133 Ranking Mature Beef Cows for Residual Intake Using an Unprocessed Grass Hay Diet and Its Relationship to Greenhouse Gas Exchange

Abstract

Abstract The objective of this study was to compare high and low residual feed intake (RFI) beef cows when consuming an unprocessed, grass hay diet (93% DM, 8.73% CP, 2.11 Mcal ME, and 59.25% TDN). Relationships of hay intake to carbon dioxide (CO2) and methane (CH4) production were also evaluated. Thirty-six mature Angus and Angus x Hereford cows were assigned randomly to five pens (n = 6 or 9) and individual feed intakes were collected for 52-d using SmartFeed intake units (C-Lock Inc., Rapid City, SD) at a stocking rate of 3 cows per feeder. An open-circuit, gas-quantification system (GreenFeed, C-Lock Inc., Rapid City, SD) was used to collect CO2 (7611.63±714.09 g) and CH4 (283.64±37.10 g) production for 9 days of collection per pen for a minimum of 20 visits per cow (n = 22 cows, 36.18±14.24 visits). Data collection on cows included (means ± SD) dry matter intake (DMI; 13.86±2.16 kg), metabolic body weight (MBW, 111.65±9.28 kg), average daily gain (ADG, 0.77±0.29 kg), and rib eye area by ultrasound (REA, 46.62±6.34 cm).To calculate RFI (0.03±1.71), a multiple regression model was used to predict DMI (PDMI = ADG + REA + MBW; R2=0.50, P = 0.001). Cows were categorized into three RFI classes: low (n = 7), moderate (n = 6), and high (n = 9). Differences between RFI and RFI class and DMI and RFI class were analyzed using PROC MIXED in SAS (SAS, Cary, NC). Low RFI cows (-1.81 kg) consumed 12.08 kg hay DM daily and high RFI cows (1.59 kg) consumed 15.27 kg hay DM daily (P < 0.001). Correlations between DMI and daily gas exchange were estimated using PROC CORR in SAS (SAS, Cary, NC). Pearson Correlation Coefficients were significant between DMI and CO2 (r=0.58, P = 0.004), and CH4 (r=0.70, P < 0.001) production. Differences in feed intake for high and low RFI rankings were similar to previously reported literature when growing cattle consumed energy-dense diets.