Description:
<jats:title>Abstract</jats:title>
<jats:p>The objective of this study was to compare residual feed intake (RFI) and greenhouse gas exchange in mature beef cows during gestation and lactation when consuming an unprocessed, grass hay diet. Twenty-six mature Angus and Angus x Hereford cows (4.5 ± 0.0 yr) were used in two voluntary intake studies. Cows were evaluated during mid-gestation (GEST) and late lactation (LACT). Unprocessed grass hay (GEST = 8.73% CP and 59.25% TDN; LACT = 8.31% CP and 54.50% TDN) and a mineral mix were provided for ad libitum consumption. Individual animal feed intake data was collected for 52 d and 71 d for GEST and LACT respectively using SmartFeed intake units (C-Lock Inc., Rapid City, SD) and 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 CH4 production for 9 days per pen for a minimum of 20 visits per cow. Hay intake averaged 13.8 ± 2.3 kg and 16.6 ± 2.0 kg during GEST and LACT, respectively. The Pearson correlation coefficient for hay intake among the two stages of production was 0.65 (P &lt; 0.01). Metabolic body weight, average daily gain, and ultrasound ribeye area explained 46% of the variation in GEST DMI (P &lt; 0.01), and milk yield, metabolic body weight, and average daily gain explained 64% of the variation in LACT DMI (P &lt; 0.01). Each one kg additional milk production resulted in 0.7 kg greater hay intake during LACT (P &lt; 0.01). There was a moderate correlation between GEST and LACT CH4 (r = 0.64) and a moderate correlation between CH4 production within each stage of production and DMI for that same stage of production (r = 0.58 and 0.63 for GEST and LACT, respectively). Hay intake was moderately correlated between stages of production although highly sensitive to milk yield.</jats:p>