Calculating Energy Efficiency Use For Nellore Bulls Divergent In Residual Feed Intake

Abstract Retained energy (RE) and energetic efficiency index were measured in Nellore bulls from divergent classes of residual feed intake (RFI). Thirty-four Nellore bulls (15 low RFI-LRFI and 19 high RFI-HRFI) were feedlot finished and slaughtered with 385 ± 40 kg of body weight (BW) and 520 ± 26.2 days of age. At the beginning of the experiment, five LRFI and three HRFI were slaughtered and used as base line. Individual dry matter intake was recorded daily; initial and final BW were recorded after 16 h of fasting. Eight bulls, four LRFI and four HRFI, were fed at maintenance, receiving 65 g of DM/kg0.75 BW, and 18 bulls (10 LRFI and eight HRFI) were fed ad libitum. Diet had 19:81 roughage:concentrate, 88% of DM and 15% of crude protein. Ultrasound measurements on the Longissimus muscle were performed at intervals of 28 days. When two ad libitum bulls reached 4 mm of subcutaneous fat thickness, one maintenance bull was randomly chosen and slaughtered on the same day. After slaughter, the centesimal composition of the empty body and carcass was measured. Data were analyzed using a random coefficients model, and RFI class was included as a fixed effect. Least-square means were used to compare the means, and significance was declared for P ≤ 0.05. The LRFI had greater protein retention than HRFI (248 vs. 142 g/d; P = 0.009), and same fat and energy retention (313 g/d and 4.12 Mcal/d, respectively). The energy efficiency indexes, heat production per metabolic energy intake (Mcal/Mcal) and gain-to-feed (kg/kg) ratio, did not differ between RFI classes. Though LRFI had the same energy efficiency index, they were leaner. These results indicated an association of RFI and maturity patterns. Further research is needed to estimate the net energy requirements of Nellore bulls classified according to RFI. Acknowledgments: FAPESP Processes 2017/06709-2, 2018/20080–2 and 2019/17714-2.