Vitamin B6 Status Assessed by Plasma Vitamers and Homocysteine in Lactating Dairy Cows and Its Relationship to Rumen Fermentation, Plasma Metabolites, and Milk Production in Different Environmental Conditions

The supply of vitamin B₆ from diet and rumen microbial synthesis is generally considered to be sufficient in dairy cows. However, increased milk yield and environmental factors may alter their vitamin B₆ status. This study aimed to clarify the vitamin B₆ status in lactating dairy cows milked with automatic milking system (AMS) under different environmental conditions. In the winter and summer feeding experiments, plasma concentrations of vitamin B₆ vitamers of pyridoxal (PL) and pyridoxal-5′-phosphate (PLP) with pyridoxic acid (PA), and homocysteine (Hcys) of the cows were assessed, along with their feed intake, milk production, rumen fermentation, and plasma metabolites. Higher PL but lower PA concentrations and PA/(PLP + PL) ratio in plasma were found in the summer experiment compared with the winter experiment, even though plasma PLP concentration, milk production, and dry matter intake were similar between the winter and summer experiments. Plasma concentration of vitamin B₆ vitamers and Hcys concentration did not correlate with milk production parameters. However, the PA/(PLP + PL) ratio in plasma negatively correlated with ruminal acetic acid composition. Summer environment and the variation of rumen fermentation could alter the turnover of vitamin B₆, even though plasma PLP concentration is likely maintained in lactating cows milked with AMS.