Consequences of accelerated gain and growth hormone administration for lipid metabolism in growing beef steers

Lipid accretion and metabolism during accelerated gain and growth hormone administration were examined in a 29-d trial with 24 beef steers. Treatments in this 2 X 2 factorial design consisted of level of feeding (restricted or ad libitum) and exogenous hormone [pituitary-derived bovine growth hormone (GH) at 38 IU/d or excipient]. Live weight gain was not affected by hormone treatment. Protein content of the 9-10-11th rib section was greatest in steers receiving GH irrespective of feeding level, whereas a reduction of lipid content due to GH was seen only with ad libitum feeding. Restricted-fed steers had the highest plasma concentrations of free fatty acids (FFA), and only at this feeding level did GH treatment result in further elevation of FFA concentrations. In vitro rates of adipose lipogenesis, esterification and lipolysis were greatest in tissue from ad libitum-fed steers. The only effect of GH on in vitro metabolism was a tendency for lower lipolytic rates with ad libitum feeding. GH did not affect adipocyte size. The mechanism for the effect of GH on lipid deposition could not be determined from incubations of adipose slices from chronically treated steers, although enhanced responsiveness to an in vivo lipolytic challenge was observed. By inference, substrate availability appears to determine the productive responses to GH.