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The objective of this study was to rank beef heifers for residual feed intake and greenhouse gas emissions using an unprocessed, grass hay diet. Fifty yearling Angus heifers were monitored for 62 days using the SmartFeed individual intake measuring technology (C-Lock Inc., Rapid City, SD) in drylot pens at the Oklahoma State University Range Cow Research Center located in Stillwater, OK. Means ± SD for dry matter intake (DMI; 6.76 ± 1.20 kg), average daily gain (ADG; 0.51 ± 0.21 kg), body weight (BW; 338.67 ± 55.24 kg), and residual feed intake (RFI; -0.04 ± 0.80 kg) were recorded while the heifers were fed unprocessed long-stem grass hay (10.3% CP, 2.0 Mcal ME, and 56% TDN). Additionally, a subset of 29 heifers were subjected to the collection of gas flux data using an open-circuit, portable, gas-quantification system (GreenFeed, C-Lock Inc., Rapid City, SD) to measure (means ± SD) oxygen consumption (O2; 3979.51 ± 510.32 g), carbon dioxide production (CO2; 5299.65 ± 610.89 g), and methane production (CH4; 202.656 ± 28.30 g). Using R, RFI was calculated using a multiple regression equation to predict DMI using BW0.75 and ADG (P &amp;gt; 0.05; R2 = 0.37). To analyze forage utilization efficiency, heifers were classified into three RFI categories consisting of efficient (n = 16), average (n = 16), and inefficient (n =18). The ADG was low for all RFI classifications and similar for all three categories (P &amp;lt; 0.05). Heifers classified in the top third of RFI rank consumed 22% less forage per day compared to the inefficient heifers. The phenotypic correlation between DMI with CO2, CH4, and O2 was 0.66, 0.18, and 0.49, respectively. These data suggest that progress in feed efficiency can be achieved when cattle are tested using a moderate-quality, unprocessed forage diet.

132 Ranking Beef Heifers for Residual Intake Using an Unprocessed Grass Hay Diet and Its Relationship to Greenhouse Gasses