Dietary starch concentration influences milk protein via the phenylalanine and tyrosine metabolism in early lactation Holstein-Friesian dairy cows

Application: Diets containing high levels of starch alter phenylalanine and tyrosine metabolism and increase milk protein concentration. This information can be used to more accurately formulate diets to alter milk protein content in dairy cows.

Introduction: Starch is frequently included in dairy cow rations to fulfil energy and protein requirements. Studies have reported changes in phenylalanine metabolism as a result of varying dietary starch concentrations in lactating Holstein-Friesian dairy cows [1] but subsequent effects on milk protein have yet to be determined. This study aimed to examine the metabolic changes in plasma associated with varying dietary starch concentration in early lactation Holstein-Friesian dairy cows.

Materials and methods: Sixteen Holstein-Friesian dairy cows that were 38.9 (±3.4) days in milk and yielding 42.6 (±1.6) kg/d were blocked according to milk yield in the week prior to being allocated to 1 of 2 dietary treatments. Cows remained on study for a period of 98 days, and the 2 diets were formulated to contain dietary starch concentrations of either 231 (HS) or 103 g/kg of DM (LS). Plasma samples collected during weeks 0, 1, 2, 4, 6, 8 and 14 were assessed using flow infusion electrospray high resolution mass spectrometry on a Q Exactive hybrid quadrupole-Orbitrap mass spectrometer for high throughput, sensitive, non-targeted metabolite fingerprinting. Variables of importance for the
projection (VIP) scores (>1) in Partial Least Squares – Discriminant Analyses (PLS-DA) were used to indicate mass-ion (m/z) values which discriminated between the classes. Performance parameters, including milk composition, were measured by t.test repeated measures using Genstat (v18).

Results: PLS-DA and hierarchical cluster analysis of the data discriminated between treatments. In total, 43 of these discriminatory metabolites were identified using the mummichog algorithm within MetaboAnalyst 4.0. Metabolite set enrichment analysis (MSEA) using over representation analysis demonstrated the significant effects (P <0.05) of dietary starch on phenylalanine and tyrosine metabolism (Figs. 1 and 2). The mean milk protein concentration of cattle on treatment HS was 3.07 g/kg (P < 0.05) higher than the LS treatment, with mean concentrations of 32.51 and 29.44 g/kg, respectively. No differences in other performance parameters (P > 0.05) were observed.

Conclusion: Feeding high starch diets decreased concentrations of multiple metabolites relating to phenylalanine and tyrosine metabolism, and was positively correlated with milk protein concentration. This suggests that phenylalanine metabolism is altered due to low amino acid supply within the low starch diets.