Reproduction in Domestic Animals

GH and IGF-I in Cattle and Pigs 33have been selected for milk production independently ofHolstein, therefore, experience similar changes in GHR1A during the post-partum period. Angus beef cattle,however, did not undergo a decrease in GHR 1A atcalving (Jiang et al. 2005).One collective interpretation of these studies is thatGHR 1A expression partially coordinates the high milkproduction phenotype that typifies dairy cattle. Aftercalving there is a decrease in liver GHR 1A mRNA thatleads to low post-partum liver GHR expression. Theloss of liver GHR causes a GH refractory state in liverwhere liver does not produce IGF-I in response to GH.This refractory state represents an uncoupling of GHand IGF-I that is caused by the inactivation of theGHR. The subsequent decrease in blood IGF-I concentrationleads to diminished negative feedback andelevated pituitary production of GH. Elevated postpartumGH supports milk production.Beef cattle may not uncouple their somatotropic axisafter calving. The failure to uncouple this axis leads torelatively higher blood IGF-I concentrations and lowerblood GH concentrations. This endocrine state (higherIGF-I and lesser GH) is relatively anabolic whencompared with the endocrine state of low IGF-I andhigh GH. The uncoupling of the somatotropic axis inpost-partum dairy cows cannot be explained by a simplenutritional phenomena involving negative energy balancebecause feed restriction in later lactation dairycows did not decrease liver GHR 1A expression whenthe question was tested by two different laboratories(Kobayashi et al. 2002; Rhoads et al. 2007).Recoupling of the GH axis after calvingThe recoupling of the GH axis after calving in highproducingdairy cows has been linked to post-partumnutrition and energy balance. Insulin infusion into earlypost-partum dairy cows increased liver GHR 1AmRNA, liver IGF-I mRNA and blood IGF-I concentrations(Butler et al. 2003; Rhoads et al. 2004). Insulin,therefore, controls the recoupling through its positiveeffects on GHR 1A expression. There was also an effectof insulin on GHR in adipose tissue (reduced inresponse to insulin) but this effect was opposite to whatwas observed in liver (Butler et al. 2003). These data foran inhibitory effect of insulin on GHR mRNA need tobe reconciled with the fact that others have shown astimulatory effect of insulin on GHR in adipose tissue(Rhoads et al. 2004). Furthermore, GHR protein inadipose tissue was reduced in low-insulin states such asthose found in early post-partum dairy cows (Rhoadset al. 2004) and underfed dairy cows (Rhoads et al.2007).Greater nutrient intake and improved energy balancein later lactation dairy cows leads to elevated insulin anda direct effect on the GH axis via the effects of insulin onGHR expression (Lucy 2004). The recoupling of the axiscauses an increase in IGF-I that acts negatively on GHsecretion. The increase in insulin and IGF-I and thedecrease in GH switches the post-partum dairy cowfrom a catabolic state (low-insulin, low IGF-I andelevated GH) to an anabolic state (high insulin, highIGF-I and low GH).Little information is available on the recoupling of thesomatotropic axis in post-partum beef cows. The failureof post-partum beef cows to undergo a decrease in GHR1A at calving (Jiang et al. 2005) may imply that the axisis either never uncoupled or minimally uncoupled postpartum.Lake et al. (2006) reported that cows in lowerbody condition had greater GH and lesser IGF-I whencompared with cows in better body condition. Theuncoupling of the somatotropic axis for a beef cow inlow body condition, however, is conceptually differentfrom the homeorhetic mechanism that occurs in postpartumdairy cows. For beef cows, feeding additionalenergy or protein will typically recouple the somatotropicaxis and elevate IGF-I (Lalman et al. 2000). For thebeef cow, either the mammary gland is too small to usethe additional nutrients or the calf is too small toconsume the additional milk. In the latter case, failure tofully evacuate the gland promotes involution andessentially redirects nutrients back toward the mother.Dairy cows are fed ad libitum post-partum. Despite thisgenerous feeding their somatotropic axis is uncoupleduntil the demand of the mammary gland for nutrients isreduced (involution of the gland) or the capacity of thecow to consume nutrients (feed intake) increases. Ananalogous scenario existed when New Zealand andNorth American dairy cows were studied. Feeding 3 kgof concentrate improved body condition in post-partumNew Zealand Holstein–Friesian cows but the sameamount of concentrate feeding had no effect on bodycondition in North American Holstein cows (Rocheet al. 2006). In this comparison, the greater capacity ofthe North American (vs New Zealand) cows to producemilk affected the partitioning of nutrients to adiposetissue.The Somatotropic Axis in Post-partum SowsDifferences in animal management for pigs and cattleaffect the somatotropic axis within the respectivespecies. As mentioned above, lactating dairy cowsare fed ad libitum post-partum and continue to lactatethrough the breeding period. For the dairy cow,lactation is typically terminated when she is in the lasttrimester of gestation. Likewise for beef, calves areweaned in the autumn when cows have a well-establishedpregnancy. In the United States system, dairycows are fed a totally-mixed ration and this ration isbalanced for ad libitum intake. For beef cows and dairycows on pasture, pasture growth and quality maydetermine the overall energetics of the animal (Kolver2003).Obesity in sows creates farrowing difficulties andmetabolic problems, and also antagonizes post-partummilk production. Sows, therefore, are fed a maintenanceration during gestation and their body condition ismanaged carefully (Aherne and Williams 1992). Afterfarrowing and during lactation, ad libitum feeding ispracticed. Obviously, newborn piglets are small andtheir capacity to consume milk is low so sow milkproduction is lowest immediately after farrowing (Nobletand Etienne 1989). Greatest levels of milk productionare achieved near the time of weaning. The lengthof lactation differs between production systems and canÓ 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag