Phenotypic relationships among methane production traits assessed under ad libitum feeding of beef cattle.

Angus cattle from 2 beef cattle projects in which daily methane production (MPR) was measured were used in this study to examine the nature of the relationships among BW, DMI, and methane traits of beef cattle fed ad libitum on a roughage diet or a grain-based feedlot diet. In both projects methane was measured using the GreenFeed Emission Monitoring system, which provides multiple short-term breath measures of methane production. The data used for this study were from 119 Angus heifers over 15 d on a roughage diet and 326 Angus steers over 70 d on a feedlot diet. Mean (+/- SD) age, BW, and DMI were 372 +/- 28 d, 355 +/- 37 kg, and 8.1 +/- 1.3 kg/d for the heifers and 554 +/- 86 d, 577 +/- 69 kg, and 13.3 +/- 2.0 kg/d for the steers, respectively. The corresponding mean MPR was 212 g/d for heifers and 203 g/d for steers. Additional traits studied included methane yield (MY; MPR/DMI), methane intensity (MPR/ BW), and 3 forms of residual methane production (RMP), which is a measure of actual minus predicted MPR. For RMPB, RMPD, and RMPDB predicted MPR were obtained by regression of MPR on BW, on DMI, and on both DMI and BW, respectively. The 2 data sets were analyzed separately using the same statistical procedures. For both feed types the relationships between MPR and DMI and between MPR and BW were both positive and linear. The correlation between MPR and DMI was similar to that between MPR and BW, although the correlations were stronger for the roughage diet (r = 0.75 for MPR vs. DMI; r = 0.74 for MPR vs. BW) than the grain-based diet (r = 0.62 for MPR vs. DMI; r = 0.66 for MPR vs. BW). The correlation between MY and DMI was negative and moderate for the roughage (r = -0.68) and grain-based (r = -0.59) diets, a finding that is different from the nonsignificant correlations reported in studies of cattle on a restricted roughage diet. The 3 RMP traits were strongly correlated (r values from 0.76 to 0.99) with each other for both the roughage and the grain-based diets, which indicates that using RMPB to lower MPR could provide a result similar to using RMPD in cattle. As feed intake (DMI) is more difficult to measure than BW, this result implies that under ad libitum feeding situations in which DMI cannot be measured, RMPB can be used to identify higher-or lower-RMP animals with similar levels of effectiveness as RMPD.