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Feed efficiency indexes in crossbred Holstein × Gyr heifers and its effects on energy and nitrogen partitioning, blood metabolic variables and gas exchanges.

J. P. Sacramento

M138
Feed efficiency indexes in crossbred Holstein × Gyr heifers and its effects on energy and nitrogen partitioning, blood metabolic variables and gas exchanges.
D. C. Silva1, J. P. Sacramento*4, L. G. R. Pereira2, J. A. M. Lima3, F. S. Machado2, A. L. Ferreira3, T. R. Tomich2, S. G. Coelho3, R. M. Mauricio4, M. M. Campos2. 1State University of Southwestern Bahia Itapetinga, Bahia, Brazil, 2Brazilian Agricultural Research Corporation EMBRAPA Dairy Cattle Juiz de Fora, MG, Brazil, 3Federal University of Minas Gerais Belo Horizonte, MG, Brazil, 4Federal University of São João del Rei São João del Rei, MG, Brazil.

The effects of divergent phenotypic classification in Holstein × Gyr (F1) heifers for residual feed intake (RFI), residual weight gain (RG) and feed conversion efficiency (FCE) in relation to digestibility, energy and nitrogen partition, CH4 emission, blood metabolic variables and heat production were evaluated in this study. Thirty-five heifers were ranked into 2 RFI, RG and FCE groups: high efficiency (HE) and low efficiency (LE). The animals were housed in a tie-stall facility and received TMR (75:25, corn silage: concentrate, 175 CP/kg, 4084 kcal/kg GE and 422 g/kg NDF in DM). Digestibility was evaluated during 5-d of total collection of feces and 2-d of urine. Gas exchanges (O2 consumption, CO2 and CH4 production) were obtained in open-circuit respirometry chambers. A completely randomized design was used and the data were analyzed by ANOVA and correlation study. The comparison of means between the divergent groups was performed by Fisher's test. High efficiency animals for RFI consumed less O2 (P = 00.4) and produced less CO2 (P = 0.05). Heat production (HP) was lower for HE-RFI animals (171 vs 178 kcal/d/BW0.75; P = 0.05). Methane production was positively correlated with RFI, (P = 00.3). The HE-RG had higher O2 consumption (2562 vs 2425 L/d; P = 00.2) and higher CO2 production (2689 vs 2562 L/d, (P = 00.4) in relation to LE-RG. The HE-FCE, had greater NFC digestibility (P = 00.1), higher positive energy balance (P = 00.4), and excreted (12.3 g/d), less N in urine (P = 00.2). HE-RFI animals had lower β-hydroxybutyrate concentrations (0.80 vs 0.88 mmol/L; P = 00.2) and HE-FCE had higher concentrations glucose (5.24 vs 4.92 mmol/L; P = 00.1) in relation to LE-FCE. High efficiency RG and FCE groups emitted less CH4 per kg of weight gain than LE animals (P = 00.1 and P = 00.4) respectively. The differences in performance, nutritional and metabolic parameters between the HE and LE groups varied according to the efficiency indexes adopted.

Keywords: digestibility, methane, residual feed intake.