Lay Summary In ruminants, the role of glucose-dependent insulinotropic polypeptide (GIP) in energy metabolism has not been widely studied. This experiment is aimed to evaluate the effect of lipids, as calcium salts of palm oil (CPO), and amount of feed offered on plasma GIP concentration and its association with the energy metabolism, measured as respiratory quotient (RQ), and marbling score (MS) in feedlot steers. Steers were fed a corn silage-based diet for 30 d and subsequently placed in a ground corn-based diet until harvest. Diets were offered at different amounts (ad libitum or restricted) with or without the inclusion of CPO. Growth performance, blood samples, and gas exchange were measured over time. Overall, steers fed diets with CPO gained more body weight when they were restrict-fed but not when they were ad libitum fed. There was no association between GIP and RQ. Insulin tended to be positively and negatively correlated with plasma GIP and RQ, respectively. There was no association between GIP and MS. Our results suggest that GIP secretion results from an interaction between dietary CPO and energy intake depending on the time relative to feeding and that GIP is not associated with MS. There is little information regarding the role of glucose-dependent insulinotropic polypeptide in ruminants. This experiment investigated the dietary inclusion of supplementary fats and the amount of feed offered as potential secretagogues of this gastrointestinal tract peptide, and its association with energy metabolism and beef quality of feedlot steers. Sixty Angus x SimAngus-crossbred steers (body weight [BW] 279 +/- 16 kg) were used to evaluate the effect of calcium salts of palm oil inclusion (CPO) and the amount of feed offered (AFO) on plasma glucose-dependent insulinotropic polypeptide (GIP) concentration and its association with energy metabolism and marbling score (MS) in feedlot steers. Steers were blocked by BW and gain to feed (G:F) and randomly assigned to individual feedlot pens. Treatments (2 x 2 factorial) consisted of ad libitum-fed steers without (ANF) or with (AWF) the inclusion of CPO or restricted-fed steers (85% of the ad libitum intake of ANF) without (RNF) or with the inclusion of CPO (RWF). After weaning, steers were adapted to individual pens and fed a corn silage-based diet for 30 d and subsequently placed in a ground corn (GC)-based diet. Diets were given ad libitum or at 85% of the ANF intake and with or without CPO. After 59 d on the finishing diet, all steers had ad libitum access to the finishing diet until harvest. Measurements of CO2 emission and O-2 consumption to estimate respiratory quotient (RQ) were taken (n = 9/treatment). Correlations between plasma GIP and insulin concentrations and RQ were analyzed. A linear regression was performed to evaluate the association of plasma GIP and MS. All data were analyzed using the PROC MIXED procedure of SAS. During the first 103 d of the trial, there were AFO x CPO interactions (P <= 0.01) for BW, dry matter intake (DMI), average daily gain (ADG), and net energy for maintenance (NEm) intake. Ad libitum-fed steers without CPO presented the greatest DMI among dietary treatments and had greater BW and ADG compared with steers in the RWF and RNF treatments. After all steers had ad libitum access to dietary treatments, steers that were previously restricted showed a 30% and 19% increase (P <= 0.01) in ADG and G:F, respectively. There was a three-way interaction time x CPO x AFO (P = 0.04) for plasma GIP concentration. There was no correlation (P = 0.96) of GIP with RQ, whereas insulin demonstrated marginal significance for a positive (P = 0.07) and negative (P = 0.08) correlation with plasma GIP and RQ, respectively. There was no association (P = 0.30) between GIP and MS. These data indicate that GIP secretion results from an interaction between CPO and energy intake depending on the time relative to feed intake that GIP might indirectly regulate energy metabolism through insulin secretion, and that GIP does not appear to be associated with MS.
