Methane production and methanogen levels in steers that differ in residual gain

Methane (CH₄) gas released by cattle is a product of fermentation in the digestive tract. The 2 primary sites of CH₄ production in ruminants are the reticulum-rumen complex and the cecum. Methane release from cattle represents a 2% to 12% loss of the energy intake. Reducing the proportion of feed energylost as CH₄ has the potential of improving feed efficiency as well as decreasing the contribution of cattle to greenhouse gas production. Feed intake and growth were measured on 132 fall-born steers for 70 d. Seven steers with extreme positive residual gain (RG) and 7 steers with extreme negative RG whose DMI was within 0.32 SD of the mean intake were selected for subsequent measurements. Enteric CH₄ production was measured via indirect calorimetry. Rumen, cecum, and rectal contents were obtained from steers at slaughter for measurement of in vitro CH₄ production and methanogen 16S rRNA levels. Enteric CH₄ production did not differ (P = 0.11) between the positive RG (112 ± 13 L/d) and the negative RG (74 ± 13 L/d) steers. In vitro rumen methane production did not differ between positive RG (64.26 × 10⁻⁵ ± 10.85 × 10⁻⁵ mmol∙g⁻¹ DM∙min⁻¹) and negative RG (61.49 × 10⁻⁵ ± 10.85 × 10⁻⁵ mmol∙g⁻¹ DM∙min⁻¹; P = 0.86). In vitro cecum methane production did not differ between positive RG (4.24 × 10⁻⁵ ± 1.90 × 10⁻⁵ mmol∙g⁻¹ DM∙min⁻¹) and negative RG (4.35 × 10⁻⁵ ± 1.90 × 10⁻⁵ mmol∙g⁻¹ DM∙min⁻¹; P = 0.97). Methanogen 16S rRNA as a percentage of the total bacteria16S rRNA did not differ between RG groups (P = 0.18). The methanogen 16S rRNA as a percentage of rumen fluid total bacteria 16S rRNA (5.3% ± 3.1%) did not differ from the methanogen 16S rRNA as a percentage of cecum content total bacteria 16S rRNA (11.8% ± 3.1%; P = 0.14). The methanogen 16S rRNA as a percentage of the rectum content total bacteria 16S rRNA (0.7% ± 3.1%) was not different from the rumen content (P = 0.29) but was less than the cecum content (P = 0.01). Methanomicrobiales 16S rRNA as a percentage of total methanogen 16S rRNA did not differ across sample sites (P = 0.81); however, steers with positive RG (10.5% ± 1.6%) were more numerous than steers with negative RG (5.1% ± 1.6%; P = 0.02). Cattle that differ in RG at the same DMI do not differ in characteristics associated with CH₄ production.