Effect of divergence in phenotypic residual feed intake on methane emissions, ruminal fermentation, and apparent whole-tract digestibility of beef heifers across three contrasting diets

This study aimed to examine the effect of divergent phenotypic ranking for residual feed intake (RFI) on ruminal CH4 emissions, diet digestibility, and indices of ruminal fermentation in heifers across 3 commercially relevant diets. Twenty-eight Limousin × Friesian heifers were used and were ranked on the basis of phenotypic RFI: 14 low-RFI and 14 high-RFI animals. Ruminal CH4 emissions were estimated over 5 d using the SF6 tracer gas technique on 3 successive occasions: 1) at the end of a 6-wk period (Period 1) on grass silage (GS), 2) at the end of an 8-wk period (Period 2) at pasture, and 3) at the end of a 5-wk period (Period 3) on a 30:70 corn silage:concentrate total mixed ration (TMR). Animals were allowed ad libitum access to feed and water at all times. Individual DMI was estimated during CH4 measurement and rumen samples were taken at the end of each CH4 measurement period. Diet type affected all feed intake and CH4 traits measured (P < 0.01) but was unavoidably confounded with animal age/size and experimental period. Correlation coefficients between RFI and DMI were significant (P < 0.05) only when animals were fed the TMR. Daily CH4 correlated with DMI (r = 0.42, P < 0.05) only when animals grazed pasture. Daily DMI was lower in low-RFI animals (P = 0.047) but only when expressed as grams per kilogram metabolic BW. Absolute CH4 emissions did not differ between RFI groups (P > 0.05), but CH4 yield was greatest in low-RFI heifers (P = 0.03) as a proportion of both DMI and GE intake. Interactions between the main effects were observed (P < 0.05) for CP digestibility (CPD), DM digestibility (DMD), ruminal propionate, and the acetate:propionate ratio. Low-RFI animals had greater (P < 0.05) CPD and DMD than their high-RFI contemporaries when offered GS but not the other 2 diets. Low-RFI heifers also had greater OM digestibility (P = 0.027). Additionally, low-RFI heifers had a lower concentration of propionate (P < 0.05) compared with high-RFI heifers when fed GS, resulting in a greater (P < 0.05) acetate:propionate ratio. However, these differences were not evident for the other 2 diets. Energetically efficient animals do not have a lower ruminal methanogenic potential compared with their more inefficient counterparts and, indeed, some evidence to the contrary was found, which may reflect the greater nutrient digestive potential observed in low-RFI cattle.