Short communication: effect of grazing on the concentrations of total sialic acid and hexose in bovine milk.

Sialic acid, which is located at the terminal end of glycoconjugates, is believed to have important biological functions. Its concentration in bovine milk varies depending on lactation stage and season. However, it remains unclear whether dietary factors, especially fresh forage, affect the total sialic acid concentration in milk. The purpose of the present study was to investigate the effect of grazing on the concentrations of total sialic acid and hexose in bovine milk. Six healthy dairy cows were used in a crossover design (3 cows fed fresh forage and 3 cows fed grass silage) for 2 wk. Individual milk samples were collected at 2 consecutive milkings (morning and evening) at 0, 1, 3, 5, 8, 11, and 14 d of the experimental period, and 2 consecutive samples in each cow were combined on each sampling day in proportion of the morning and evening milk yields. No differences in body weight, milk yield, or milk composition were observed between the 2 groups during the experimental period. The hexose concentration in milk did not differ between these groups during the experimental period. Conversely, the total sialic acid concentration in the milk of each grazing cow significantly increased at 11 and 14 d of the experimental period compared with that at 0 d. In the grass silage group, the total sialic acid concentration at the end of the experimental period tended to be lower than that at 0 d, but the decrease was not significant. These results indicate that grazing management could have increased the concentration of sialoglycoconjugates in milk. This suggests that grazing may increase the biological function of milk because it is thought that sialic acid is significant in many ways.