Abstract
The objectives of this study were: i) to classify animals into groups of high and low feed efficiency (FE) using three FE indexes (Residual feed intake (RFI), Residual weight gain (RG) and Feed conversion efficiency (FCE)), and ii) to evaluate whether crossbreed Holstein x Gyr heifers divergent for FE indexes exhibit differences in nutrient intake and digestibility, energy partitioning, heat production, methane emissions, nitrogen partitioning and blood parameters. Thirty-five heifers were housed in a tie-stall, received ad libitum TMR (75:25, corn silage: concentrate) and were ranked and classified into high (HE) or low efficiency (LE) for RFI, RG and FCE. The number of animals for each HE group were 13 (< 0.5 standard deviation (SD) for RFI, 11 for RG and 11 for FCE (> 0.5 SD) and for the LE were 10 (> 0.5 SD) for RFI, 11 for RG and 12 for FCE (< 0.5 SD). Gas exchanges (O2 consumption, CO2 and CH4 production) in open-circuit respiratory chambers and whole tract digestibility trial was performed. A completely randomized experimental design was used and the data were analyzed by ANOVA and correlation study. High efficiency animals for RFI produced less CO2, consumed less O2 and had lower heat production (HP). Methane production was positively correlated with RFI. High efficiency RG had higher O2 consumption and CO2 production in relation to LE-RG. High efficiency FCE had greater NFC digestibility, higher positive energy balance (EB) and excreted (11.4 g/d) less nitrogen in urine. High efficiency RG and FCE groups emitted less CH4 per kg of weight gain than LE animals. Animals HE for RFI and FCE had lower β-hydroxybutyrate and higher glucose concentrations, respectively. The differences in intake, digestibility, energy and nitrogen partition, CH4 emission, blood metabolic variables and heat production between the HE and LE groups varied according to the efficiency indexes adopted. The HP (kcal/d/BW0.75) was lower for HE animals for RFI and FCE indexes.
Highlights
Increasing the efficiency of livestock production systems is essential to improve productivity and to reduce negative environmental impacts
The hypothesis of the present study were: i) Despite of which Feed efficiency (FE) index used to classify animals in high efficiency (HE) and low efficiency (LE), the groups will present physiological differences that justify a better use of energy and protein in HE group, and ii) Classifications by different feed efficiency indexes will result in different physiological responses between animals classified HE and LE
The differences of intake, digestibility, energy and nitrogen partition, CH4 emission, blood metabolic variables and heat production between HE and LE groups varied according to FE index adopted
Summary
Increasing the efficiency of livestock production systems is essential to improve productivity and to reduce negative environmental impacts. Feed efficiency (FE) is a highly important economic trait in milk production systems[1,2]. The most efficient animals have lower production costs[3,4]. According to Arthur and Herd[4], there is individual variation in nutrient utilization efficiency among animals with similar characteristics. Differences in efficiency between animals can be dependent on body weight, stage of production, growth composition, environmental conditions and their interactions with other factors such as rumen kinetics, digestion, absorption and efficiency of energy and protein utilization[5,6]. The relationships between groups of high and low FE and the effects on energy and nitrogen partitioning, blood metabolic variables and gas exchanges are not well understood
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