Similar feed-intake levels yield no differences in energy utilisation between beef heifers identified as low (efficient) and high (inefficient) for residual feed intake

Context

Improving cattle feed efficiency would reduce feed costs and increase herd profitability. Residual feed intake (RFI) is commonly used to rank cattle for feed efficiency, with low-RFI animals being more efficient than high-RFI animals. However, RFI classification merits further investigation because observed differences in heat energy (HE) production between low- and high-RFI cattle may be related to heat associated with differences in dry-matter intake (DMI) rather than maintenance-energy requirements.

Aims

To determine energy partitioning of beef heifers with low and high residual feed intake.

Methods

Angus crossbred heifers (n = 60) were fed a grower diet (metabolisable energy (ME) = 2.17 Mcal/kg DM) for 70 days. Feed intake was recorded daily using the GrowSafe system, and bodyweight (BW) was recorded every 14 days. Residual feed intake was calculated as the residual from the regression of DMI on mid-test BW^0.75 and average daily gain (ADG) (R² = 0.31). Low-RFI (n = 8) and high-RFI (n = 8) heifers were selected for a metabolism experiment to determine energy partitioning at three feed-intake levels, namely, ad libitum intake, and 1.0× and 0.5× expected maintenance-energy requirement. Apparent nutrient digestibility was determined using indigestible neutral detergent fibre (iNDF) as an internal marker. The sulfur hexafluoride (SF₆) tracer and oxygen-pulse techniques determined methane emissions and heat production respectively. Metabolisable energy required for maintenance (MEm) and fasting heat production (H_eE) were then calculated from the regression of log HP on ME intake (MEI). Efficiencies of ME used for maintenance and growth were calculated from H_eE, MEm, and retained energy at ad libitum intake.

Key results

Residual feed intake was strongly correlated with DMI (0.83). Low-RFI heifers consumed 31% less (P = 0.01) feed than high-RFI heifers during the performance experiment. Heifers with low RFI had greater MEm, but similar efficiencies of ME use for maintenance and gain as did high RFI heifers.

Conclusion

These data indicated that selection based on RFI may not lead to improved energy efficiency in growing heifers.

Implications

The results of this study indicated that low-RFI cattle may not have lower maintenance-energy requirements or differences in efficiencies of ME use than do high-RFI cattle.