Abstract
The objective of this study was to evaluate the plasma concentrations of insulin-like growth factor-I (IGF-I), and the mRNA hepatic expression of IGF-I and of the growth hormone receptors GHR and GHR 1A, in postpartum beef cows. Four Angus and four crossbred (Angus x Nelore) postpartum suckled beef cows were used. Liver and blood samples were collected every 10 days, from calving to 40 days postpartum, for gene expression and for β-hydroxybutyrate and IGF-I assays, respectively. Samples for progesterone assay were collected every other day, from day 10 to 40 postpartum. Three cows ovulated before 40 days postpartum. IGF-I concentration was higher in Angus x Nelore than in Angus cows. There was no difference in the expression of GHR, GHR 1A and IGF-I according to breed or ovulatory status. IGF-I concentrations were higher in crossbred cows, but have not changed according to postpartum ovulatory status. Moreover, changes in postpartum IGF-I concentrations are not associated with changes in liver GHR, GHR 1A and IGF-I mRNA expression in either breed.
Highlights
The main objective of cow‐calf rearing is to produce one calf per cow annually
During the early postpartum period, there is a simultaneous reduction in growth hormone receptor (GHR) 1A and insulin‐like growth factor I (IGF‐I) mRNA expression in the liver of dairy cows due to intense negative energy balance (Radcliff et al, 2003a), no similar condition was observed for beef cows (Jiang et al, 2005)
The most important observation of this study was the marked increase in plasmatic IGF‐I concentration in postpartum crossbred cows Angus x Nelore, in comparison to Angus cows, despite no difference in the hepatic expression of GHR/IGF‐I mRNA
Summary
The main objective of cow‐calf rearing is to produce one calf per cow annually. knowledge of cow reproductive biology has increased (Santos et al, 2004), factors involved in the resumption of postpartum cyclicity in cows are still unclear. The stress of pregnancy, parturition, onset of lactation and suckling negatively affect the energy intake (Ciccioli et al, 2003). This induces a postpartum negative energy balance, which is associated with a prolonged interval from calving to first ovulation (Stagg et al, 1998). During the early postpartum period, there is a simultaneous reduction in GHR 1A and IGF‐I mRNA expression in the liver of dairy cows due to intense negative energy balance (Radcliff et al, 2003a), no similar condition was observed for beef cows (Jiang et al, 2005). The reduced expression of these genes in liver led to reduced plasma IGF‐I concentration, which is restored about three weeks postpartum due to decreasing intensity of the negative energy balance (Kobayashi et al, 1999)
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