Reducing GHG emissions through genetic improvement for feed efficiency: effects on economically important traits and enteric methane production

Genetic selection for residual feed intake (RFI) is an indirect approach for reducing enteric methane (CH(4)) emissions in beef and dairy cattle. RFI is moderately heritable (0.26 to 0.43), moderately repeatable across diets (0.33 to 0.67) and independent of body size and production, and when adjusted for off-test ultrasound backfat thickness (RFI(fat)) is also independent of body fatness in growing animals. It is highly dependent on accurate measurement of individual animal feed intake. Within-animal repeatability of feed intake is moderate (0.29 to 0.49) with distinctive diurnal patterns associated with cattle type, diet and genotype, necessitating the recording of feed intake for at least 35 days. In addition, direct measurement of enteric CH(4) production will likely be more variable and expensive than measuring feed intake and if conducted should be expressed as CH(4) production (g/animal per day) adjusted for body size, growth, body composition and dry matter intake (DMI) or as residual CH(4) production. A further disadvantage of a direct CH(4) phenotype is that the relationships of enteric CH(4) production on other economically important traits are largely unknown. Selection for low RFI(fat) (efficient, −RFI(fat)) will result in cattle that consume less dry matter (DMI) and have an improved feed conversion ratio (FCR) compared with high RFI(fat) cattle (inefficient; +RFI(fat)). Few antagonistic effects have been reported for the relationships of RFI(fat) on carcass and meat quality, fertility, cow lifetime productivity and adaptability to stress or extensive grazing conditions. Low RFI(fat) cattle also produce 15% to 25% less enteric CH(4) than +RFI(fat) cattle, since DMI is positively related to enteric methane (CH(4)) production. In addition, lower DMI and feeding duration and frequency, and a different rumen bacterial profile that improves rumen fermentation in −RFI(fat) cattle may favor a 1% to 2% improvement in dry matter and CP digestibility compared with +RFI(fat) cattle. Rate of genetic change using this approach is expected to improve feed efficiency and reduce enteric CH(4) emissions from cattle by 0.75% to 1.0% per year at equal levels of body size, growth and body fatness compared with cattle not selected for RFI(fat).