Reproduction in Domestic Animals

34 MC Lucybe 18–21 days (North American systems) or closer to28 days (European systems). After weaning, sows arereturned to a lower level of intake (approximately onehalfto one-third of their lactation ration).The preceding paragraph illustrates the complexity ofthe question when an attempt is made to understand theimpact of nutrition on the somatotropic axis in alactating sow. Feeding and management practices havea major effect on the outcomes of studies of metabolichormones. It is necessary to examine managementdetails carefully when inspecting the scientific literatureon the somatotropic axis in sows.Growth hormone, IGF-I and insulinFew studies have examined intensively the concentrationsof GH, IGF-I, IGFBP and insulin in the sowduring lactation. There is an increase in blood GHconcentrations after farrowing (Schams et al. 1994;Mejia-Guadarrama et al. 2002; Govoni et al. 2007).The increase in blood GH is associated with an increasein blood NEFA concentrations during lactation. Theincrease in NEFA implies that GH is mediating lipidcatabolism during lactation. The increase in GH duringlactation is caused partly by the suckling stimulus fromthe piglets (Rushen et al. 1993). The suckling-inducedGH release explains why a relatively high blood GHconcentration is sustained throughout lactation even inwell-fed sows. With regard to GH, the lactating sow issimilar to the lactating dairy cow because both specieshave elevated GH during lactation. The causes ofelevated GH may be different (lactating dairy cows arenot suckled) but they may also share some commonmechanisms because catabolic states typically elevateGH.One apparent difference between cows and sowsinvolves the recoupling of the somatotropic axis postpartum(Fig. 1). Post-partum sows have elevated IGF-Iafter farrowing (Schams et al. 1994; Govoni et al. 2007)and this endocrine state suggests that the increase in GHafter farrowing can stimulate the liver to synthesize andsecrete IGF-I. In the dairy cow there is a large decreasein IGF-I after calving that is associated with anuncoupling of the somatotropic axis (Radcliff et al.2003a; b). Little is known about changes in IGF-I inbeef cattle around calving but there appears to be atleast some reduction in IGF-I on day 3 post-partumrelative to day 6 post-partum (Lake et al. 2006).There are discrepancies with respect to insulin concentrationsin lactating sows. Some studies reportgreater insulin concentrations in sows after farrowing(Guedes and Nogueira 2001), whereas others reportlower insulin concentrations after farrowing (Revellet al. 1998). The latter case (lower insulin) is similar towhat is reported for cattle after calving and this lowerinsulin appears to be a consequence of metabolic glucoseconsumption during lactation. Sows may become insulin-resistantduring lactation (Quesnel et al. 2007).Sows fed more energy have greater blood IGF-I postpartum(de Braganca and Prunier 1999; van den Brandet al. 2001). The increase in IGF-I post-partum may bepartly explained by the fact that gestating sows are fedmaintenance diets during late pregnancy. In the gestatingsow, therefore, feeding level may limit liver IGF-Iproduction. The combination of elevated GH andad libitum feeding after farrowing drives the somatotropicaxis and increases liver IGF-I production. Theincrease in IGF-I may be a consequence of elevated GHbut also may arise from feed-dependent mechanismssuch as greater liver GHR concentration, an enhancedcapacity for GH to signal through the GHR orGH-independent mechanism through which feedingincreases IGF-I.During lactation, IGF-I concentration may remainhigh (well-fed sows) or may decrease over time (underfedsows) (van den Brand et al. 2001). Presumably, inthe underfed sow, IGF-I concentrations decrease duringlactation because the somatotropic axis becomes uncoupled.The uncoupling may occur in the second and thirdweek of lactation when litter milk consumption and sowmilk production are greater (Noblet and Etienne 1989).The sow enters into negative energy balance because sheFig. 1. Conceptual diagram for changes in blood GH, blood IGF-I and liver GHR after parturition (P; farrowing or calving) in sows (leftdiagram) and dairy cows (right diagram). In sows, the somatotropic axis remains coupled after farrowing and GH concentrations increasebecause of suckling. This leads to elevated GH and IGF-I during lactation. During the latter half of lactation, the somatotropic axis becomesuncoupled perhaps because there is negative energy balance and the amount of GHR in liver is less. Weaning (W) causes a decrease in GH andIGF-I. The blood IGF-I concentration increase after weaning as the sow approaches breeding (B). In dairy cows, the liver GHR decreases aftercalving and this causes uncoupling of the somatotropic axis. Loss of the GHR leads to a decrease in blood IGF-I. The decrease in blood IGF-Imay alleviate GH negative feedback and cause elevated GH in early lactation. Improved energy balance and insulin may increase the GHR andIGF-I and reduce GH concentrations as cows approach the breeding period (B).Ó 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag