Serum amino acid concentrations in high and low efficiency finishing steers

Abstract Text: We hypothesized that circulating AA concentrations differ between finishing cattle that vary in efficiency of feed utilization. The objective of this study was to investigate serum AA concentrations in steers classified as high and low efficiency based on residual feed intake (RFI). Hereford-Angus crossbred steers (n = 75, 412 ± 3.8 kg initial BW) from a single contemporary group (birth through slaughter) were used in this study. Steers were fed a finishing diet (13.2% CP, 1.8 Mcal NE_m/kg, 1.19 Mcal NE_g/kg; DM basis) for 80 d using the GrowSafe system to monitor intake.  Residual feed intake was calculated as the difference between actual feed intake and expected feed intake of each individual, where expected intake was determined by regressing ADG and metabolic midweight on actual intake. At the end of the feeding period, the 20% most efficient (low RFI, n = 8) and 20% least efficient (high RFI, n = 8) steers with 12^th rib fat thickness ≥ 1.02 cm were selected for slaughter 6 or 8 d after the end of the feed intake test. During exsanguination at slaughter, blood was collected for serum AA analysis. Data were analyzed with PROC MIXED in SAS 9.2 using RFI class (high vs. low efficiency) as a fixed effect. Serum relative histidine concentration (% of total AA) tended to be greater (P = 0.08) in high efficiency than low efficiency steers, whereas all other relative AA were unaffected (P ≥ 0.23) by RFI class. The serum concentrations of 19 individual common AA (cysteine was not analyzed), total essential AA, and total AA in addition to ornithine, citrulline, taurine, hydroxproline, 3-methylhistidine, 1-methylhistidine, carnosine, β-alanine, α-amino-N-butyric acid, and cystathione were unaffected (P ≥ 0.13) by RFI class. All relative essential AA (% of total essential AA) were also unaffected (P ≥ 0.16) by RFI class. Data suggest that histidine metabolism may differ in high and low efficiency cattle, resulting in altered circulating relative histidine concentrations.

Keywords: amino acids, metabolites, residual feed intake