# Effects of cow efficiency phenotype on performance, energy partitioning, ultrasound carcass composition, and gas exchange during gestation and lactation

## TL;DR

This study shows that efficient beef cows consume less feed without sacrificing productivity, but their efficiency rankings change during different life stages.

## Contribution

The study reveals that efficiency classifications for beef cows are not stable across physiological stages and should be assessed within specific production phases.

## Key findings

- RFI-efficient cows consumed less forage during gestation and lactation without negative effects on body weight or carcass composition.
- RNE-efficient cows allocated more energy to tissue accretion during gestation and showed a similar trend during lactation.
- Methane emissions per kilogram of dry matter intake were higher in RFI-efficient cows during gestation but not during lactation.

## Abstract

This study investigated the effects of residual feed intake (RFI) and residual net energy recovered (RNE) classifications on the performance, carcass composition, milk production, and gas exchange of crossbred Angus beef cows consuming an unprocessed forage diet during gestation and lactation. Thirty-six multiparous fall-calving cows were monitored for feed intake, body weight, body condition, ultrasound-based carcass traits, milk yield and quality, carbon dioxide and methane emissions, and oxygen consumption using automated feeding and gas measurement systems. Cows were retrospectively classified as efficient, moderate, or inefficient for RFI and RNE during each physiological stage. RFI-efficient cows consistently consumed less forage, both in absolute terms and as a percentage of body weight during gestation (both, P < 0.01) and lactation (both, P ≤ 0.03). Divergence in RFI did not have negative effects on body weight, average daily gain (ADG), body condition score or carcass composition at gestation (P ≥ 0.13) or lactation (P ≥ 0.13). However, RFI classification significantly affected allocation of energy to maintenance in gestating (P < 0.01) and lactating (P = 0.02) cows, with inefficient cows allocating more metabolizable energy to maintenance (MEm) compared to moderate and efficient cows. RNE-efficient cows had greater ADG during gestation (P < 0.01) and had a similar tendency in lactation (P = 0.10), allocating more net energy to tissue accretion (P ≤ 0.05, both stages). Neither RFI nor RNE classification affected milk yield or composition (P ≥ 0.18), except for RNE-efficient cows having greater milk lactose concentration (P = 0.01) than RNE-moderate cows. Methane emissions per kilogram of DMI were greater in RFI-efficient cows during gestation (P < 0.01) but not during lactation (P = 0.22). Notably, while feed intake was more persistent across stage of production (r = 0.66), efficiency classifications were not stable across physiological stages (r = 0.33 and 0.25 for RFI and RNE, respectively). In addition, there was a moderate relationship between RFI and RNE at gestation (r = 0.36) and weak relationship during lactation (r = 0.24). These findings suggest that RFI and RNE are complementary but distinct measures of efficiency, and that selection for either trait can improve feed efficiency and a more targeted achievement of objectives in beef cow herds without compromising productivity or maternal performance.

These results suggest that intake and maintenance requirements exhibit greater persistence across physiological stages, whereas efficiency rankings are less stable. This suggests the need to assess forage-use efficiency within biologically relevant phases of the cow-calf production cycle rather than assuming a single, stable measure of lifetime efficiency.