Metabolic differences in Angus steers divergently selected for residual feed intake

Residual feed intake measures variation in feed intake independent of liveweight and liveweight gain. First generation steer progeny ( n = 33) of parents previously selected for low or high post-weaning residual feed intake were examined to determine metabolic processes contributing to variation in residual feed intake. Blood samples were taken from the steers from weaning through to slaughter. These samples were analysed for key metabolites and hormones. Total urine and total faecal collections were taken from the steers in an animal-house experiment to estimate dry matter digestibility, microbial protein production and protein turnover. At weaning, there were phenotypic correlations between concentrations in plasma of ß-hydroxy butyrate ( r = 0.55, P <0.001), aspartate aminotransferase ( r = 0.34; P <0.001), urea ( r = 0.26, P <0.1) and total plasma protein ( r = 0.26, P <0.1), and subsequent residual feed intake over the whole experiment (feedlot plus animal-house phases), but no evidence of associations with genetic variation in residual feed intake. At the start of the feedlot residual feed intake test period plasma levels of glucose, creatinine and aspartate aminotransferase were correlated with residual feed intake over the experiment ( r = 0.40, –0.45 and 0.43, respectively, P <0.05), providing evidence of phenotypic associations with residual feed intake, and concentrations of urea and triglycerides were correlated with sire estimated breeding values for residual feed intake ( b = 1.20 and –0.08, respectively, P <0.05), providing evidence for genetic associations with residual feed intake. At the end of the experiment, concentrations of plasma insulin, cortisol and leptin were correlated with residual feed intake over the experiment ( r = 0.43, –0.40 and 0.31, respectively, P <0.05). Plasma concentrations of urea, insulin and cortisol illustrated trends for an association with sire estimated breeding values for RFI ( b = –0.35, 0.98 and 12.19, respectively, P <0.1). The ratio of allantoin : creatinine in urine, as a measure of rumen microbial production, tended to be correlated with residual feed intake in the animal house ( r  = 0.32, P <0.1) but not with residual feed intake over the entire experiment ( r = 0.10, P >0.05). Neither the ratio of 3-methyl histidine : creatinine in urine, as a measure of rate of muscle breakdown, nor the dry matter digestibility measured in the animal house were correlated with residual feed intake in the animal house ( r = 0.04, P >0.05), or residual feed intake over the whole experiment ( r = – 0.22, P >0.05), and neither were associated with genetic variation in residual feed intake. It is hypothesised that high-RFI (low-efficiency) steers have higher tissue energy requirements, are more susceptible to stress and utilise different tissue substrates (partly as a consequence of differences in body composition) to generate energy required in response to exposure to a stressful stimulus.