Effect of calving season on mineral status and metabolic disorders in postpartum grazing dairy cows in Uruguay.

In grazing dairy systems, seasonal changes in climate and forage conditions may influence feed intake, mineral supply, and energy balance during the transition period. Thus, to evaluate the association between calving season and the odds of hypomagnesemia, hypophosphatemia, elevated non-esterified fatty acids (NEFA), and low body condition score (BCS) in Holstein dairy cows, a cross-sectional study was conducted on 865 cows from 122 commercial dairy farms in Uruguay. Blood samples were collected once from each cow between 1 and 3 days postpartum, and multivariable logistic regression was used to assess associations between calving season and metabolic outcomes, accounting for parity and its interaction with season. Compared with summer-calving cows, autumn- and winter-calving cows had higher odds of hypomagnesemia, and winter-calving cows also had increased odds of hypophosphatemia and elevated NEFA concentrations. A significant interaction between parity and season was observed for low BCS, with the greatest odds in autumn- and winter-calving multiparous cows and in winter-calving primiparous cows. In conclusion, calving season, particularly winter, was associated with greater mineral and metabolic imbalances and inadequate BCS in grazing dairy cows in Uruguay.