UC Davis

The present thesis consists of two chapters. The first chapter focuses on reviewing previously conducted research pertaining to feed efficiency, nitrogen metabolism, and fatty acids utilization in dairy cows to underscore the key points investigated in the research conducted during my master’s. The second chapter describes a study aimed to identify biomarkers associated with residual feed intake (RFI) and improve predictive models for feed efficiency. The study used a subset of 24 lactating Holstein cows representing extremes of least feed-efficient (LFE, n = 12, RFI = 2.44) and most feed-efficient (MFE, n = 12, RFI = -2.69) that had no difference in 3 energy sinks (body weight change, metabolic body weight, and energy secreted in milk) from a population of 454 genotyped cows with full phenotype for feed efficiency. Rumen fluid and serum samples were collected between 60 and 90 DIM. Rumen fluid samples were collected using an oroesophageal tubing procedure. Serum samples were used to measure fatty acids using a two-step assay. The fatty acid methyl ester was performed using solid phase extraction and quantified using chromatographic peak area and internal standard-based calculations. Ruminal ammonia nitrogen was performed using a phenol-hypochlorite assay, while serum urea was measured using a commercial ELISA test validate for the bovine species. 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 acids concentration differed between groups, with myristic acid, palmitic acid, cis-heptadecenoic acid, stearic acid, and total saturated fatty acids in greater concentrations in the MFE group than in the LFE group. Total polyunsaturated fatty acids concentration was lower in the MFE group than in the LFE group. Myristic acid, palmitic acid, total polyunsaturated fatty acids (PUFA), and saturated fatty acids (SFA) were correlated with RFI. A model incorporating myristic acid, palmitic acid, palmitoleic acid, ante iso heptadecanoic acid + palmitoleic acid, oleic acid, cis-vaccenic acid, petroselinic acid, stearic acid, linoleic acid, dihomo-γ-linolenic acid (DGLA), cis-monounsaturated fatty acids, omega 6 PUFA, total PUFA, total SFA, other and unknown fatty acids resulted in a R2 = 0.74. When rumen ammonia nitrogen was added to the previous model, an improvement was observed (Full R2 = 0.84). The findings of the current study provide evidence that ruminal ammonia and fatty acids in serum are correlated with RFI, and the integration of these metabolites may improve the prediction of RFI.