Comparison of different residual carbon dioxide formulations as a means to select feed-efficient dairy cows

Improving feed utilisation efficiency and environmental sustainability by the selection of superior animals are amongst the widely studied topics during the last decade. For the evaluation of individual's feed utilisation efficiency, residual feed intake (RFI) has become the common metric and is defined as the difference between actual and expected feed intake. Lately, a new metric for carbon dioxide (CO₂) called residual CO₂ (RCO₂) is being developed and similarly defined as RFI. However, the partial regression coefficients for expected feed intake obtained by regressing DM intake (DMI) on energy sinks may not be biologically plausible and this could also be the case for CO₂. The objective of this study was to compare RCO₂ and RFI formulations calculated using different partial regression coefficients of energy sinks obtained either from regression on energy sinks or from different energy requirement formulations used nationally or internationally. The correlations between these different formulations as well as production, efficiency, and BW measurements were also calculated. Repeated daily measurements of CO₂ production (n = 51 977) using two GreenFeed Emissions Monitoring system and records of DMI from 83 primiparous Nordic Red dairy cows were used. Three types of RCO₂ and RFI formulations were calculated. The first was by fitting a multiple linear regression (RCO_2MLR and RFI_MLR) whereas the second and third were based on the Finnish energy requirement formulation (RCO_2FIN and RFI_FIN) and National Research Council 2021 (NRC, 2021; RCO_2NRC and RFI_NRC), respectively. Correlations between different RCO₂ and RFI formulations were lower (from 0.37 to 0.44) than the correlation between CO₂ production and DMI (0.58) implying that selection based on different RFI formulations may lead to selection of different sets of animals. Selection based on RCO₂ formulations would lead to improvement in energy conversion efficiency (ECE) albeit with a slightly lower rate compared to selection based on RFI formulations. However, the decline in the trend of CO₂ production would be enhanced when selection is based on RCO₂ rather than RFI. Of all the residual formulations studied in Finnish dairy cows, the use of RCO_2FIN is preferred because it had higher favourable correlations with ECE, CO₂ and methane emission per unit of energy-corrected milk. Due to its high correlation with DMI, the conventional RFI could favour cows with lower DMI, regardless of their milk production. More data are needed to further verify the correlation between CO₂ production and feed intake.