Evaluation of the association between plasma glucose-dependent insulinotropic polypeptide, respiratory quotient, and intramuscular fat deposition in feedlot cattle fed different levels of dry matter intake.

The objectives of this trial were to evaluate the association between different levels of dry matter intake (DMI) on gas exchange, plasma glucose-dependent insulinotropic polypeptide (GIP) concentration, and intramuscular (IM) fat deposition. We used 60 individually fed backgrounded Angus × SimAngus-crossbred steers ( n = 30) in a randomized complete block design. Steers (paired by body weight [BW] and gain to feed ratio [G:F]) were randomly allocated to one of the following treatments: ad libitum intake (AI) or restricted intake (RI; the same diet fed at 85% of the AI) of a finishing diet. The diet contained 61% cracked corn, 9% corn silage, 15% distillers' dried grains with solubles, 5% soyhulls, and 10% of a protein-mineral-vitamin premix. Measurements of CO 2 emission and consumption of O 2 , and respiratory quotient (RQ) were taken using the GreenFeed system ( n = 15/treatment). Plasma and gas samples were collected 10 d before slaughter, 1 h before and 2 h after feeding. Plasma glucose, non-esterified fatty acids, GIP, and insulin concentration and gasses (O 2, CO 2, and RQ) were analyzed using the MIXED procedure of SAS evaluating the fixed effect of treatment, time (repeated measurement) and their interaction, and the random effect of the block. Final BW and carcass characteristics were analyzed with a similar model, without the time statement and its interaction. Compared with RI, AI steers had greater ( P < 0.01) DMI and average daily gain (ADG). Steers on AI had greater final BW ( P = 0.02), tended to have a greater ribeye area ( P = 0.09), and had lower plasma GIP concentration ( P = 0.04). There was no treatment effect ( P ≥ 0.11) on G:F, subcutaneous backfat (BF), and IM fat, O 2 consumption, CO 2 emission, and RQ. Plasma glucose concentration of AI steers was greater before and after feeding than RI ( P < 0.05). In conclusion, feeding steers ad libitum increased DMI, ADG, and plasma glucose and GIP concentration but does not affect G:F, BF, IM fat, CO 2 emission, and O 2 consumption. Plasma GIP concentration and RQ are not associated with IM fat deposition.