Reproduction in Domestic Animals

Reprod Dom Anim 43 (Suppl. 2), 31–39 (2008); doi: 10.1111/j.1439-0531.2008.01140.xISSN 0936-6768Functional Differences in the Growth Hormone and Insulin-like Growth Factor Axisin Cattle and Pigs: Implications for Post-partum Nutrition and ReproductionMC LucyDivision of Animal Sciences, University of Missouri, Columbia, MO, USAContentsGrowth hormone (GH) and insulin-like growth factor-I(IGF-I) control growth and lactation in cattle and swine.Insulin participates in the endocrinology of growth andlactation because insulin and GH are antagonistic in theiractions. Dairy cows experience a period of negative energybalance during the first 4–8 weeks post-partum. During thisperiod, their somatotropic axis (comprised of GH, the GHreceptor and IGF-I) becomes uncoupled and there is elevatedGH and diminished IGF-I in the circulation. Blood insulinconcentrations are low as well. Sows are different from dairycows because their somatotropic axis remains coupled duringlactation and both GH and IGF-I are elevated. Nonetheless,sows that become catabolic during lactation will havereduced IGF-I concentrations. Sows are inseminated afterweaning so their metabolic state is different from post-partumbeef and dairy cows that are inseminated when they arelactating. Dairy cows are fed ad libitum and naturally havelow IGF-I during lactation. Sows have elevated IGF-I whenthey are well-fed. A threshold of IGF-I protein in follicularfluid may be met by local ovarian (paracrine ⁄ autocrine) andendocrine sources of IGF-I. Nutritionally induced changes ininsulin and in liver IGF-I secretion that arise from perturbationsof the somatotropic axis have a direct effect on theovary through the endocrine actions of insulin and IGF-I.Sows and cows that are nutritionally compromised have lowconcentrations of insulin and IGF-I in their blood and thistheoretically reduces ovarian responsiveness to gonadotropins.Although sows are inseminated after weaning, thereappear to be carry-over effects of the previous lactation onthe ovarian follicular populations that develop after the sowis weaned. Understanding the mechanisms through whichmetabolic hormones control ovarian function may lead toimproved reproductive management of both pigs and cattlebecause lactation and post-partum reproduction are closelytied in both species.IntroductionGrowth hormone (GH) is a pituitary hormone thatcontrols growth and lactation. Insulin-like growthfactor-I (IGF-I) is released from liver in response toGH and is believed to control growth and lactation aswell (Renaville et al. 2002). There are a series of wellstudiedIGF-binding proteins (IGFBP) that inhibit IGFactivity (Le Roith et al. 2001). In addition to causingIGF-I release, GH antagonizes insulin action (Ethertonand Bauman 1998). Antagonizing the actions of insulinhas a nutrient partitioning effect through which thedevelopment of lean tissue and the production of milkare favoured. A complete understanding of nutrientpartitioning during growth, lactation and reproductiontypically involves an appreciation of GH, IGF-I, IGFBPand insulin because collectively they act on the individualanimal (Chagas et al. 2007).Reproductive events are controlled primarily bygonadotropins (LH and FSH). Growth hormone,IGF-I and insulin can promote gonadotropin actionby potentiating gonadotropin receptor function (Webbet al. 2004; Scaramuzzi et al. 2006). Each hormone canalso act directly through their respective hormonereceptors on the ovary. Metabolic events such aslactation, therefore, are tied to reproductive eventsthrough a hormonal link that involves GH, IGF-I andinsulin. Although not the primary focus of this review,additional metabolic hormones including leptin andadiponectin should be considered when nutrition, reproductionand lactation are discussed.The comparison of cattle and swine is interesting andworthwhile because it may give insights into underlyingbiological mechanisms through which lactation andreproduction are linked. Beef and dairy cattle aredistinctly different in terms of lactation and nutrientpartitioning. The underlying biology of GH, IGF-I andinsulin are different as well. In many respects, swine mayshare more similarities to beef cattle than dairy cattlebecause beef cattle and swine nurse their young.Machine milking systems for dairy cows were neverintended to recapitulate the physiology or lactationcurve of the nursed mother.Cattle and swine are very different with respect to thecoincidence of lactation and post-partum breeding(insemination). Post-partum beef and dairy cattle areinseminated while they are lactating. Post-partum swine,however, are inseminated after weaning. Although bothcattle and swine share common physiology with regardto the local actions of GH, IGF-I, IGFBP and insulinon the ovary, breeding a weaned animal compared witha lactating animal represents a distinct physiologicaldifference, particularly when metabolic hormones areconsidered. This review will examine the somatotropicaxis within cattle and swine during lactation andsubsequent changes in the axis after weaning. Implicationsfor reproduction will be examined with an eyetoward understanding nutrition and reproduction collectively.The Somatotropic Axis in Post-partum CattleDairy cows are well-known for the period of negativeenergy balance that they experience during the first 4–8 weeks post-partum (Drackley 1999). This period ofnegative energy balance is caused by the tremendouscapacity of the mammary gland to sequester nutrientsand synthesize milk. Cows cannot consume enough feedto meet the energy demands for milk production so theyenter into a negative energy balance. Negative energyÓ 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag