Nutrient partitioning in dairy cattle. Matthew C. Lucy. Department of Animal Sciences, University of Missouri

Transcription

1 NUTRIENT PARTITIONING AND REPRODUCTIVE PERFORMANCE IN DAIRY COWS Matthew C. Lucy Department of Animal Sciences, University of Missouri Take Home Messages Blood growth hormone (GH) concentrations increase shortly after calving. Greater blood GH coordinates nutrient partitioning; the process through which nutrients are preferentially targeted for milk production. Blood insulin and insulin-like growth factor (IGF)-I remain low when blood GH concentrations increase. Blood GH concentrations are elevated in cows in negative energy balance. Blood insulin and IGF-I concentrations are low in cows in negative energy balance. Postpartum dairy cows experience a variety of reproductive abnormalities. Many of reproductive abnormalities can be linked to negative energy balance through hormonal mechanisms involving GH, IGF-I, and insulin. Introduction The initiation of lactation and the metabolic transition to peak milk production occurs rapidly in dairy cattle (Bell, 1995). Nutrient partitioning is the process through which nutrients are preferentially targeted for milk production. The rapid increase in nutrients required for lactation causes a negative energy balance during the early postpartum period that may last for several weeks after calving. Nutrient partitioning and negative energy balance affect hormonal concentrations that ultimately control reproductive function (Butler 2000; Lucy, 2003). This review will examine the endocrine mechanisms involved and how they relate to reproduction in dairy cattle. Nutrient partitioning in dairy cattle Blood GH concentrations increase shortly after calving (Bauman, 1999). Greater blood GH coordinates nutrient partitioning; the process through which nutrients are preferentially targeted for milk production. Multiple tissues are affected by GH in the lactating cow but coordinated events in liver and adipose tissue may be most important. In liver, the postpartum increase in GH stimulates gluconeogenesis. The increase in gluconeogenesis is believed to involve a direct effect of GH on the gluconeogenic pathway as well an indirect effect of GH through an antagonism of insulin action. The actions of GH on liver gluco- 139

2 neogenesis in periparturient dairy cattle are essential for meeting glucose demands for milk production. In adipose tissue, GH increases lipolysis which in turn increases blood NEFA concentrations. The NEFA may be oxidized in liver or extra-hepatic tissues or may be incorporated directly into milk fat. Blood glucose concentrations are low during this period because the mammary gland uses glucose for energy and the synthesis of lactose. Low glucose concentrations in postpartum cows are associated with low blood insulin concentrations. There appears to be insulin resistance during this period; particularly in high producing dairy cows. Low insulin concentrations and partial insulin resistance redirect existing glucose pools to the mammary gland where glucose uptake is independent of blood insulin. The relationship between blood insulin and blood growth hormone (GH) is important because each hormone affects lactation in an opposite way. High producing cows have high blood GH concentrations, low blood glucose concentrations, and low blood insulin concentrations (Bauman, 1999). The high blood GH concentrations promote adipose tissue mobilization and increase blood nonesterified fatty acids (NEFA) concentrations. The NEFA can be used for milk fat synthesis. Low blood insulin concentrations redirect blood glucose toward the mammary gland (mentioned above). Low producing cows have lower blood GH and higher blood insulin concentrations. Their capacity to mobilize NEFA is less and more glucose is partitioned to tissues outside the mammary gland. These features lead to lower milk production. Mechanisms Controlling Nutrient Partitioning Around Calving Growth hormone binds to the growth hormone receptor (GHR) in liver and controls the secretion of insulin-like growth factor (IGF)-I. Insulin-like growth factor-i then acts on the hypothalamus and pituitary to control GH secretion through a negative feedback loop. A series of physiological events involving GH, the GHR, IGF-I and insulin coordinate metabolic events during early lactation in cattle. The GHR in liver decreases about 2 days before calving, remains low for approximately one week, and slowly increases during the second week after calving (Figure 1; Radcliff et al., 2003). The decrease in GHR after calving is associated with the GH-resistant period of early lactation because GH must act through the GHR. The profile for liver IGF-I is similar to GHR but the decrease in IGF-I occurs slightly later than the decline in GHR. The delay may reflect the fact that liver IGF-I synthesis depends on GH acting through the GHR. Blood insulin concentrations decrease as well during the periparturient period. The decrease in insulin occurs about 2 to 3 days after the decrease in GHR and coincides with the decrease in IGF-I. Blood GH concentrations increase during the first week after calving because blood IGF-I concentrations decrease and negative feedback on GH is reduced. The increase in blood GH causes lipid mobilization and elevated blood NEFA (Figure 1). Remarkably, the signal that initiates adipose tissue mobilization (decreased GHR) occurs coincident with lactogenesis and well before peak milk production. Thus, mechanisms for nutrient partitioning are triggered ahead of the major 140

3 nutrient demand for lactation (a feed-forward system). The decrease in GHR is a phenomenon that is unique to dairy cattle because beef cattle do not undergo the same changes in GHR around calving. The changes in GHR for dairy versus beef may be a consequence of differential sensitivity to periparturient hormones within beef and dairy breeds or may be a consequence of greater lactogenic drive in dairy cows. The respective roles that parturient signals and lactogenic signals play in controlling GHR needs to be clarified. The decrease in GHR before calving causes the uncoupling of the GH axis in postpartum cows. There is a subsequent recoupling of the GH axis during early lactation. The recoupling process has been linked to postpartum nutrition and energy balance and is probably dependent on GHR. If postpartum nutrition and energy balance affects recoupling (GHR expression) then what is the nature of the signal? Insulin infusion into early post-partum dairy cows increased liver GHR (Butler et al., 2003). The uncoupling of GHR immediately before calving may not be insulin-dependent because GHR decreases before the decrease in blood insulin (Figure 1). The coincident increase in blood insulin and liver GHR, however, suggests that the recoupling process depends on insulin. A Model For Gh And Insulin Controlling Nutrient Partitioning During Lactation The aforementioned effects of insulin on GHR and IGF-I expression can be combined with recent data on type 1 and type 2 diabetes mellitus to create a model for early lactation. The model includes the hypothalamus/pituitary, liver, pancreas (β cell; insulin source), adipose tissue, and the mammary gland. The GHR decreases in liver shortly before calving. The decrease in GHR leads to a decrease in liver IGF-I synthesis and a decrease in blood IGF-I concentrations. The decrease in blood IGF-I causes reduced negative feedback on GH and an increase in blood GH concentrations. Greater blood GH increases liver gluconeogenesis and promotes lipolysis in adipose tissue. Insulin concentrations decrease during this period because glucose concentrations are low. Low insulin concentrations keep liver GHR low. The decrease in insulin, however, has an opposite effect on adipose tissue where GHR expression increases in response to a decrease in blood insulin. The biological mechanism controlling the adipose tissue response is unclear. The increase in GHR increases the responsiveness of adipose tissue to GH and enhances lipid mobilization. Elevated GH and elevated NEFA during this period antagonize insulin action and create a state of insulin resistance. The low blood insulin and insulin resistance blunt glucose utilization by non-mammary tissues and conserve glucose for milk synthesis. The cycle described above (low liver GHR, low IGF-I, high GH, low glucose, low insulin, and insulin resistance) is gradually turned off during the first 4 to 8 weeks of lactation. The critical event may be an increase in blood glucose. The increase in blood glucose occurs when glucose synthesis exceeds glucose demand. Greater blood glucose increases blood insulin concentration; the increase in insulin increases liver GHR and liver 141

4 Figure 1. Relative amounts of liver GHR, blood insulin, and blood IGF-I (upper graph) and liver GHR, blood GH, and blood NEFA (lower graph) in postpartum dairy cows sampled for 15 days before through 15 days after parturition. Data are smoothed means from the original data published by Radcliff et al. (2003) GHR 1A IGF -I Insulin GHR 1A GH NEF A

5 IGF-I; the increase in blood IGF-I concentrations feeds back negatively on GH; and the decrease in GH reduces adipose tissue mobilization. The increase in insulin also decreases adipose tissue GHR and reduces adipose tissue responsiveness to GH. The link between the insulin and somatotropin systems, therefore, insures a coordinated response to changing nutrient demand and availability during early lactation. Mechanisms Linking Nutrient Partitioning to Reproduction in Cattle Many of the mechanisms that control reproduction are linked directly to the nutrition of the animal. A global visualization of the basic concepts will be presented (Figure 2) and the reader is referred to reviews for more information on specific topics (Butler, 2000; Lucy, 2003). Interval to First Ovulation Follicular growth in postpartum cattle is controlled by a combination of LH and FSH. Essential mechanisms controlling follicular growth are similar whether or not the cow is undergoing anestrus or estrous cycles. Classically described, FSH is viewed as responsible for initiating follicular growth and LH is responsible for final maturation of the dominant/ preovulatory follicle. Secretion of LH and FSH is controlled primarily (LH) or in part (FSH) by GnRH from the hypothalamus. The factors that control the release of GnRH from the hypothalamus are major components of a conceptual model for LH secretion (Figure 2). The duration of the postpartum anestrus period depends on suckling stimulus, body condition, and the depth of negative energy balance. Dairy cows may not experience the suckling stimulus but they do experience a period of negative energy balance early postpartum. Energy require-ments for milk and maintenance exceed energy consumed in the feed. The resulting negative energy balance is associated with a decrease in LH pulsatility (Beam and Butler, 1999). Postpartum cows will begin to cycle once LH pulsatility reaches a critical level. The increase in LH pulsatility stimulates the maturation of a dominant follicle. The dominant follicle produces estradiol that reaches a threshold level to trigger an LH surge. There is a coordinated series of events that act to promote follicular development and eventually ovulation. Insulin and IGF-I concentrations gradually increase postpartum. Cows in negative energy balance have lower blood concentrations of insulin and IGF-I. Insulin and IGF-I influence GnRH and LH secretion. The hormonal control (endocrine) arises from tissues that respond to the metabolic or nutritional status of the animal (e.g., insulin from the pancreas and IGF-I from the liver). It makes sense that these peripheral cues would act upon the hypothalamus to convey information from metabolically important tissues. A variety of metabolites (glucose, fatty acids, etc.) and other hormones acting as blood-borne messengers may 143

6 Figure 2. Conceptual model for the mechanisms through which nutrition affects reproduction in postpartum cows (Lucy, 2003). Hormones and metabolites from the gastrointestinal tract and nutrient-responsive tissues affect GnRH and LH secretion through their actions on the central nervous system (CNS) and hypothalamus. These same hormones and metabolites may have direct effects on ovarian function (both follicles and corpora lutea) as well as the oocyte, oviduct and uterus. The combined effects of each axis determine postpartum fertility. 144

7 also be involved. The same metabolites and hormones that influence GnRH secretion and ultimately LH and FSH secretion may act directly on the ovary to influence the sensitivity of the ovary to LH and FSH. Ovarian cells treated with either insulin or IGF-I have greater numbers of gonadotrophin receptors and greater activation of second messenger pathways in response to gonadotropins (Lucy, 2000). There is also the potential for insulin and IGF-I effects that are completely independent of LH and FSH. It may be impossible to dissect apart the relative importance of gonadotropins, locally and peripherally produced growth factors, and metabolites for controlling reproduction in the postpartum cow. Cows that are nutritionally compromised have low concentrations of metabolites and metabolic hormones in their blood. The lower metabolic hormone concentrations theoretically reduce ovarian responsiveness to gonadotropins (see above). At the same time, postpartum cows have low blood LH concentrations, in part because of the effects of metabolic hormones on GnRH secretion from the hypothalamus. Thus, the effects of nutrition on reproduction are manifested at the ovary and at the pituitary and hypothalamus. Overcoming one limitation will not necessarily recover ovarian function. Abnormal estrous cycles The resumption of estrous cyclicity in cattle is not necessarily synonymous with the resumption of normal fertility. There are of course infertile short estrous cycles whose physiology has been extensively studied. Reproductive physiologists have viewed estrous cycles after the short estrous cycles as essentially normal with normal fertility. We now know, however, that at least in high producing dairy cattle the resumption of estrous cyclicity is not synonymous with the resumption of regular estrous cycles with normal fertility (see below). The factors that control regular estrous cyclicity with normal fertility represent the biggest challenge to reproductive biologists working with modern cattle. Studies have identified an increased incidence of irregular or abnormal estrous cycles in dairy cattle (Roche et al., 2000; Table 1). These abnormalities can be manifested through a variety of estrous cycle pathologies that include temporary cessation of luteal phases as well as long luteal phases. Factors known to affect postpartum cows such as negative energy balance, periparturient disorders, and postpartum diseases are known risk factors for delayed cyclicity and prolonged luteal phases (Opsomer et al., 2000). The incidence of twinning has also increased in modern dairy cattle because there are positive genetic correlations between the incidence of twins and level of milk production (Kinsel et al., 1998). The increased incidence of anestrus, abnormal estrous cycles, and twinning share a common LH-mediated mechanism (Figure 3). Presumably, the increase in anestrus is caused by a decrease in LH pulsatility that is secondary to negative energy balance in dairy cattle selected for high milk production. Lower metabolic hormone concentrations (e.g., insulin and IGF-I) may contribute to a decrease in ovarian LH responsiveness and create gonadotrophin insensitivity at the ovary (see above). It is possible that the compromised state of LH 145

8 Table 1. Summary of estrous cyclicity based on analyses of progesterone profiles in traditional and modern lactating dairy cows (Roche et al., 2000). Item Number of cycles Normal pattern, % Anestrus, % Temporary cessation of cycle, % Prolonged luteal phase, % Short cycles, % Other irregular patterns, % Traditional Modern Figure 3. Mechanisms linking poor LH secretion and suboptimal follicular growth to estrous cycle and ovarian abnormalities (Lucy, 2003). Low blood estradiol concentrations may be created by low follicular estradiol secretion (secondary to low LH pulsatility and [or] low blood growth factor concentrations) and enhanced estradiol metabolism during high nutrient intake. A variety of estrous cycle and ovarian abnormalities may be linked to low blood estradiol concentrations (see text for details). 146

9 secretion and sensitivity continues in the cyclic animal and disrupts functional aspects of the dominant follicle. Early phases of luteolysis are initiated by estradiol. Long luteal phases in dairy cattle selected for milk production, therefore, may be caused by dominant follicles that are developmentally compromised and produce insufficient estradiol to initiate the luteolytic cascade. Temporary cessation of estrous cycles could be caused by a similar estradiol-mediated mechanism where dominant follicles are incapable of producing enough estradiol to trigger an LH surge for ovulation. We found that 21% of luteal-phase postpartum dairy cows treated with a luteolytic dose of PGF2α failed to ovulate the preovulatory follicle (J.M. Borman and M.C. Lucy, unpublished). The phenomenon was completely reversed by estradiol treatment after PGF2α injection. Insufficient blood estradiol concentrations, therefore, may be a causative factor leading to ovulation failure following spontaneous luteolysis or luteolysis induced in estrous synchronization programs. Oocyte quality Snijders et al. (2000) found that in vitro fertilized oocytes from dairy cows in low body condition had a lower cleavage rate and a lower developmental rate when compared with oocytes from dairy cows in better body condition. The exact period of nutritional im-printing of the oocyte is not known but many have speculated that it occurs during the two months that it takes for a follicle to progress from the primordial to preovulatory stage. The possibility that modern dairy cattle have poor oocyte quality and low fertilization capacity in vivo has been raised by recent work comparing cleavage stage embryos from lactating and nonlactating dairy cattle (Wiltbank et al., 2001). Percent normal embryos 4 to 5 days after estrus was low (58%) for lactating cattle and less than historical values reported in the literature. Nonlactating dairy cattle had a percentage normal embryos that was comparable to historical values for normal lactating cattle (82%). The percentage of early stage embryos in lactating cows approached that expected for repeat-breeder cattle described in the 1970 s (cows with four or more inseminations and failing to achieve pregnancy). Poor oocyte quality and poor early embryonic development may reflect a compromised state of follicular development in postpartum cattle. The compromised follicular development may be ultimately tied to factors linking nutrition to reproduction. Size and steroidogenic capacity of the corpus luteum Undernutrition may compromise pregnancy through its effects on the corpus luteum. There is a positive association between blood progesterone concentrations and pregnancy (Lamming and Darwash, 1998). Cattle that are underfed have smaller corpora lutea and lower blood progesterone concentrations (Gombe and Hansel, 1973). The effect of nutrition on corpora lutea size is likely a consequence of nutritional effects on the follicle before breeding. 147

10 Cyclic cattle that are underfed have progressively smaller and less estrogenic dominant follicles before they succumb to anestrus (Bossis et al., 1999). The smaller dominant follicles give rise to smaller corpora lutea. Steroidogenic capacity of luteal cells is also dependent on hormones such as GH, insulin and IGF-I that are controlled by the nutrition of the cow (Lucy, 2000). Uterine function Seemingly normal embryos may fail to develop within the uterus because cattle in poor body condition may not synthesize adequate amounts of embryotrophic growth factors that are required by filamentous embryos. Mapletoft et al. (1986) examined pregnancy rates after embryo transfer and found that recipients with low body condition score had lower conception rates when compared to recipients with high body condition score. Embryonic loss after day 28 of pregnancy was highest in cows losing the greatest amount of body condition (Silke et al., 2002). The IGF system is nutritionally regulated and is clearly resident within the uterus and embryo. Treating cows with GH after insemination increased conception rates perhaps through an embryotrophic mechanism involving IGF-I. (Bilby et al., 1999; Moreira et al., 2000). There-fore, the IGFs may be tied to a nutritional (body condition) effect on the embryo. Conclusions The reproductive physiology of postpartum cows is complex because of lactation. Blood GH concentrations increase shortly after calving. Greater blood GH coordinates nutrient partition-ing; the process through which nutrients are preferentially targeted for milk production. Blood GH concentrations are elevated and blood insulin and IGF-I concentrations are low in cows in negative energy balance. Many reproductive abnormalities can be linked to nutrient partitioning and negative energy balance through hormonal mechanisms involving GH, IGF-I, and insulin. Understanding the signals through which GH, IGF-I and insulin regulate ovarian function, uterine function and early embryonic development will reveal critical control points that can be manipulated to improve reproductive efficiency in cattle. REFERENCES Bauman DE Bovine somatotropin and lactation: from basic science to commercial application. Domestic Animal Endocrinology 17: Beam SW, Butler WR Effects of energy balance on follicular development and first ovulation in postpartum dairy cows. Journal of Reproduction and Fertility Supplement 54:

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