Evaluating Forage-Based Feed Efficiency Metrics in Angus Replacement Heifers

Abstract The objective of this study was to evaluate forage utilization efficiency in Angus replacement heifers. A two-year study was conducted using 109 fall-born, weaned Angus heifers (n = 55 in Year 1; n = 54 in Year 2) at the Oklahoma State University Range Cow Research Center. Pens were equipped with individual feed intake monitoring systems (C-Lock Inc., Rapid City, SD) fitted with hay baskets. The study included a 21-day adaptation period followed by 71-days of data collection. Heifers had ad libitum access to bermudagrass hay (12% CP, 56% TDN) and a vitamin/mineral supplement. Multiple regression analysis was used to determine residual feed intake (RFI) for each animal, which were then grouped through K-means clustering based on RFI efficiency classification within contemporary groups: efficient (n = 23, Year 1; n = 17, Year 2), moderate (n = 19, Year 1; n = 20, Year 2), and inefficient (n = 13, Year 1; n = 17, Year 2). Growth performance was evaluated using linear mixed models in R (version 4.3.1) to analyze the effects of RFI and RFIgain classifications. In the original RFI analysis, RFI-efficient heifers consumed less forage (6.19 kg/d) than moderate (6.87 kg/d) and inefficient (7.67 kg/d) groups (P 0.01), with no differences in body weight or hip height (P 0.13). Average daily gain showed a tendency to differ among efficiency groups (P = 0.07), with efficient heifers gaining slightly less (0.45 kg/d) compared to moderate (0.53 kg/d) and inefficient (0.51 kg/d). It was decided to incorporate an ADG threshold (RFIgain) to exclude animals gaining less than one standard deviation below the group mean. Applying RFIgain eliminated 22% and 35% of the originally classified "efficient" heifers in Year 1 and Year 2, respectively. Similar adjustments were made in the moderate group (16% in Year 1; 20% in Year 2), with no disqualifications among inefficient heifers. After removing these animals, no differences in ADG, body weight, or hip height were found across efficiency groups (P 0.15). However, intake differences across groups remained (P 0.01). These findings suggest that, although the RFI index is effective in identifying heifers with lower forage intake, it frequently classifies cattle exhibiting unacceptable forage-based ADG (17% of all heifers) as either ‘efficient’ or ‘moderate’. Consequently, sole reliance on RFI might be inadequate for selecting biologically efficient animals suited for forage-based systems. Incorporating a threshold for performance may improve efficiency while also identifying replacements capable of thriving in a strict forage environment.