Physiological mechanisms controlling anestrus and infertility in postpartum beef.pdf

R E Short, R A Bellows, R B Staigmiller, J G Berardinelli and E E Custercattle.

Physiological mechanisms controlling anestrus and infertility in postpartum beef

1990, 68:799-816.J ANIM SCI

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PHYSIOLOGICAL MECHANISMS CONTROLLING ANESTRUS AND INFERTILITY IN POSTPARTUM BEEF

R. E. Short3 R. A. Bellows3, R. B. Staigmiller3, J. G. Berardinelli4 and E. E. Custep

U. S. Department of Agriculture, Miles City, MT 59301 and Montana State University, Bozeman 59717

ABSTRACT

Postpartum infertility is caused by four factors: general infertility, lack of uterine involution, short estrous cycles and anestrus. The general infertility component is common to any estrous cycle and reduces potential fertility by 20 to 30%. Incomplete uterine involution prevents fertilization during the first 20 d after calving but is not related to anestrus. Short estrous cycles prevent fertility during the first 40 d after calving by causing the cow to return to estrus before pregnancy recognition occurs. Anestrus is the major component of postpartum infertility and is affected by several minor factors: season, breed, parity, dystocia, presence of a bull, uterine palpation and carryover effects from the previous pregnancy as well as two major factors: suckling and nuhition. These major factors have direct effects on anestrus but also interact with one or more other factors to control postpartum anestrus. Physiological mechanisms associated with anestrus involve blockage of the GnRH “pulse generator” in the hypothalamus, but other pathways also must be involved because bypassing the pulse generator is not an effective treatment for all cows. The primary cause of anestrus probably is different for different stages of anestrus. The mediating mechanisms for anestrus are not involved with prolactin, oxytocin, the adrenal or direct neural input from the mammary gland but are at least partially involved with blood glucose and the endogenous opioid peptide system. Management options to decrease the impact of anestrus and infertility include: 1) restrict breeding season to 545 d; 2) manage nutrition so body condition score is 5 to 7 before calving; 3) minimize effects of dystocia and stimulate estrous activity with a sterile bull and estrous synchronization; and 4) judicious use of complete, partial or short-term weaning. (Key Words: Cattle, Beef, Postpartum Interval, Infertility, Anestrus.)

J . Anim. Sci. 1990. 68:7’)9-816

‘This research is a contribution 10 Western Regional Re- search Project W-112, Reproductive Performance in Do- mestic Ruminants. Conuibution No. 2288 from Montana Agric. Exp. Sta. Presented ai a symposium titled “Postpar- tum Rebrdiiig in Cattle” at the ASAS Both Annu. Mtg.. New Brunswick, NJ.

2The authors express appreciation to former graduate students P. F. Clemente, D. L. Walters and G . L. Faltys for their contributions to this work and to The Upjohn Company. Kalamazoo, MI for financial support and generous contributions of Lutalyse and to American Breeders Service, De- Forest, WI for semen used in some of these studies.

3USDA-ARS. Ft. Keogh Livest. and Range Res. Lab., Rt. 1 Box 2021, Miles City, MT.

4Dept. of Anim. and Range Sci., Montana State Univ., Bozeman.

Received January 3.1989. Accepted June 12,1989.

Introduction

Reproduction is the main factor limiting production efficiency of beef cattle (Dickerson, 1970; Dziuk and Bellows, 1983; Koch and Algeo, 1983). The largest loss of the potential calf crop occurs because cows fail to become pregnant (Wiltbank et al., 1961; Bellows et al., 1979). Postpartum anestrus or the interval from parturition to estrus (postpartum interval) largely determines the likelihood of cow’ becoming pregnant during the breeding season (Symington, 1969; Wiltbank, 1970). The pur- pose of this review is to enumerate factors that affect the postpartum interval of cows and to show how these factors affect the endocrine system.

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800 SHORT ET AL.

TABLE 1. EARLY HISMRY OF POSTPARTUM RESEARCH

Referena Location commentsa

Hammmd (1927) Guilben and McDonald (1933) Clapp (1937)

Lasley and Bogan (1943) Casida and Wisnicky ( 1950) Jouben (1954) Wamick (1955) Bunis and =ode (1958) Wiltbank and Cook (1958) Foote et al. (1960a.b)

Biswal and Rao (1960) Wiltbank et al. (1962, 1963) Perkms and Kiddw (1 %3) Foote and Hunter (1964) BhaIla et al. (1966,1967) Oxenrcider ( 1968) Saiduddii et al. (1%8a) Casida (1%8); Graves et al.

England California Wisconsin

Missouri Wisconsin S. Africa Oregon USDA USDA Wisconsin

India USDA W. Virginia Nevada India Iowa Nevada Wisconsin

Herdsmen reported nurse cows had longer PPI. Distribution of PPI in beef cows. Four times milked or suckled dairy cows has longer PPI than those two times milked. PPI of Hereford range cows. Diethylstibcsterol (DES) did not affect PPI. Nutritional effects on calving interval, beef and daily. PPI for Angus and Hereford and its relationship to day of calving. Date of calving vs pregnancy rate. PPI of suckled and milked cows. Uterine involution not related to PPI; pretreatment did not affect uterine involution but delayed estrus. Weaning reduced PPI and effect decreased with increasing parity. Re- and post-calving nutritional effects on PPI in Hereford cows. Uterine involution did not affect conception rate, PPI more important. P4 and E2 treclunents reduced PPI, short estrous cycles. Parity and weaning decreased PPI. Suckling effects in beef cows. PPI shortened by steroid treatment. Most definitive study in postpartum cow.

(1968) Lauderdale et al. (1968); Riesen et al. 1968; Saiduddin et al. (1968b) Shocl et al. (1972) USDA Suckling effect independent of nutrition; mastectomy enhances wean-

ing effect.

*PPI = Interval from calving to estrus.

Postpartum infertility and anestrus were first recognized as problems over 60 yr ago (Hammond, 1927). Research on postpartum reproduction progressed slowly for a period of time (see Table 1 for a summary of the early work in this area). A tremendous increase in postpartum research occurred during the last 20 yr; the majority was reported in the last decade (see Figure l5 for summary of research activity during the last 60 yr). In spite of the research activity during the last decade, postpartum infertility is still a significant problem in the industry, but, as shown in Figure 1, research activity in this area is decreasing rapidly.

Over the years, several review papers summarizing reproduction and anestrus research in cattle have been published either as independent articles (Baker, 1969; Wiltbank,

5A computer database of postpartum cow literature with over 700 references was compiled by R. E. Short and B. W. Knapp and is available from the authors on computer disc or hard copy with any of the following fields: authors, year, title. journal, volume and page, keywords, author address and abstract.

1970; Tervit et al., 1977; Kilkenny, 1978; Lamming et al., 1981; Garcia, 1982; Peters, 1984a; Hanzen, 1986; Short and Adams, 1988) or as part of various books or proceedings of symposia (Ninth Animal Reproduction Sympo- sium: Casida, 1971; W. C. Foote, 1971; W. D. Foote, 1971; Tucker, 1971; Wagner and Oxenreider, 1971 - BARC Symposium on Animal Reproduction: Bellows et al., 1979; Inskeep and Lishman, 1979 - Fourteenth Animal Reproduction Symposium: Wette- mann, 1980; Kiracofe, 1980; Dunn and Kalten- bach, 1980 Edgerton, 1980; Christensen, 1980 - Diamond Jubilee Symposium for American Society of Animal Science: Koch and Algeo, 1983; Dziuk and Bellows, 1983 - Current Topics in Vet. Med. and Animal Science: Terqui, 1985; Schallenberger and Walters, 1985 - Reproductive Endocrinology of Do- mestic Ruminants Symposia: Nett, 1987). However, there are no recent reviews that are comprehensive and address the problem of postpartum infertility and anestrus in beef cattle from a practical point of view. There- fore, the emphasis of this review is to cover this subject for beef cattle with relevant literature included from both beef and dairy cattle because the difference between these two



POSTPARTUM ANESTRUS AND INFERTILITY 80 1

80

70 C n I fertility by 20 to 30% for any estrus regardless of whether it occurs after calving or at any other reproductive state. This component will not be discussed further, but it must be recognized as a factor in postpartum infertility. The other three factors are related specifically to the postpartum period and will be discussed. Shown in Figure 2 are the time relationships that several components have to infertility; the order of the four factors listed does not indicate relative importance.

25 35 45 55 65 75 85 95 YEAR (19--)

Figure 1. Numbers of citatim reviewed on postpartum cow research from 1927 to 1988 (673 citations, complter database; Short and Knapp5).

is due primarily to management and not to physiology. Literature that pertains mainly to the management aspects of postpartum dairy cattle is not included (e.g., use of milk progesterone for detecting ovarian cycles).

Factors Affecting Padparturn Infertility

After parturition, cows are infertile for a variable period of time. Several factors contribute to that infertility. Illustrated in Figure 2 are 1) the relationship between probability of a pregnancy occuning (infertility vs fertility) and time after calving and 2) the four main factors that contribute to infertility. A general infertility component exists that decreases

Uterine Involution

Kiracofe (1 980) thoroughly reviewed the literature on this subject and concluded that uterine involution has no relationship to length of the anestrous period. Nevertheless, uterine involution is a barrier to fertility during the early postpartum period. Graves et al. (1968) recovered ova or palpated for pregnancy in cows that were bred at the first postpartum estrus. They found that fertilization rates and pregnancy rates were very low when cows were bred <20 d after calving, but fertility returned to normal between 20 to 40 d after calving.

In several experiments, we bred cows that showed estrus very early after calving (Short et al., 1974). Half the cows were bred by normal AI procedures, and half were bred by surgi- cally &positing semen into the tip of the uterine horns. Ova were recovered 3 d after insemination to determine fertilization rates. Inseminations into the tip of the uterine horn reduced the interval to conception by increas-

Figure 2. Relationship of f d l i t y (probability of a pregnancy occurring) 10 time after calving. The factors contributing to infertility arc shown above the fmility probability line.



802 SHORT ET AL.

ing the proportion of cows with fertilized ova when inseminations were done before 20 d after calving. Fertilization rates after 20 d were similar for both groups. We concluded that infertility during the first 20 d after calving is caused by a physical barrier to sperm transport and not by any inherent defect in the ova or other physiolcgical mechanisms. We do not have data on whether an early postpartum uterus actually could maintain a pregnancy if a fertilized ovum were present. The nonin- voluted uterus may be a barrier not only to sperm transport but also to implantation.

From a practical point of view, uterine involution is not a problem for beef cattle because it does not affect anestrus. Very few cows would exhibit estrus early enough after calving for uterine involution to interfere with conception so long as disease conditions do not prevent or delay normal involution.

Short Estrous Cycles

Short estrous cycles also contribute to postpartum infertility during the first 30 to 40 d after calving (Figure 2). Most estrous cycles after 40 d have normal length, although some evidence exists that short estrous cycles will occur later (Lishman et al., 1979). The short estrous cycle phenomenon was one of the first abnormalities observed in the postpartum cow (Foote and Hunter, 1964; Morrow et al., 1968; Lauderdale et al., 1968; Casida, 1971; Short et al., 1972). Ovulations following esms des- tined to start a short cycle are normal with ova released that can be fertilized. However, no pregnancies were detected, apparently because the corpus luteum (CL) regressed before the ovary received a signal from the uterus that a pregnancy existed (Graves et al., 1968; Short et al., 1972, 1974; Odde et al., 1980; Ramirez- Godinez et al., 1982a,b).

Why is this first estrous cycle short? Is it because the CL is not capable of functioning normally, or is it because regression signals are being given prematurely? The CL formed during a short estrous cycle is smaller, secretes less progesterone (P4) and is less responsive to stimulation (Short et al., 1974; Lishman et al., 1979; Kesler et al., 1981; Rutter and Randel, 1984; Duby et al., 1985; Carmthers et al., 1986). Gonadotropic stimulation of the short- cycle CL appears to be normal and not to be a limitation to its development (Manns et al., 1983; Carmthers et al., 1986 Garverick et al.,

1988), although FSH concentrations may be low preceding this ovulation (Ramirez-Godi- nez et al., 1982b). The short-cycle CL is not hypersensitive to prostaglandin (Copelin et al., 1988), nor is there any evidence of a problem with receptors or enzymes (Smith et al., 1986). So far, this evidence doesn’t answer the question asked earlier; it merely reinforces the evidence that the short-cycle CL is function- ally deficient.

Prostaglandin F2a (PGF) appears to be the normal physiological signal whereby the uterus causes regression of the CL at the end of the estrous cycle. During the early postpartum period, high PGF concentrations probably prolong the interval to normal CL function because hysterectomy reduces PGF concentrations and maintains normal CL function (Edqvist et al., 1978; Lindell et al., 1982a,b; Madej et al., 1984; Copelin et al., 1987). The early postpartum uterus produces and metabo- lizes large amounts of PGF, and suppression or infusions of PGF further implicate a role of PGF in the control of CL function in the postpartum cow (Guilbault et al., 1984, 1987a,b). Therefore, evidence exists to answer the question raised earlier. The functional capabilities of the early-postpartum CL are normal, but the CL is caused to regress prematurely by abnormally high PGF concentrations from the uterus. Regression occurs before the signal can be given that a pregnancy exists. These high FGF concentrations presumably are a part of the mechanisms involved in involution of the uterus. Uterine involution is involved with prevention of pregnancy, not only blocking fertilization as described earlier, but also by preventing normal CL function.

Normal CL function during an early postpartum estrous cycle can be obtained by pretreating with a progestin (Ramirez-Godinez et al., 1981; Pratt et al.. 1982; Sheffel et al., 1982; Rutter et al., 1985; Smith et al., 1987). The mechanism associated with the progestin effect involves a reduction in PGF (Troxel and Kesler, 1984) with the effect mediated via the ovary rather than the pituitary. Some evidence exists for a LH-follicle involvement (Williams et al., 1982b; Garcia-Winder et al., 1986b). Even ovarian or follicular involvements could be mediated indirectly through alteration in uterine endocrinology.

The short-estrous-cycle syndrome is a practical beef cattle problem because some cows exhibit estrus and are bred during the period



POSTPARTUM ANESTRUS AND INFERTILITY 803

that these estrous cycles occur, especially in more intensive operations where postpartum intervals are short and estrous synchronization is used. For early postpartum cows, estrous synchronizing treatments that include a progestin may have an advantage (see review by Odde, 1990).

Postpartum Anestrus

In beef production systems, postpartum anestrus can affect infertility for a long time (Figure 2); therefore, anestrus is a more serious problem than either uterine involution or short estrous cycles. P o s t p m anestrus commonly is referred to as postpartum interval (PPI), the interval from pamuition to first estrus. Inter- vals to other endpoints such as to ovulation or to conception also are important, but from a practical point of view, estrus is the most logical measure of the commencement of potential fertility.

Ovulation may precede the first observed estrus and often is called a “silent” or “quiet” heat (Casida and Venzke, 1936; Labsetwar et al., 1963). The true incidence of this phenomena will be affected by how well we “listen” (Casida, 1971). but even under ideal estrous detection conditions, some ovulations are not accompanied by a detectable estrus. This condition is much more prevalent in milked dairy cows and when ovulation occurs very early after calving.

The opposite condition, estrus without ovulation, has been observed at puberty (nonpuberal estrus, Nelsen et al., 1985; Rutter and Randel, 1986), but it has been reported only once during postpartum anestrus in cattle, and that was in diury canle (Casida, 1968). The appropriate experiments have not been con- ducted to determine the extent to which this condition exists in beef cattle.

Postpartum anestrus or PPI is a€fected by several minor factors, including season, breed, age or parity, dystocia, presence of a bull, uterine palpation and carryover effects from the previous pregnancy, as well as by the two major factors of suckling and nutrition. These factors all have direct effects on PPI, but each factor can interact with one or more of the other factors. Because of the many factors and interactions that affect PPI, control and management of postpartum anestrus is complex. Minor Factors. Season was one of the first factors to be associated with postpartum

anestrus (Buch et al., 1955; Warnick, 1955). In the study reported by Warnick, season was included in the analysis as day-of-year calved with spring-calving cows. Cows calving later in the spring had shorter PPI. In this case, day- of-year effects could be true season effects associated with light and temperature differences, or the effect could be confounded with nutritional changes as year progressed. Subse- quent work has confirmed that day-of-year decreases PPI as the spring calving season progresses and has shown that cows calving from late spring to early fall have shorter PPI than cows calving from late fall to early spring (Bellows and Short, 1978; Peters and Riley, 1982a,b; King and MacLeod, 1983; Smeaton et al., 1986). These seasonal effects are not due to nutritional and management differences (Boyd, 1977; Bellows and Short, 1978) but are true seasonal effects related to changes in light (Hansen and Hauser, 1984; Hauser, 1984; Garel et al., 1987). However, seasonal effects are modified by numtion and other factors such as genotype and suckling (Hansen and Hauser, 1983; Hansen, 1985; Montgomery et al., 1985). From recent evidence, it appears that seasonal effects are mediated via the pineal because injections of melatonin will increase PPI (Sharpe et al., 1986). Not much can be done through management to correct seasonal effects on PPI other than to adjust for seasonal effects.

Breed and genotype effects on postpartum anestrus can be seen by making comparisons between studies. Dairy breeds that are milked have shorter PPI than suckled beef breeds, but when dairy cows are suckled, they have longer PPI than beef cows. Comparisons also have been made within studies that have shown that, when managed comparably, dairy genotypes have longer PPI than beef genotypes and that these effects are more pronounced at first parity and at lower dietary intakes (Kropp et al.. 1973; Hansen et al., 1982). Differences also have been reported between beef breeds, with the effect being more pronounced at lower &etary intakes (Dunn et al., 1969; Bellows and Short, 1978). How genotype affects PPI is not known; effects may be due to true physiological differences among breeds, and(or) they may be due to confounding effects such as differences in amount of milk produced or appetite and feed intake. The effect of breed on PPI is an important factor, and even though breed usually is predeter-



804 SHORT ET AL.

mined, this effect must be considered when managing postpartum cows.

The effect of age and parity on PPI is due mostly to the difference between 2- and 3-yr-old dams or older, with young cows having longer PPI and lower reproductive potential. Dystocia also is associated with age and will increase PPI and delay rebreeding (Brinks et al., 1973; Laster et al., 1973; Bellows and Short, 1978; Doornbos et al., 1984; Izaike et al., 1984). The adverse effects of dystocia can be overcome at least partially by providing early obstemcal assistance (Bel- lows et al., 1988).

Presence of a bull decreases PPI (2alesky et al., 1984; Alberio et al., 1987; Berardinelli et al., 1987; Scott and Montgomery, 1987; Custer, 1988). The mechanism whereby presence of a bull accelerates the physiological processes that initiate resumption of estrous cycles is not understood; the available evidence implicates no involvement with either P4 or LH (Custer, 1988). Putting sterile teaser bulls with cows during the early postpartum period could be a useful tool in managing postpartum anestrus in beef cattle.

Postpartum reproduction also may be affected by other factors such as rectal palpation, treatment with a synthetic prostaglandin (Tol- leson and Randel, 1988) and effects of the calf. The effects of the calf apparently are related to differences in growth rate and amount of milk consumed; fast-growing, larger calves and(or) calves consuming more milk will have dams with longer PPI (Rice et al., 1961; Reynolds et al., 1978; Bellows et al., 1982). These effects are more subtle or are fixed and are less likely to be manipulated by management. However, they should be considered when devising management strategies.

Major Factors. Suckling and nutrition are by far the most important factors determining the length of postpartum anestrus. Suckling probably has the most dramatic effect on PPI and was one of the first factors to be related to postpartum reproduction (Clapp, 1937; Wilt- bank and Cook, 1958). Cows that have their calves weaned at birth have shorter PPI than do cows that are suckled (Graves et al., 1968; Oxenreider, 1968; Short et al., 1972). If calves are weaned at some time after birth but before estrous cycles begin (usually between 20 and 40 d after calving), cows will return to estrus in a few days (Walters et al., 1982b). Regulation of the suckling and lactation

stimulus is a viable management option to decrease PPI. Postpartum intervals can be decreased by complete weaning, short-term weaning (48 h) or partial weaning (restricting suckling to short periods of time each day). However, response to weaning treatments will vary with other factors such as age, nutrition, genotype of the cow and age of the calf. The effects of suckling and lactation have been reviewed in this symposium as well as in an earlier symposium (Edgerton, 1980 Williams, 1990).

Nutritional effects were firmly established as a factor controlling postpartum anestrus by the classical work of Wiltbank (Wiltbank et al., 1%2, 1963; Dunn et al., 1969). Reviews on the effects of nutrition have detailed these effects (Dunn and Kaltenbach, 1980; Short and Adams, 1988; Randel, 1990). Nutritional effects are elicited via a complex interplay among many variables such as quantity and quality of feed intake, nutrient reserves stored in the body and competition for nutrients from other physiological functions besides reproduction. The effects of nutrition have been most commonly measured using energy as a variable. Other nutrients also may affect reproduction, but their effects are not well documented.

The allocation of nutrients to various body functions is commonly referred to as nutrient partitioning. A schematic representation of these relationships is shown in Figure 3 (Short and Adams, 1988). Illustrated in Figure 3 are several points about nutritional and production interrelationships. The first is that cattle (and other ruminants) have a unique niche in animal agriculture because they have the ability to convert lowquality forages to useful products. The second is that excess nutrients can be stored during excess periods to be retrieved at a later time to maintain production. However, extended periods of limited nutrient availability because of low quantity and(or) poor quality will decrease production. The third is that nutrients are partitioned by a priority to first maintain life of the cow and then to propagate the species. The approximate order of priority for partitioning of nutrients is as follows: 1) basal metabolism, 2) activity, 3) growth, 4) basic energy reserves, 5) pregnancy, 6) lactation, 7) additional energy reserves, 8) estrous cycles and initiation of pregnancy and 9) excess reserves. The relative priority of these functions can change depending on what functions are present and at what level.




Figure 3. Partitioning of numents in a cow with nutrient intake varying in quantity and quality (from Shon and Adams, 1988).

Energy reserves can constitute almost 50% of an animal’s maximum possible weight (Short et al., unpublished data). One of the simplest ways to estimate nutrient reserves is the use of body condition scores (BCS; Lowman et al., 1973; Bellows et al., 1982; Richards et al., 1986). As shown in Figure 3, estrous cycles usually can be maintained if BCS is 4 or greater, although this may differ depending on other factors such as breed and whether an animal is entering into or coming out of anestrus (Hansen et al., 1982; Louw et al., 1988).

The effect of nutrition on postpartum reproduction depends somewhat on whether nutritional differences exist before or after calving. In general, the differences that exist at calving as estimated by precalving BCS are more important than those that exist after calving. These relationships are depicted in Figure 4. The effects of BCS at calving on PPI are nonlinear, with the effects being greatest at

very low scores (<4), becoming less as BCS increases, with very little effect as BCS increases above 7. Postcalving dietary intake can alter these relationships, but postcalving differences in diets have their main effect when BCS at calving is <6 and when the feed intake is low vs adequate (100% of NRC) rather than adequate vs high.

A degree of caution should be exercised when interpreting simple correlation and regression relationships among BCS and PPI. Both of these variables are physiological responses in a system in which the relationships may not be cause-effect but merely are cogenerated. This type of relationship has been shown between age at first estrus and weight at first estrus in heifers (Greer et al., 1983). Analyses of postpartum data using a path hagram approach and generalized least squares procedures to estimate parameters of a simultaneous equations model have shown that BCS at calving and PPI are recursively



806 SHORT ET AL.

Figure 4. Relationship of interval from calving to estrus (PPI) and body condition score (BCS) as affected by levels of postpartum diets.

dependent; the relationship is not a simple cause and effect (R. C. Greer, personal communication).

Control Mechnnlrms of Postpartum Anestrus

Control of the resumption of estrous cycles in the postpartum cow is accomplished by a complex relationship among the hypothalamus, pituitary and ovary that is influenced by a variety of external and internal signals. A major portion of this relationship is accomplished by hormone signals. A model for the hormonal control of estrus and ovulations is shown in Figure 5. Postpartum anestrus exists because the various factors discussed earlier prevent postpartum cows from proceeding through the normal steps (Steps 1 to 6, Figure 5) leading to estrus and ovulation. This series of events leading to estrus and ovulation is similar for postpartum cows and cows that are having estrous cycles (Walters et al., 1982b). Somehow, the postpartum cow is prevented from continuing through these steps by one or more of the factors discussed earlier. We will now examine some evidence for how each of these steps is involved with postpartum anestrus.

The functional competence of the hypothalamus and pituitary is decreased for a 10- to 204 period after calving. During this time, the amount of LH in the pituitary is lower, less LH is released in response to either estradiol (E2) or GnRH, and the bioactivity of LH is lower. These differences do not seem to be related to suckling, pituitary LH content, hypothalamic GnRH content or pituitary GnRH receptors (Nancarrow and Radford, 1977; Webb et al.. 1977; Radford et al., 1978; Short et al., 1979; Foster et al., 1980; Irvin et al., 1981a,b; Smith et al., 1981; Wettemann et al., 1982; Williams et al., 1982a; Stevenson et al., 1983; Peters, 1984b; Allrich et al., 1985; Hinshelwood et al., 1985; Moss et al., 1985; Schallenberger and Prokopp, 1985; Leung et al., 1986; Jaeger et al., 1987; Weesner et al., 1987; Nett et al., 1988). However, these negative effects are not sustained and do not appear to account for the normal delay in resumption of estrous cycles observed in the suckled postpartum beef cow (Short et al., 1979).

The primary defect that exists in the postpartum cow is a low serum concentration of LH (Short et al., 1972; Ingalls et al., 1973) caused by a low-frequency pulsatile secretion



POSTPARTUM ANESTRUS AND INFERTILITY

ANESTRUS and

807

Figure 5. A model depicting the hormonal control of esttu and ovulation in postpartum cows.

pattern of LH and, presumably, GnRH (Steps 2 and 3, Figure 5). During postpartum anestrus, the pulse pattern of LH typically has a very low frequency (<1 pulse/4 h). The frequency increases during the time preceding estrus to about one pulse every 1 to 2 h (Carruthers and Hafs, 1980 Carruthers et al., 1980; Walters et al., 1982a, Schallenberger. 1985). The GnRH “pulse generator“ somehow must be inhibited during the early postpartum period.

A pulsatile pattern of LH release can be reproduced by giving low-dose injections of GnRH every 1 to 2 h. Postpartum cows treated in such a manner have shorter postpartum intervals (Riley et al., 1981; Walters et al., 1982c; Jagger et al., 1987; Wildeus et al., 1987). However, all cows do not respond to such treatment, especially if they are early in the postpartum period or on a low dietary intake (Short et al., 1981; Edwards et al., 1983; Peters et al.. 1985; Jagger et al., 1987; Short and Adams, 1988). The inconsistency in the response to these treatments leads us to conclude that inhibition of the GnRH “pulse generator” is not the only cause of anestrus.

The mechanisms for control of anestrus may be different depending on the cause (i.e., nutrition vs suckling), or the primary defect may change as the postpartum cow progresses from “deep” anesnus to “shallow” anestrus with time after calving. The amount of time required to make this transition varies with the amount of input from each of the controlling factors.

A pulsatile pattern of GnRH release may be only the strategy evolved to maintain a certain level of secretion because continuous infusions of GnRH will elicit a response similar to pulse injections (Lofstedt et al., 1981; Jagger et al., 1987). Prolonged infusions of GnRH result in a decreased release of LH over time, but this decreased sensitivity to GnRH is not reflected by the endogenous pulsatile release of LH (Lamming and MacLeod, 1988).

At present, there is no evidence implicating FSH as a limiting factor in postpartum anestrus (Walters et al., 1982a; Moss et al., 1985).

The ovarian component of anestrus (Step 4, Figure 5 ) is highly dependent on the status of the preceding steps, but in general the ovary



808 SHORT ET AL.

per se does not Seem to be a limiting factor because it will respond to exogenous stimulation (Casida et al.. 1943). Follicular develop ment and function are normal, although there are some carryover effects of the CL of pregnancy. Follicles are selected from a pool of medium-sized follicles, and as follicles mature, they show an increase in number of LH receptors and switch from P4 to E2 production. Atresia is not related to gonadotropin binding (Braden et al., 1986; Spicer et al., 1986a,b,c; Inskeep et al., 1988). The main evidence for the ovary’s involvement in anestrus is that the pituitary-hypothalamus is hypersensitive to negative feedback of estrogen rather than an absence of the positive feedback mechanism discussed earlier (Acosta et al., 1983; Garcia-Winder et al., 1986a; Chang and Reeves, 1987).

This hypersensitivity to estrogen gradually is overcome to allow Steps 2 to 4 to proceed, resulting in the continuation through preovula- tory LH release, estrus and ovulation (Steps 5 and 6, Figure 5) . Then the cycle repeats itself or pregnancy occurs (Steps 7a or 7b).

The stimuli that are detected by control mechanisms to retard or block the normal progression through these steps in Figure 5 are not understood, but several possibilities have been investigated. A role of the adrenal was implicated by some circumstantial evidence, but careful experimentation has ruled this out (Wagner and Oxenreider, 1971; Dunlap et al., 1981a,bcc; Ellicot et al., 1981; Smith et al., 1974; Whisnant et al., 1985; Faltys et al., 1987; Spicer and Zinn, 1987). Prolactin also was explored as a possible mediator but was ruled out (Smith et al., 1974; Cummins et al., 1977; Clemente et al., 1978; Schallenberger et al., 1978; Holness and Jeffers, 1980 Williams and Ray, 1980; Webb and Lamming, 1981; Weiss at al., 1981; Montgomery, 1982; Whee- ler et al., 1982). Oxytocin was injected three times daily in an experiment, and we found no evidence that oxytocin was involved with anestrus (Short et al., 1972); this was con- fumed by Stewart and Stevenson (1987). A direct neural involvement from the udder does not seem to be present (Short et al., 1976; Williams, 1990).

The only positive evidence for a control pathway involves glucose (see review by Short and Adams, 1988) and the endogenous opioid peptide (EOP) system. Even though evidence exists that the EOP system is involved in the

control of LH release during the postpartum period (Gregg et al., 1986; Malven et al., 1986; Short et al., 1986; Whisnant et al., 1986a,b,c,d; Cross et al., 1987; Peck et al.. 1988; Trout and Malven, 1988), this system is only a part of the control system because prolonged treatment with an EOP antagonist only temporarily will release LH without subsequent ovulation (Short et al., 1986).

Conclusions and Management Considerations

Postpartum infertility and anestrus are complex phenomena controlled by many factors that act either individually or in concert to decrease the production potential of beef cattle. Many of the factors that affect postpartum reproduction cannot be eliminated, but they can be considered when one makes management decisions relative to other factors that can be manipulated. In making a management decision about postpartum anestrus, one must consider not only the potential benefits of a treatment on postpartum fertility but also the cost of implementing this treatment. For most production systems, maximum fertility will not be the most profitable. Optimum fertility rate for a given production unit is that which returns the greatest profit (or some other defined measure of success) over a period of time. Unfortunately, only limited data are available on the costs of most treatments that affect postpartum fertility, so that aspect of the decision making process is left to the individ- ual producer.

The first management decision that has 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 examination of Figure 6 reveals an even more important reason for limiting the breeding season to 45 d or less (Figure 6 is Figure 2 redrawn with the addition of variable lengths of breeding seasons). The three examples of breeding seasons shown (45, 60 and 82 d) illustrate stages of the postpartum period for cows when the next breeding season starts. This assumes a gestation period of 280 d and an interval of 365 d between the start of the two breeding seasons. If breeding seasons are 45 d or less, than all cows have a high probability for pregnancy at the beginning of the breeding season because they are beyond the effects of uterine involution and short




py qftar Calnng

Figure 6. Relationship of tbe length of the breeding Season to fertility during the postpartum period.

estrous cycles and beyond the major effects of anestrus if proper management has been accomplished (see later points on this). Con- versely, as breeding seasons extend beyond 45 d, more and more cows will have a lower potential fertility at the beginning of the next breeding season. Cows that calve in the last 2 wk in a 60-d breeding season system or in the last 6 wk in a 90-d breeding system will have increasing problems with anestrus, short estrous cycles and uterine involution. Any cow that is bred after 82 d in a long breeding season system will not have calved by the time the next breeding season starts. If producers presently are using a long breeding season (A5 d), it can be a slow but beneficial process to change to a shorter season. Because cows that presently are in a long breeding season program are difficult to convert to a short breeding season, it is best to start with the replacement heifers and gradually work the breeding season shorter as these heifers and each subsequent year’s replacements enter the herd. By using this method and culling nonpregnant and late-calving cows, the whole herd can be shifted to a breeding season of 45 d or less in a few years.

The second management decision to consider for decreasing postpartum infertility is nutrition. Nutritional levels and BCS have major impact on anestrus and potential fertility. The best way to monitor a nutritional

program is by following body weight changes and BCS. Often it is not practical to measure body weight in many production settings, but in most situations a procedure can be devel- oped for monitoring BCS. For optimum reproduction, it is best to manage for a BCS at calving of 5 to 7 (on a 9- or 10-point scale). Condition scores higher than 7 are not necessary and probably waste valuable re- sources unless severe nutritional deficiencies are anticipated after calving. Condition scores lower than 5 will lower potential fertility unless there is a high availability of numents after calving. It is difficult and expensive to correct nutritional problems that exist at calving by increasing dietary intake after calving. When selecting a target condition score at calving for managing a specific group of cows, it is important to consider other factors unique to that herd such as breed, amount of milk produced and incidence of dystocia and then adjust the precalving BCS as necessary.

The third management decision to minimize postpartum infertility is to consider several options that include minimizing stress from dystocia, putting a sterile bull with cows during the early postpartum period before the breeding season starts and using estrous synchronizing treatments that include a progestin to increase estrous response and decrease the incidence of short estrous cycles. The



810 SHORT

deleterious effects of dystocia can be mini- mized by careful attention to the following: selection of replacement heifers, selection of sires to be used on replacement heifers, management of both cows and heifers before and after calving and providing appropriate obstetrical assistance at parturition. Putting a bull with cows during the postpartum period to reduce postpartum infertility is a new approach. More information is needed concern- ing the optimum bukcow ratio, when the bulls should be turned in and whether other forms of social interaction will elicit the same response (e.g.. androgenized cows). However, the relative cost of using bulls is low and should be considered by producers. If estrous synchronization is a viable alternative in a production system, it can be used to shorten the effective breeding season and take advantage of possible stimulatory effects of a progestin.

The fourth and last management alternative to consider is decreasing the suckling stimulus. We view this alternative as a last resort because this alternative potentially has more serious economic and management conse- quences, such as an increased potential for calf diseases and for reduced calf weaning weights. All the other management alternatives should be considered and dealt with before resorting to this option. The suckling stimulus can be decreased by complete weaning, limiting the time during which suckling can occur to once or twice a day or weaning for a short period of time (48 h) associated with an estrous synchronizing treatment. In order for these treatments to be effective for reducing infertility, they must be accomplished at some point before the breeding season starts. However, in some cases of limited feed supplies later in lactation, weaning can be an effective tool for managing BCS going into the winter in order to minimize fertility problems in the subsequent year. In all cases of partial or complete weaning, the potential economic and management disadvantages should be carefully weighed against the potential advantages of increased fertility.

Implications

Postpartum infertility can be a serious problem that decreases production efficiency of beef production systems. Many factors contribute to infertility through a complex interaction of physiological and endocrine

ET AL.

mechanisms. Anestrus is the main component of postpartum infertility, but uterine involution and short estrous cycles as well as a general infertility component also are involved. Man- agement options to increase fertility during the postpartum period include shorter breeding seasons, using BCS to monitor nutritional management, minimizing dystocia, using a sterile teaser bull before breeding, synchronizing esms and decreasing the suckling stimulus.

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