Relationship of efficiency indices with performance, heart rate, oxygen consumption, blood parameters, and estimated heat production in Nellore steers

The objective of this study was to examine the relationship of efficiency indices with performance, heart rate, oxygen consumption, blood parameters, and estimated heat production (EHP) in Nellore steers. Eighteen steers were individually lot-fed diets of 2.7 Mcal ME/kg DM for 84 d. Estimated heat production was determined using oxygen pulse (O2P) methodology, in which heart rate (HR) was monitored for 4 consecutive days. Oxygen pulse was obtained by simultaneously measuring HR and oxygen consumption during a 10- to 15-min period. Efficiency traits studied were feed efficiency (G:F) and residual feed intake (RFI) obtained by regression of DMI in relation to ADG and midtest metabolic BW (RFIREG). Alternatively, RFI was also obtained based on equations reported by the NRC's Nutrient Requirements of Beef Cattle to estimate individual requirement and DMI (RFI calculated by the NRC [1996] equation [RFINRC]). The slope of the regression equation and its significance was used to evaluate the effect of efficiency indices (RFIREG, RFINRC, or G:F) on the traits studied. A mixed model was used considering RFIREG, RFINRC, or G:F and pen type as fixed effects and initial age as a covariate. For HR and EHP variables, day was included as a random effect. There was no relationship between efficiency indices and back fat depth measured by ultrasound or daily HR and EHP (P > 0.05). Because G:F is obtained in relation to BW, the slope of G:F was positive and significant (P < 0.05). Regardless of the method used, efficient steers had lower DMI (P < 0.05). The initial LM area was indirectly related to RFIREG and RFINRC (P < 0.05); however, the final muscle area was related to only RFINRC. Oxygen consumption per beat was not related to G:F; however, it was lower for RFIREG- and RFINRC-efficient steers, and consequently, oxygen volume (mL.min(-1).kg(-0.75)) and O2P (mu L O-2.beat(-1).kg(-0.75)) were also lower (P < 0.05). Blood parameters were not related to RFIREG and RFINRC (P > 0.05); however, G:F-efficient steers showed lower hematocrit and hemoglobin concentrations (P < 0.05). Differences in EHP between efficient and inefficient animals were not directly detected. Nevertheless, differences in oxygen consumption and O2P were detected, indicating that the O2P methodology may be useful to predict growth efficiency.