Associations of serum fatty acids, serum urea nitrogen, and ruminal ammonia nitrogen with residual feed intake in lactating dairy cows.

Feed efficiency is critical in dairy farming, affecting production costs and environmental sustainability. The development of the trait residual feed intake (RFI) has provided an opportunity to select dairy cows that are more efficient in converting nutrients into milk. Note that RFI requires individual daily intake records, which are typically collected on a limited number of research farms. In this context, the identification of biomarkers that can be used to identify and select more feed-efficient cows is of great interest. As such, this study aimed to identify ruminal and serum biomarkers associated with RFI in mid-lactation Holstein cows. A selected subset of 24 out of 454 Holstein cows was used in this study. This subset was strategically selected to represent extremes of least feed-efficient (LFE; n = 12, RFI = 2.44) and most feed-efficient (MFE; n = 12, RFI = -2.69) cows with no difference in the 3 energy sinks, namely BW change, metabolic BW, and energy secreted in milk. Rumen fluid and serum samples were collected between 60 and 90 DIM. Rumen fluid samples were collected using an oro-esophageal tubing procedure. Serum samples were used to measure fatty acids using a 2-step assay. The fatty acid methyl ester was assessed using solid-phase extraction and quantified using the chromatographic peak area and internal standard-based calculations. Ruminal ammonia nitrogen was measured using a phenol-hypochlorite assay, and serum urea was measured using a commercial ELISA kit. Cows in the MFE group had higher ruminal ammonia nitrogen concentrations than cows in the LFE group. There were no differences in serum urea concentration between MFE and LFE cows. Serum fatty acid concentrations differed between groups, with myristic acid (C14:0), palmitic acid (C16:0), cis-heptadecenoic acid (cis-9-17:1), stearic acid (C18:0), and total SFA having greater concentrations in the MFE group than in the LFE group. The total PUFA concentration was lower in the MFE group than in the LFE group. A model incorporating C14:0, C16:0, palmitoleic acid (trans-9-C16:1), anteiso-heptadecanoic acid plus palmitoleic acid (C17:0+trans-13-C16:1), oleic acid (cis-9-C18:1), cis-vaccenic acid (cis-11-C18:1), petroselinic acid (cis-12-C18:1), C18:0, linoleic acid (C18:2n-6), dihomo-γ-linolenic acid (C20:3n-6), cis-MUFA, n-6 PUFA, total PUFA, total SFA, and other or unknown fatty acids was used to assess goodness-of-fit for RFI and showed an adjusted R of 0.74. When ruminal ammonia nitrogen was added to the previous model, the adjusted R improved to 0.84. Our findings provide evidence that ruminal ammonia nitrogen and serum fatty acids are associated with RFI, thus suggesting that these metabolites might be helpful in identifying more feed-efficient dairy cows.