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Relationships between feed efficiency and carcass characteristics, meat quality, heart rate and oxygen consumption in beef cattle There is growing interest in the use of feed efficiency in beef cattle, due to its relationship to production costs and environmental impact. Identification of variation in energy losses can explain differences in efficiency. The objective of this study was to evaluate measures of feed efficiency (residual feed intake – RFI and residual intake and gain _ RIG) and their relationships with performance, carcass characteristics, meat quality, heart rate and oxygen use in Nellore bulls. A group of 84 animals were evaluated for performance, carcass characteristics (weight, yield, loin eye area, fat thickness, weight of visceral and internal fat) and meat quality in fresh and aged meat samples (pH, shear force, cooking loss, water retention capacity, myofibrilar fragmentation index and intramuscular fat). To estimate heat production (HP) by O2 pulse methodology (O2P), 39 animals were monitored for heart rate (HR). A subgroup of 18 animals which was calibrated for O2 consumption determined over in 18 of them. HP was estimated multiplying daily heart beats by O2 volume per beat, assuming 4.89 kcal/LO2 as constant. The slope coefficient for each variable against RFI and RIG was studied. There was no association of RFI with weight (P>0.05) and weight gain (P>0.05), but efficient animals consumed 16.1% less feed (P<0.0001). Efficient animals for RIG consumed 11.7% less food (P<0.0001), but gained more weight than inefficient animals (P<0.01). There was no association between RIG and average metabolic weight (P>0.05), however RIG was related to final weight (P=0.06). HP estimated by the difference between retained energy and metabolic energy intake was higher for inefficient animals (P<0.001), for both RFI and RIG. However there was no effect on HP estimated by the O2P methodology (P>0.05). Heart rate (HR) and O2 consumption were not associated with RFI and RIG (P>0.05), but HR was lower in efficient animals during O2 calibration (P<0.05). There was a positive association of efficiency with a leaner gain composition, where efficient animals for RFI had of lower ultrasound subcutaneous fat (P=0.08), while efficient animals for RIG had higher loin eye area (P<0.05). There was no effect on meat quality parameters for RFI but in efficient animals for RIG, there was an effect on meat and fat color (P<0.05) compared to inefficient animals. Internal fat and liver weights were decreased in efficient animals according to RFI (P<0.05), but unchanged according to RIG (P>0.05). Efficient animals for RFI and RIG had lower cost of production resulting in increased profit (P<0.01). Profit was strongly associated with feed intake, RIG and weight gain (r =-0.81, 0.72 and 0.61, P<0.0001), whereas its association with RFI was lower (r =-0.46, P<0.0001). Efficient animals for RFI and RIG are more profitable with small decreases in fat and small increases in muscle content. Profit is better related to RIG than RFI. However, RIG had the disadvantage of increasing final test weight which could be genetically associated with mature weight.

Relações entre eficiência alimentar e características de carcaça, qualidade de carne, batimentos cardíacos e consumo de oxigênio em bovinos