**2. Aethiology of RBC syndrome**

338 A Bird's-Eye View of Veterinary Medicine

really get this objective? Studies carried out in US indicate that only between 50-70% of cows reach this goal. In Spain, the production system presents notable differences with respect to reproductive indexes. Under intensive farming, beef cows usually calve for the first time at 28 months old, while in dairy cows the first parturition occurs at 26 months old. In beef

Fig. 1. Evolution of the lactation curves in normal and repeat breeder cows. Note that RBCs

The preovulatory follicle size is correlated with fertility in beef cattle, and it has been postulated that the implementation of management to optimize the size of the ovulatory follicle could improve the fertility (Perry & Perry, 2008). For getting good reproductive

Then, the reproductive efficiency can be measured throughout the number of weaned calves/number of cows in reproduction x 100. This percentage ranges between 60-70%. The majority of cows that failure to get pregnant (30-40% of cows in reproduction) has infertility problems during the breeding season, and many of these cows are defined as RBC. This syndrome should be treated in beef cattle farms, since expenses may rise due to the decrease of the number of calves per season and a prolonged breeding season. If beef cows are mated for a long time (for a long season), a low production will be obtained: calves will be less heavy and then its price will be lower, the health management of different ages calves will be more difficult, and the calving interval will be longer. A higher conception rate means that a higher number of calves will be weaned, and then, a higher beef yield is obtained. The costs are the same in pregnant or open cows, but profits are different. It has been reported that dystocia provokes higher economical expenses associated with perinatal mortality of

have fewer calves per cow and therefore milk yield is also reduced.


results it is recommended to select those animals that:



cattle exists a tendency to reduce the calving interval, in contrast with dairy cows.

The aethiology of RBC syndrome is unclear and multifactorial. The cow, the bull and several environmental/handling factors are incriminated. All of them are often overlapped and it is difficult to determine the primary origin.

## **2.1 Maternal factors involved in RBC syndrome**

Causes provoking repeat estrus in cattle are often related to maternal defects, which used to be at individual-level and make more difficult the clinical assessment and diagnosis. Age, genetic defects, genital tract infections, conformational defects, hormonal disorders, embryo mortality and nutritional defects have been reported. But usually there is no clear aethiology and several concomitant causes often appear, what makes difficult to characterize the problem.

#### **2.1.1 Influence of maternal age**

It has been widely reported that age impacts negatively on fertility (Hodel et al., 1995), and higher RBC rates have been described in old cows (Hewett, 1968). It is attributed to alterations in hypothalamic or pituitary hormonal levels or to inability of the ovary response (Bullman & Lamming, 1978). It has also been demonstrated the relationship between old age and low oocytes viability, what explains the fertility decline (Lanman, 1968). Studies in RBCs showed that age and breed affect the FSH and LH levels. FSH was higher in cows with six or more lactations (1.03 ± 0.12 ng/ml) and LH ranged from 1.31 ± 0.21 ng/ml in heifers and 2.19 ± 0.28 ng/ml in cows after three calves, falling to 0.94 ± 0 25 ng/ml in six or more calves (Santana et al. 2000). It is supported that fertility in dairy cattle improves after the 1st or 2nd parturition, and decreases from the 4th or 5th, but it should be taken into account the time required for uterine involution or problems associated to puerperium (Dominguez, 1989).

#### **2.1.2 Genetic factors in repeat breeder cows**

Individuals inherit their parent´s genetic merit, and then chromosomal or genetic abnormalities of parent, or those that occurred during the differentiation process may compromise fertility. RBC syndrome has been described in cows with chromosomal abnormalities as translocation 1/29 or trisomy X (Roberts, 1971; Lafi & Kaneene, 1988; Bruyas et al., 1993). A study regarding Robertsonian translocation 1/29 in cattle (Rodriguez et al., 2000) informed that cows diagnosed with this chromosomal defect were classified as RBC. Few embryos (less than 10%) exhibit chromosomal or genetic abnormalities, and are usually associated with high inbreeding or aged gametes. Humblot (1986) observed higher RBC rate in Holstein and Charolais breeds than in Norman or Frisian breeds, considering breed as a risk factor for this syndrome.

The utilization of bulls to produce bigger calves could be considered as a genetic- or management-based problem when dystocia or postpartum disease appear, and metritis, infertility or repeat estrus are developed.

Clinical Approach to the Repeat Breeder Cow Syndrome 341

Oviductal abnormalities, that complicate and frequently inhibit the reproduction, are present in 6-15% of adult cows and can reach up to 80% in those with a history of infertility or repeat breeding. Adhesions between the ovary, fallopian tubes or ovarian bursa, unilateral or bilateral obstructions, moderate degree of hydrosalpinx and inflammation (perisalpingitis, peritonitis) have been described in RBC syndrome (Figure 2). López-Gatius (1995), based on previous studies, proposes the use of intraperitoneal insemination in RBC

Acquired uterine alterations, as metritis, are critical to the resumption of the normal cyclicity during postpartum period, provoking RBCs (Shresta et al., 2004). Other non-infectious abnormalities, as uterine degeneration and neoplasia, could also be involved in this

Cervix is a defensive barrier and a sperm reservoir, and may undergo structural changes associated with inflammation. Cervical traumatic stenosis and obstruction, prolapse of cervical rings, adhesions or functional incompetence can be detected associated with RBCs. Vagina acts as receptacle for semen and is one of the uterine defensive barrier. Infectious disorders alter vaginal pH and bacterial flora, allowing the infection and reducing the sperm vitality. Congenital anomalies, conformation defects (urovagina and pneumovagina) and infections (vaginitis) could be diagnosed. Vulvitis or vestibulitis may also modify the

Following are showed some alterations that affect the reproductive hormonal function and produce RBCs, although their diagnosis is usually difficult and uncertain. Perez-Marín & España (2007) reported hormonal dysfunctions found in RBCs in Spanish farms (Table 1). Hypofunctional CL provokes a decrease of progesterone and affects negatively the fertility. Perez-Marín & España (2007) reported that hypofunctional CL was diagnosed in a 17,3% of RBCs, and Gonzalez-Stagnaro et al. (1993) affirm that this pathology appears in 23,7% of cows. CLs are small and poorly developed, with low progesterone production and LH peak asynchrony. Therefore, inadequate uterine environment is formed and this increases the abnormalities and the loss of embryos. As Kimura et al. (1987) suggested, a delayed increase

to avoid the detrimental effect of uterine diseases on the sperm transit.

Fig. 2. Piosalpingitis observed at slaughterhouse.

syndrome, although their incidence is low.

normal reproductive function.

**2.1.5 Hormonal dysfunctions** 

## **2.1.3 Uterine infection and repeat estrous cycles**

The uterine environment promotes the normal embryonic development. So, any disorder compromises the survival of the embryo and induces the RBC syndrome. Some authors have observed a correlation between repeat estrus and endometrial abnormalities (Roine & Saloniemi, 1971; Francos, 1979; Santana et al., 1998b). In fact, reproductive failure appears after metritis, and Francos (1979) observed that from 3.5 to 5.7% of cows with metritis had repeat estrus. Arguably, uterine infections (specific and nonspecific) will adversely affect the reproductive indexes by enlargement of the uterine and cervical postpartum involution, by alteration of follicular development (Lewis, 1997), and by increased embryo mortality and repeat estrus rates (Santana et al., 1998).

Subclinical endometritis should be considered when pregnancy failure or repeat estrus is observed. However, clinical signs are difficult to detect; it is not easy to do the diagnoses by rectal palpation and the bacteriological analysis of cervical mucus does not reflect the endometrium status. Endometrial biopsies and uterine microbiological culture can enhance the diagnosis. Leukocyte infiltration in moderate degree with lymphocytes, neutrophils, plasmocits, eosinophils and macrophages are histopathological findings that have been reported in the endometrium of RBCs

Many authors have identified the germs located in the reproductive tract of RBCs:


Barbato et al. (1994) studied histopathological changes in repeat breeder zebus, describing focal chronic endometritis, adenomyosis, endometrial hyperplasia, glandular hyperplasia of the "rete ovarii", hyperplasia of the ovarian serous membrane, oophoritis and granulosa cells tumour. It was concluded that uterine infections are the main causes of RBC in zebus.

### **2.1.4 Anatomical defects of the genital tract**

The reproductive tract of cow provides a suitable environment for oocyte growth, as well as for sperm transport, fertilization and implantation. Uterus is a suitable habitat for the embryonic and fetal development. A complex communication between hormones, proteins, etc. will be necessary for obtaining reproductive success. Anatomical or functional changes of these structures can drive to gestational failure and infertility. Therefore, it is essential to carry out a proper reproductive assessment for discarding animals with congenital or acquired defects.

The uterine environment promotes the normal embryonic development. So, any disorder compromises the survival of the embryo and induces the RBC syndrome. Some authors have observed a correlation between repeat estrus and endometrial abnormalities (Roine & Saloniemi, 1971; Francos, 1979; Santana et al., 1998b). In fact, reproductive failure appears after metritis, and Francos (1979) observed that from 3.5 to 5.7% of cows with metritis had repeat estrus. Arguably, uterine infections (specific and nonspecific) will adversely affect the reproductive indexes by enlargement of the uterine and cervical postpartum involution, by alteration of follicular development (Lewis, 1997), and by increased embryo mortality and

Subclinical endometritis should be considered when pregnancy failure or repeat estrus is observed. However, clinical signs are difficult to detect; it is not easy to do the diagnoses by rectal palpation and the bacteriological analysis of cervical mucus does not reflect the endometrium status. Endometrial biopsies and uterine microbiological culture can enhance the diagnosis. Leukocyte infiltration in moderate degree with lymphocytes, neutrophils, plasmocits, eosinophils and macrophages are histopathological findings that have been

Hartigan et al. (1972): Stafilococos aureus, Gram+, Streptococos haemolitic,

Murthy et al. (1974): Pseudomonas, Aerobacter sp, Klebsiella sp, Paracolobactrum sp,

 Palangala et al. (1978): Streptococos sp, Escherichia coli, Bacillus sp, Corynebacterium sp; others less frequent as Proteus sp, Klebsiella sp, Pasteurella sp, Neiseria sp,

Santana et al. (1998a): Escherichia coli, Stafilococos non-haemolitc sp, Acinetobacter sp,

Barbato et al. (1994) studied histopathological changes in repeat breeder zebus, describing focal chronic endometritis, adenomyosis, endometrial hyperplasia, glandular hyperplasia of the "rete ovarii", hyperplasia of the ovarian serous membrane, oophoritis and granulosa cells tumour. It was concluded that uterine infections are the main causes of RBC in zebus.

The reproductive tract of cow provides a suitable environment for oocyte growth, as well as for sperm transport, fertilization and implantation. Uterus is a suitable habitat for the embryonic and fetal development. A complex communication between hormones, proteins, etc. will be necessary for obtaining reproductive success. Anatomical or functional changes of these structures can drive to gestational failure and infertility. Therefore, it is essential to carry out a proper reproductive assessment for discarding animals with congenital or

Many authors have identified the germs located in the reproductive tract of RBCs:

Proteus sp, Micrococos sp, Stafilococos sp, Corinebacterium sp, Bacillus sp.

 Singla et al., (1993): Stafilococos aureus, Escherichia coli, Klebsiella pneumoniae Vasconcelos et al. (1995): Streptococos sp, Enterobacterias, Stafilococos, Yeast.

Sagartz y Hardenbrook (1971): Stafilococos, Corinebacterium.

Branhamella sp, Actinobacter sp, Haemophilus sp, Kurthia sp.

Streptococos -haemolitic, Enterobacter cloacae.

**2.1.4 Anatomical defects of the genital tract** 

**2.1.3 Uterine infection and repeat estrous cycles** 

repeat estrus rates (Santana et al., 1998).

reported in the endometrium of RBCs

Streptococos microaerofiles.

Harvey (1993): Nocardia

acquired defects.

Oviductal abnormalities, that complicate and frequently inhibit the reproduction, are present in 6-15% of adult cows and can reach up to 80% in those with a history of infertility or repeat breeding. Adhesions between the ovary, fallopian tubes or ovarian bursa, unilateral or bilateral obstructions, moderate degree of hydrosalpinx and inflammation (perisalpingitis, peritonitis) have been described in RBC syndrome (Figure 2). López-Gatius (1995), based on previous studies, proposes the use of intraperitoneal insemination in RBC to avoid the detrimental effect of uterine diseases on the sperm transit.

Fig. 2. Piosalpingitis observed at slaughterhouse.

Acquired uterine alterations, as metritis, are critical to the resumption of the normal cyclicity during postpartum period, provoking RBCs (Shresta et al., 2004). Other non-infectious abnormalities, as uterine degeneration and neoplasia, could also be involved in this syndrome, although their incidence is low.

Cervix is a defensive barrier and a sperm reservoir, and may undergo structural changes associated with inflammation. Cervical traumatic stenosis and obstruction, prolapse of cervical rings, adhesions or functional incompetence can be detected associated with RBCs.

Vagina acts as receptacle for semen and is one of the uterine defensive barrier. Infectious disorders alter vaginal pH and bacterial flora, allowing the infection and reducing the sperm vitality. Congenital anomalies, conformation defects (urovagina and pneumovagina) and infections (vaginitis) could be diagnosed. Vulvitis or vestibulitis may also modify the normal reproductive function.

#### **2.1.5 Hormonal dysfunctions**

Following are showed some alterations that affect the reproductive hormonal function and produce RBCs, although their diagnosis is usually difficult and uncertain. Perez-Marín & España (2007) reported hormonal dysfunctions found in RBCs in Spanish farms (Table 1).

Hypofunctional CL provokes a decrease of progesterone and affects negatively the fertility. Perez-Marín & España (2007) reported that hypofunctional CL was diagnosed in a 17,3% of RBCs, and Gonzalez-Stagnaro et al. (1993) affirm that this pathology appears in 23,7% of cows. CLs are small and poorly developed, with low progesterone production and LH peak asynchrony. Therefore, inadequate uterine environment is formed and this increases the abnormalities and the loss of embryos. As Kimura et al. (1987) suggested, a delayed increase

Clinical Approach to the Repeat Breeder Cow Syndrome 343

Irregular and delayed ovulations have been associated with asynchrony between estrus and ovulation (Duchens et al., 1994), asynchrony of LH peak and ovulation (Lee et al., 1983), or incapacity for LH release (Duchens et al., 1995). The LH peak characteristics are altered in RBCs and estrous signs are less intense than in normal cows. As described above,

**Pregnant** 9 24,32 0,37 0,20 1,79 0,33

Table 1. Endocrine dysfunctions and reproductive management defects detected in RBC in

The bovine embryo releases a substance of trophoblastic origin (interferon tau) into the uterus around day 16-18 that prevents luteolysis (Sheldon, 1997; Kerbler et al., 1997) and maintains the luteal function and pregnancy. EED has been attributed to irregular LH and progesterone profiles that induce failures in the maintenance of CL (Swanson & Young, 1990). EED is associated with poor quality of gametes and zygotes, uterine alterations, hormonal imbalances and defects in the immunitary mechanisms (Bruyas et al., 1993). The EED occurs between days 8 and 16 post-mating (Diskin & Sreenan, 1980), before cow returns to estrus. As a result, no variation at interestrus interval is observed and clinicians cannot differentiate between embryonic resorption and other pregnancy failures. The

Kimura et al. (1987) consider that EED and fertilization failure are recurrent causes of RBC, and genetic, environmental and endocrine influences are risk factors involved. De la Fuente et al. (1988) observed a lower embryo production in RBCs. In this study, FSH-p was administrated as superovulatory treatment and the good results suggested that non-ovarian factors (hormonal, uterine...) directly influence the reproductive failure in RBCs. Results

Prolonged luteal phase 3 8,11 0,34 0,10 1,60 0,31

AI mis-timmed 1 2,69 2,47 ---

progesterone 1 2,71 1,78 3,2 Late ovulation 2 5,41 0,53 0,01 2,03 0,25 Anovulation 1 2,70 0,49 2,15

anoestrus 2 5,41 0,18 0,10 1,33 0,04

II 1 2,70 0,12 1,65

II 3 8,11 0,24 0,16 1,40 0,26

**aethiology 14 37,84** 0,28 0,23 1,92 0,32

**Progesterone on day 0** 

**Preovulatory follicle size** 

suprabasal progesterone levels can be involved.

**Reproductive failure** 

**RBC Diagnosis n %** 

Ovarian cysts and

Luteal dysfunction type

Suprabasal levels of

Luteal dysfunction type

**WITHOUT diagnosed** 

**2.1.6 Early Embryonic Death (EED) causes RBC syndrome** 

incidence of EED is highly variable, from 10.6 to 39.7%.

Spain (Pérez-Marín & España, 2007).

in progesterone levels may indicate late or insufficient corpus luteum formation following ovulation or short luteal phase. Shelton (1997) argues that luteal inadequacy, due to a diminished response to circulating luteotrophic hormones, may contribute to embryo mortality in subfertile cows. A delayed and diminished post-ovulation progesterone curve has been associated with low conception rates in cattle, and a low progesterone curve has been shown to be related to significantly reduced production of interferon-tau by bovine embryos recovered on Day 16 of pregnancy (Figure 3).

Fig. 3. Luteal phase deficiency in cow. Red line indicates normal progesterone levels, and yellow line indicates the progesterone level in cows with luteal hypofunction.

Suprabasal progesterone level around estrus has been described in RBCs (Duchens et al., 1995; Bage et al., 1997; Perez-Marín & España, 2007). It is associated with low gonadotrophin levels and with incomplete luteal regression after luteolysis, which prolongs the follicular growth and damages the oocyte. A longer estrus-ovulation interval usually appears and premature insemination is then carried out. Duchens et al. (1995, 1996) have experimentally induced this pathology using retard release progesterone. It has been informed that these levels reduce the number of blastomeres around day 3, accelerate the zygote progress though the oviduct and affect negatively the fertility.

Anovulation has been reported in 2-16% of RBCs and it is characterized by prolonged basal progesterone after estrus (Gonzalez-Stagnaro et al., 1993; Perez-Marín & España, 2007). The LH release pattern is modified and follicle does not get the stimulus for ovulation to occur. The follicle continues growing and releases estradiol, which induces the formation of persistent follicles and delayed ovulation. Also, defective follicle recruitment during the middle- and late-luteal phases has been suggested as a cause of anovulation. An abnormal hormone environment may promote continuous development in the dominant follicle, impairing follicular function and oocyte quality, and thus reducing fertility (Odde, 1990; Stock & Fortune, 1993).

in progesterone levels may indicate late or insufficient corpus luteum formation following ovulation or short luteal phase. Shelton (1997) argues that luteal inadequacy, due to a diminished response to circulating luteotrophic hormones, may contribute to embryo mortality in subfertile cows. A delayed and diminished post-ovulation progesterone curve has been associated with low conception rates in cattle, and a low progesterone curve has been shown to be related to significantly reduced production of interferon-tau by bovine

Fig. 3. Luteal phase deficiency in cow. Red line indicates normal progesterone levels, and

Suprabasal progesterone level around estrus has been described in RBCs (Duchens et al., 1995; Bage et al., 1997; Perez-Marín & España, 2007). It is associated with low gonadotrophin levels and with incomplete luteal regression after luteolysis, which prolongs the follicular growth and damages the oocyte. A longer estrus-ovulation interval usually appears and premature insemination is then carried out. Duchens et al. (1995, 1996) have experimentally induced this pathology using retard release progesterone. It has been informed that these levels reduce the number of blastomeres around day 3, accelerate the zygote progress

Anovulation has been reported in 2-16% of RBCs and it is characterized by prolonged basal progesterone after estrus (Gonzalez-Stagnaro et al., 1993; Perez-Marín & España, 2007). The LH release pattern is modified and follicle does not get the stimulus for ovulation to occur. The follicle continues growing and releases estradiol, which induces the formation of persistent follicles and delayed ovulation. Also, defective follicle recruitment during the middle- and late-luteal phases has been suggested as a cause of anovulation. An abnormal hormone environment may promote continuous development in the dominant follicle, impairing follicular function and oocyte quality, and thus reducing fertility (Odde, 1990;

yellow line indicates the progesterone level in cows with luteal hypofunction.

though the oviduct and affect negatively the fertility.

Stock & Fortune, 1993).

embryos recovered on Day 16 of pregnancy (Figure 3).

Irregular and delayed ovulations have been associated with asynchrony between estrus and ovulation (Duchens et al., 1994), asynchrony of LH peak and ovulation (Lee et al., 1983), or incapacity for LH release (Duchens et al., 1995). The LH peak characteristics are altered in RBCs and estrous signs are less intense than in normal cows. As described above, suprabasal progesterone levels can be involved.


Table 1. Endocrine dysfunctions and reproductive management defects detected in RBC in Spain (Pérez-Marín & España, 2007).

#### **2.1.6 Early Embryonic Death (EED) causes RBC syndrome**

The bovine embryo releases a substance of trophoblastic origin (interferon tau) into the uterus around day 16-18 that prevents luteolysis (Sheldon, 1997; Kerbler et al., 1997) and maintains the luteal function and pregnancy. EED has been attributed to irregular LH and progesterone profiles that induce failures in the maintenance of CL (Swanson & Young, 1990). EED is associated with poor quality of gametes and zygotes, uterine alterations, hormonal imbalances and defects in the immunitary mechanisms (Bruyas et al., 1993). The EED occurs between days 8 and 16 post-mating (Diskin & Sreenan, 1980), before cow returns to estrus. As a result, no variation at interestrus interval is observed and clinicians cannot differentiate between embryonic resorption and other pregnancy failures. The incidence of EED is highly variable, from 10.6 to 39.7%.

Kimura et al. (1987) consider that EED and fertilization failure are recurrent causes of RBC, and genetic, environmental and endocrine influences are risk factors involved. De la Fuente et al. (1988) observed a lower embryo production in RBCs. In this study, FSH-p was administrated as superovulatory treatment and the good results suggested that non-ovarian factors (hormonal, uterine...) directly influence the reproductive failure in RBCs. Results

Clinical Approach to the Repeat Breeder Cow Syndrome 345

open days. McClure (1995) reported that reproductive disorders could occur at 3 levels: the synthesis and release of LH from hypothalamus, at ovarian function, or at ovulation,

Homeostasis of nutritional elements can be maintained through correct animal diet (deficit or excess depends on the production status). Nutritional variations may be due to an excess or deficiency of certain elements or imbalances of their concentrations in diet, and provoke alterations in their absorption or utilization. Nutritional deficiencies are more significant in high production animals, as occur in dairy cattle. In the 70's, Payne et al. (1973) developed the theory of metabolic profiles, which aimed to monitor the metabolic status and health of

Factors related to the bull and sperm quality must also be taken into account when RBCs are evaluated. It is essential to evaluate the sperm function in both natural mating and AI. Frozen semen straws should be carefully stored and managed. Repeat pregnancy failure could be linked to the mentioned bull factors, in which estrus is repeated and interestrus

Optimal bull fertility (by natural breeding or AI) is necessary to achieve high pregnancy rate and normal calving interval. Semen doses for AI must contain at least 6 million of motile sperm after thawing, and fertility drops if sperm concentration is reduced (Foote & Parks, 1993). Currently, frozen semen doses are packed with 15-25 million of motile sperm prefreezing, because around 50% of spermatozoa recover motility after thawing. However, despite semen doses may fulfill all requirements, there are donors of sperm with erratic fertility. Eid et al. (1994) found that embryos formed from high fertile bulls reached earlier the S phase of DNA synthesis and the 2-cell phase, showing high blastocyst viability. Embryos from low fertile bulls showed a longer G2 phase associated with sperm DNA damage or DNA replication failure during the S3 phase (Eid et al., 1995). The strict selection of semen donors based on the semen quality after thawing and sexual behavior, among other variables, has reduced defects in bulls used for the creation of frozen semen banks.

At natural breeding, it is necessary to assess the reproductive performance of the bulls at least twice a year, carrying out semen assessment (macroscopic and microscopic) and physical evaluations. It is also interesting to evaluate libido and the sexual behavioral pattern for mating to diagnose reproductive failures in bulls. On the other hand, it is essential to maintain an appropriate male:female ratio for mating. For guaranteeing optimal

Sperm that has just been ejaculated into the vagina by the bull or placed into the uterus by the inseminator should reach the oviducts, where fertilization occurs. Sperm begins its upward through the tubular tract of the female and reaches, within minutes, the fallopian tubes to find the oocyte. It has been reported that the utero-tubal isthmus acts as a reservoir

fertilization and development of the fertilized egg, embryo and fetus.

interval has normal duration. Some relevant aspects are mentioned below.

**2.2.1 Influence of bull fertility and semen quality on repeat breeding** 

ejaculates, bulls should do 10 mating throughout a week.

**2.2.2 Site of semen deposition and estrus return** 

**2.2 Bull factors involved in the RBC syndrome** 

the herd.

obtained after embryo transfer in RBC suggest that uterine environment is the most important factor in the EED, reporting abnormal concentrations of ions and proteins in the uterus. Intrinsic embryo factors are also related with this syndrome. Linares (1982) observed that blastocysts collected in RBCs had more abnormalities than those of control cows (3:1), and the oocyte collection rate was also lower, possibly due to a degenerative process initiated after fertilization. It was determined that the reduction of embryo viability could be associated with the reduction of nutrients exchange capacity and other regulatory substances. They could disrupt the process of cell differentiation, initiate degenerative processes, and slightly reduce the number of cilliar cells in the endometrium. High and early endometrial progesterone receptors have been detected 3 days after estrus in RBC that suggest hormonal and cellular changes in the uterus (Almeida et al., 1987, 1995).

Abnormal embryonic development by hormonal asynchrony during estrus and metaestrus have been involved in the RBC syndrome. Inadequate levels of estrogen and abnormal interactions with gonadotrophins injure the oocyte maturation, resulting in abnormal embryonic development. The altered hormonal pattern in RBCs could cause the incompetence of oocytes that will suffer abnormal embryonic development (Gustafsson et al., 1986; Albihn et al., 1991). However, other authors consider that the majority of embryonic abnormalities occur during the way through the oviducts, although those abnormalities are not evident until 6-7 days post-AI (i.e. in blastocyst stage). As oocytes of RBCs are competent to reach the blastocyst stage and to continue the development, embryo production (IVM-IVF-IVC-ET) could improve the reproductive success in these animals.

EED has usually no effect on the normal length of the estrous cycle. In occasions, anamnesis in RBC shows normal estrous cycle and fertilization rate, and the "apparent infertility" is due to earlier embryo mortality (day 7). No significant differences have been detected in these animals regarding estrus duration, ovulation time and the incidence of anovulation or silent estrus (Linares et al., 1984).
