growth of follicles in cattle occurs in distinct wave-like patterns18 were scientists able to embark on the third generation systems for estrus synchronization. Controlling follicular waves with a single injection of GnRH at random stages of the estrous cycle involves release of a surge of luteinizing hormone (LH) that causes synchronized ovulation or luteinization of dominant follicles.19–21 Consequently, a new follicular wave is initiated in most (>60%) cows within 1–3 days of GnRH administration. Luteal tissue that forms after GnRH administration will undergo PGF2α-induced luteolysis 6–7 days later.22 A drawback of this method of estrus synchronization is that approximately 5–15% of cows are detected in estrus on, or before, the day of PGF2α treatment, reducing the proportion of females that are detected in estrus during the synchronized period.23–25 Preliminary studies identified significant improvements in fertility among cows that received MGA prior to the administration of PGF2α compared with cows that received only PGF2α.26 When cows received a CIDR for 7 days and an injection of PGF2α the day before CIDR removal, estrus synchrony and pregnancy rates were improved.17 When GnRH was given 6 or 7 days prior to PGF2α, 70–83% of cows were in estrus within a 4-day period.22 The use of GnRH to control follicular wave emergence and ovulation and PGF2α to induce luteolysis led to the development of the Ovsynch protocol for dairy cows.27 Combining the second injection of GnRH with TAI (CO-synch) proved to be more practical than estrus detection for beef producers because it had no negative effects on fertility.28 However, a disadvantage of this protocol is that approximately 5–15% of suckled beef cows exhibit estrus prior to, or immediately after, the PGF2α treatment.24 Unless these cows are detected in estrus and inseminated, they will fail to become pregnant to TAI. Therefore we hypothesized that the addition of a CIDR to a GnRH-based protocol would prevent the premature occurrence of estrus and result in enhanced fertility following TAI. Overall pregnancy rates were enhanced by the addition of a CIDR to a GnRH-based TAI protocol (59% vs. 48%, respectively). The CIDR delayed the onset of ovulation, resulting in more synchronous ovulation, and induced cyclicity in noncycling cows.24 However, the efficacy of these CIDR-based TAI protocols had not been evaluated concurrently with AI protocols requiring detection of estrus in suckled beef cows. Therefore, we implemented and coordinated a multi-state, multi-location experiment to discern whether a GnRH-based plus CIDR protocol for TAI could yield pregnancy rates similar to protocols requiring detection of estrus.29 Results demonstrated that the TAI protocol yielded pregnancy rates that were similar to the estrus detection protocol, even though 35% of the cows were in postpartum anestrus at the time of treatment. A detailed version of current estrus synchronization and TAI protocols reviewed by the Beef Reproduction Task Force is available in Figure 78.1. Utilizing a similar protocol on recipients using FTET would be practical and effective in yielding high pregnancy rates in recipients. Figure 78.1 Current protocols for estrus synchronization and TAI of beef cows reviewed by the Beef Reproduction Task Force. Available at http://beefrepro.unl.edu. Early studies in beef heifers demonstrated that feeding MGA for 14 days followed by PGF2α 17 days later was an effective method of estrous cycle control in heifers.16,30 However, when heifers were treated with PGF2α 19 days after the 14-day MGA feeding period, there was no difference in fertility but estrus was more synchronous.31 Following the success of this protocol, researchers began to include GnRH in estrus synchronization protocols for TAI. However, addition of GnRH to the above protocol failed to increase pregnancy rates following TAI in heifers.32 Estrus synchronization using GnRH followed by PGF2α successfully synchronized heifers, but the above MGA–PGF2α protocol led to greater synchrony of estrus and therefore tended to be more effective.31 Development of a TAI protocol in beefs heifers has not been as straightforward as in cows, especially considering that at the time of estrus synchronization a majority (>85%) of heifers have attained puberty.33 The primary reason for failure of TAI in heifers appears to be the inability to synchronize follicular waves with GnRH. After an injection of GnRH at random stages of the estrous cycle, 75–90% of postpartum beef cows ovulated,34,35 whereas only 48–60% of beef and dairy heifers ovulated in response to the same treatment.27,36,37 We have found no difference in synchrony of estrus or pregnancy rate in CIDR-treated heifers whether or not GnRH is administered at CIDR insertion, suggesting that response to GnRH in heifers at CIDR insertion may be of limited value.33 In a large multi-location study using GnRH, PGF2α, and CIDR, GnRH did not enhance pregnancy rates following estrus detection but the addition of a CIDR to a GnRH-based TAI protocol yielded similar pregnancy rates to those utilizing estrus detection.33 Nevertheless, a bewildering fact remains that the average pregnancy rate for these protocols ranged from 53 to 58%, whereas pregnancy rates in MGA (with PGF2α administered 19 days after MGA removal) or long-term CIDR (with PGF2α administered 16 days after MGA removal) protocols followed by PGF2α have been reported to range from 60 to 75%.23,31,33,38 A detailed version of current estrus synchronization and TAI protocols reviewed by the Beef Reproduction Task Force is available in Figure 78.2. Utilizing a similar protocol on recipients using FTET would be practical and effective in yielding high pregnancy rates in heifer recipients. Figure 78.2 Current protocols for estrus synchronization and TAI of beef heifers reviewed by the Beef Reproduction Task Force. Available at http://beefrepro.unl.edu. Reinsemination of nonpregnant cows at the first eligible estrus can be facilitated by resynchronization of the estrous cycle,39 which would have wide application in intense embryo transfer programs. To most effectively condense the calving season, the second round of estrus synchronization needs to begin before the pregnancy status of the animals is known. Although resynchronization with a progestin increased synchronized return rates of nonpregnant females,40,41 resynchronization with CIDR devices and estradiol cypionate or estradiol benzoate decreased subsequent conception rates to AI.40 In contrast, further studies did not note a decrease in fertility when estrogens were utilized for resynchronization with a CIDR.42 Furthermore, insertion of a CIDR for 13 days on the day of embryo transfer 7 days after estrus43 or from 5 days after TAI until day 2129 was effective in resynchronizing estrus in nonpregnant cows, but insertion of a CIDR failed to enhance fertility compared to controls. The procedure of removing an embryo from its natural uterine environment, and in many cases freezing and thawing, increases the stress experienced by those embryos resulting in a decreased survival rate following transfer. Our findings of a decrease in pregnancy rate from 83% with fresh embryos (n = 122) to 69% with frozen-thawed embryos (n = 326) are similar to the 10–15% decrease in pregnancy rates reported previously,44,45 which is similar to the difference in averages reported by the American Embryo Transfer Association and the International Embryo Transfer Association (Savoy, IL). Results from two studies reveal that pregnancy rates among cows receiving a Grade 1 or Grade 2 fresh embryo46,47 were not different. Previous reports48–51 noted a decrease in pregnancy rate with each corresponding decrease in quality score. Pregnancy rates have been shown to vary with the synchrony of the donor and recipient. Higher pregnancy rates were observed when recipients were in estrus coinciding with the donor or 12 hours before the donor. Pregnancy rates decreased in recipients in estrus 12 hours after the donor50 but not until 24 hours in other reports.45–47 The variability in progesterone concentrations in recipients reflects a combination of different rates of CL development and the fluctuation of progesterone secretion during the early luteal phase. It has been suggested that the optimum circulating concentration of progesterone for establishing pregnancy ranges between 2.0 and 5.0 ng/mL.52 However, a recent study has revealed that the minimum threshold progesterone concentration on the day of embryo transfer essential for the establishment and maintenance of pregnancy may be lower than previously reported; there were no differences in pregnancy rates when progesterone concentrations were as low as 0.58 ng/mL or exceeded 16.0 ng/mL (n = 448).47 In another study, 8 of 177 pregnant recipients had concentrations of progesterone of less than 0.5 ng/mL on days 10, 11, and 12 of the transfer cycle.53 In addition, the diameter and volume of the CL differed among recipients that received embryos from 6.5 to 8.5 days after estrus,47 increasing as days after estrus increased. However, pregnancy rates did not differ among recipients receiving embryos 6.5–8.5 days after estrus. Recipient movement while the embryo transfer gun is in the uterus increases the risk of damage to the endometrium, causing the release of PGF2α. Pregnancy rates have been inversely correlated with the time spent in the uterus during embryo transfer.54 In addition, when the prostaglandin synthesis inhibitor flunixin meglumine was administered at the time of embryo transfer, pregnancy rates were reportedly enhanced, but only when poorer-quality embryos were transferred.43,55 However, the FDA has released a reminder that flunixin meglumine is only approved for intravenous use in cattle and extra-label drug use has resulted in increased residues found at slaughter. Selection and identification of high-quality recipients is not simple. Many prefer the use of virgin heifers, whereas others choose cows with a known history of high fertility. When heifers are to be used for recipients, the selection criteria should be the same as for high-quality replacement heifers. Heifers need to be cycling (which can be assessed indirectly by using reproductive tract scores56), on a high plane of nutrition, have an adequately sized and normally shaped pelvic canal, and have no history of receiving growth implants. Lactating recipients have an advantage of a known reproductive history. Since the health of the calf is dependent on the recipient, records should be kept of calf health and weaning performance. Recipients that carry an embryo transfer calf to term but do not raise a normal calf to weaning should be reevaluated as a recipient prospect. Similarly, open cows with an unknown reproductive history need to be carefully examined prior to being included in a recipient herd or program.57 The reproductive tract needs to be thoroughly examined via rectal palpation or transrectal ultrasonography for pregnancy or uterine anomalies such as fluid or fetal remnants or evidence of metritis or endometritis and the ovaries examined for normal follicular or luteal structures. In addition, recipients should have good teeth and eyes and a good udder and be less than 8 years of age and structurally sound. Highest fertility occurs in herds where handling facilities are designed to ensure that cattle are handled with a minimum of stress. It is wise to keep the new arrivals separate from the breeding herd until sufficient time has elapsed for diagnostic screening tests to return and any incubating disease to become apparent. Many purebred producers and embryo transfer companies take blood samples to test for exposure to bovine leukosis virus (BLV), Mycobacterium paratuberculosis (Johne’s disease), bovine viral diarrhea virus (BVDV), anaplasmosis, and Neospora caninum. Brucellosis testing or vaccination is no longer required in many areas but it is prudent to test cattle from areas where the disease is present in wildlife. Many of these pathogens have been associated with decreased fertility, by preventing fertilization or by causing embryonic death, fetal loss or ovarian dysfunction.58 The use of vaccinations to control livestock diseases is a common and proven practice. Conventional recommendations suggest that modified-live virus vaccines be given at least 30 days prior to breeding. However, recent research has shown that in previously vaccinated cattle, there are no detrimental effects of vaccinating cattle at the time of PGF2α treatment or initial GnRH treatment.58 Cattle with an unknown or questionable history of vaccination should receive primary and booster vaccinations at least 30 days prior to breeding.59 Once the recipient is confirmed pregnant, she needs to be managed to remain pregnant. There are several environmental risks that cause abortion depending on the area of the country where the cow is maintained. Some of these include N. caninum, locoweed, ponderosa pine needles, fescue toxicity, nitrates, and mycotoxins and/or moldy feed. Handling stress has been shown to cause heifers to abort, but no reliable data have focused on the ideal interval after embryo transfer to transport recipients. However, pregnancy rates were lower in females that were transported between 8 and 33 days after AI as compared with those transported within the first 4 days.60 Therefore, it appears to be more desirable to transport recipients prior to embryo hatching than after hatching. Infectious agents (e.g., BVDV, infectious bovine rhinotracheitis and, rarely, bluetongue virus) are also a threat to the developing fetus. Often it is not possible to diagnose the cause of abortions but diagnostic success can be increased if the proper samples and history are submitted.61 To protect the herd from the introduction of infectious diseases that could cause abortion or reduce the value of the calves, a well-designed biosecurity program needs to be in place. This includes a test and quarantine program for all new cattle introduced into the herd. A commonsense program of single-use needles and rectal sleeves, disinfection of equipment between cows (drench applicators, tattoo pliers, and other instruments), external parasite control, the use of clean coveralls for employees and visitors as well as footbaths or shoe coverings, and not allowing equipment or clothing to be taken from one farm to another without first thorough washing and disinfection can minimize risk of introducing pathogens onto a facility. Knowing when cows conceive and when they will calve helps concentrate calving supervision. Ultrasonography can be used to accurately determine the presence of a conceptus as early as 28 days, but it is recommended to recheck all cows after 45 days to confirm pregnancy.62 Through the use of ultrasonography and breeding dates to determine the estimated date of calving, cows can be sorted into calving groups and managed to save on feed, labor, and veterinary expenses. Pre-calving vaccinations can also be timed to insure the most effective response. Also, avoiding overcrowding of calving pastures and/or calving cows on “fresh” pastures that have not been grazed by cows with calves has been shown to reduce calf morbidity and mortality due to infectious calfhood scours.63 Effective management of a recipient herd requires preparing the recipient to receive an embryo and identifying and preparing open cows to be resynchronized and reused or inseminated. In any group of synchronized recipients, a small percentage will not be detected in estrus and not all detected in estrus will receive an embryo, due to either an asynchronous estrus or lack of a suitable CL at the time of transfer. If 80% of the synchronized recipients are detected in estrus and 90% of those receive embryos and 60% become pregnant, then less than 45% of any group of recipients will become pregnant. Therefore, it is important to devise a strategy to resynchronize recipients as soon as possible. In large herds with extended calving seasons, it is necessary to synchronize more than one group of recipients. Resynchronization strategies vary depending on the resources and capabilities of the ranch. With the use of ultrasonography, nonpregnant recipients may be confidently identified and resynchronized as early as 3 weeks after embryo transfer.64 A CIDR can also be inserted at the time of transfer and removed 12–13 days later; ultrasonographic pregnancy diagnosis can then be performed before reuse. We do not transfer an embryo to a recipient more than twice in any season. Unpublished data from Granada BioSciences Inc., Marquez, Texas demonstrated that the pregnancy rate between recipients that became pregnant following one or two transfers was not significantly different, but less than 20% of the recipients that received a third embryo transfer became pregnant. However, a single report has reported up to a 12% decrease in pregnancy rates for recipients receiving a second embryo transfer.65 Many purebred producers contract with commercial cow/calf producers to use cows as recipients and purchase the weaned embryo transfer calf. Normally, the cooperator synchronizes and observes cows for estrus, sorts recipients 6–8.5 days after estrus, and provides the labor to move the cows through the working facility for a technician to transfer embryos. The purebred producer pays the synchronization expenses and the embryo transfer technician, and pays the cow owner a premium for every embryo transfer calf weaned. This arrangement can be a win–win for both producers. However, there are issues that arise when first beginning this program. Not all commercial producers have the satisfactory management; a commercial producer already familiar with AI is a good candidate for such a program. It is essential that a producer has a good record-keeping system, is able to weigh and identify each calf at birth, be comfortable with estrus detection, and have a good herd health program. The purebred producer may expect the cooperator to screen cows for the aforementioned diseases for biosecurity purposes. In 2011, we conducted a survey of all members of the American Embryo Transfer Association (AETA), regarded as leaders in embryo transfer of cattle in the United States. The goal of the survey was to assess the utilization of current embryo transfer practices, the perception by AETA members of each practice, and whether perceptions were supported by research. A total of 218 professional members received the survey, of whom 63 (29%) responded to the survey; 47 (75%) of the members who responded were certified AETA members, while 27 (43%) classified their embryo transfer business as primarily beef cattle related, 13 (21%) as dairy cattle related, 21 (34%) as a mixture of beef and dairy, and one (2%) member was not cattle related. We asked the survey participants to rate their perception regarding several factors that could potentially affect the fertility of the recipients to embryo transfer, and the results are shown in Figure 78.3. Embryo quality was rated as the factor with the greatest impact on fertility. Quality grade of the embryo has a huge impact on fertility; in a study comparing pregnancy rates of embryo transfer using embryos graded from 1 to 3 (1 being excellent and 3 being poor), recipients receiving grade 1 embryos had greater pregnancy rates than recipients receiving grade 3 embryos (56.1% vs. 33.3%, respectively).66 Embryo transfer technician was rated the second most important factor affecting fertility of embryo transfer; indeed differences in pregnancy rates can be seen across embryo transfer technicians67 and pregnancy rates have been inversely correlated to the time spent in the uterus during embryo transfer.54 Figure 78.3 Perception of the relative impact on fertility of recipients to embryo transfer (1, least impact to 5, greatest impact). BCS, body condition score; ET, embryo transfer. Recipient BCS at embryo transfer was rated of moderate impact on fertility and the effect of nutrition on the success of embryo transfer programs has been discussed in the section on nutritional management. Although rated as moderate, the use of fresh or frozen embryos as well as embryo stage at transfer and recipient history has a great impact on the fertility of embryo transfer programs.67 The diameter of the CL is perceived as having a low impact on fertility of recipients of embryo transfer. Indeed, in a study evaluating luteal characteristics and plasma concentrations of progesterone in pregnant and nonpregnant recipients, no differences were found in CL diameter, luteal volume, and plasma progesterone concentrations.47 Surprisingly, 61% of the survey participants claimed to have no experience with the use of human chorionic gonadotropin (hCG), while 23% claimed that the use of hCG is unsuccessful, and only 16% believed that the use of hCG is successful in embryo transfer programs. Nevertheless, our recent work68 indicated that treatment with hCG at embryo transfer increased the incidence of accessory corpora lutea formation, increased progesterone in pregnant recipients, and increased transfer pregnancy rates. At three locations, purebred and crossbred Angus, Simmental, and Hereford recipients (n = 719) were assigned alternately to receive hCG 1000 IU i.m. or 1 mL saline (control) at embryo transfer. Pregnancy rates at the first diagnosis on day 33 were 61.8 and 53.9% for hCG and controls, respectively (Figure 78.4) and were 59.0 and 51.4%, respectively at the second diagnosis on day 68. In agreement with our findings, the use of hCG at embryo transfer also increased pregnancy rates of Holstein recipient cows receiving fixed-time embryo transfer.69 Figure 78.4 Effects of hCG 1000 IU at embryo transfer on pregnancy rates at day 33 (fresh and frozen embryos combined). **, hCG differs from saline, P = 0.03). Adapted from Wallace L, Breiner C, Breiner R et al. Administration of human chorionic gonadotropin at embryo transfer induced ovulation of a first wave dominant follicle, and increased progesterone and transfer pregnancy rates. Theriogenology 2011;75:1506–1515.
Selection and Management of the Embryo Recipient Herd for Embryo Transfer
Introduction
Nutritional management
Body condition score as an indicator of reproductive efficiency
Prepartum nutritional effects on reproduction
Postpartum nutrition
Estrous cycle control
Development of estrus synchronization protocols
Advances in protocols for beef cows
Advances in protocols for beef heifers
Resynchronization of estrus
Recipient-related factors
Embryo transfer factors
General recipient considerations
Management after confirmation of pregnancy
Biosecurity
Pregnancy diagnosis
Management for efficient recipient utilization
American Embryo Transfer Association Survey
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