The phenotypic relationship between residual feed intake and ultrasound carcass traits in Santa Gertrudis steers

Numerous studies have been conducted characterizing phenotypic and genetic variation in RFI in beef cattle, but limited studies have examined effects of diet and/or stage of maturity on phenotypic reranking of cattle for feed efficiency traits. For this study, 2 trials were conducted with Santa Gertrudis steers (N = 233). Steers were fed a roughage-based diet (2.1 Mcal ME/kg DM) during the growing phase and a highgrain diet (3.0 Mcal ME/kg DM) during the finishing phase. Steers were weighed at 14-d intervals and DMI was measured (Calan gate or GrowSafe) for 70 d during both the growing and finishing phases, with ultrasound measurements obtained on Day 70 of each feeding phase. RFIp was calculated as the difference between actual DMI and expected DMI based on regression of DMI on ADG and mid-test BW0.75. Stepwise regression revealed that final back fat depth (BF) accounted for additional variation in RFI in both the growing (R2 = 0.43 vs. 0.46) and finishing (R2 = 0.54 vs. 0.56) phases. Therefore, RFIc was computed for both phases based on regression of DMI on ADG, mid-test BW0.75, and final BF depth. During the growing phase, RFIp was positively correlated (P < 0.01) with DMI (0.75), feed:gain ratio (0.49), and final BF depth (0.13) but not with ADG or BW. Feed:gain ratio was negatively correlated (P < 0.01) with ADG (-0.70) but was not correlated with DMI. During the finishing phase, RFIp was positively correlated (P < 0.01) with DMI (0.65), feed:gain ratio (0.51), and final BF depth (0.17) but not with ADG or BW. Feed:gain ratio was negatively correlated (P < 0.01) with ADG (-0.69) but was not correlated with DMI. Correlations between RFIp and RFIc were 0.91 and 0.92 during the growing and finishing phases, respectively. Spearman rank correlations revealed that ADG and feed:gain ratio measured during the growing phase were weakly associated with ADG (0.21) and feed:gain ratio (0.12) measured during the finishing phase, whereas rank correlations were moderate for DMI (0.47), RFIp (0.45), and RFIc (0.28) measured during the growing and finishing phases. Although these results indicate that a moderately positive rank correlation exists between RFI measured when steers are fed high-roughage vs. high-grain diets, these two feed efficiency traits may not be biologically similar. It is unclear as to whether the lack of a strong correlation between RFI measured during growing vs. finishing phases was due to influences of diet and/or stage of maturity.