132. 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.