Examining Relationships Between Residual Feed Intake Classification, Enteric Emissions, and Apparent Total Tract Digestibility in Yearling Beef Heifers

Abstract Improving feed efficiency in beef heifers may help reduce enteric emissions and improve economic returns for producers. The objective of this project is to assess the relationship between traditional residual feed intake (RFI) efficiency classification, enteric gas emissions, and apparent total tract nutrient digestibility in growing yearling heifers. Angus crossbred heifers (n = 74, BW = 408 ±27.1 kg) were blocked by BW and randomly assigned to pen groups. All heifers were fed a common haylage-based diet for ad libitum intake. Insentec feeding stations (Insentec B.V., Marknesse, The Netherlands) were used to measure individual feed intake and feeding behavior data. To measure enteric gas emissions of methane (CH4), carbon dioxide (CO2), and oxygen (O2), heifer groups were rotated through pens with GreenFeed trailers (C-Lock Inc., Rapid City, SD, USA) weekly. Apparent total tract digestibility was determined by collecting fecal samples from each heifer every 9 h over three days, composited and dried at 65 °C for 96 h, and analyzed for nutrient values (A&L Canada Laboratories, London, ON, Canada). Nutrient digestibility was determined for each heifer using acid-insoluble ash (AIA) and undigestible neutral detergent fiber (uNDF) as internal markers. RFI was determined using a regression of midpoint BW and ADG and then heifers were ranked by RFI quartile group. Data were analyzed using PROC GLIMMIX in SAS, with efficiency quartile group as the fixed effect and pen as a random effect, comparisons between quartile groups were assessed with a Tukey test. As expected, efficient (least RFI) heifers had decreased dry matter intake (8.5 vs 10.1 ± 0.20 kg/d, P < 0.01), less bunk visits (70 vs 112 ± 6 visits/day, P < 0.01), spent greater time per visit (2.46 vs 1.31 ± 0.249 min/visit, P < 0.01), and consumed more feed per visit (145 vs 104 ± 11.9 g DM/visit, P < 0.03), which resulted in a faster eating rate (65 vs 98 ± 5.0 g DM/min, P < 0.01), and produced decreased enteric CO2 emissions (7,653 vs 8,120 ±122.6 g/d, P = 0.05) compared with inefficient (greatest RFI) heifers. Additionally, when using AIA as the internal marker, efficient heifers showed greater digestibility of organic matter (89.8 vs 91.4 ± 0.49 %, P = 0.02) and NDF (20.2 vs 30.0 ± 2.66 %, P = 0.04) apparent total tract digestibility, while using uNDF as the internal marker showed no difference (P > 0.20) between efficiency groups. These data indicate that more efficient yearling heifers consume less feed, have reduced enteric gas emissions, and have improved organic matter and NDF apparent total tract digestibility.