Effects of mild heat stress periods on milk production, milking frequency and rumination time of grazing dairy cows milked by a mobile automatic system

Grazing dairy cows milked by an automatic system (AS) experienced mild heat stress (HS) periods twice during the summer. The daily temperature humidity index (THI) during these periods were higher than 72. Milk production, as well as milking frequency, rumination time and milk fat to protein ratio (F/P) during these periods were compared with adjacent periods with mean THI of 61. The daily milking frequency, the total number of visits to the AS, and milk production were significantly higher in HS periods (2.12 vs. 1.97, 2.99 vs. 2.69, and 19.7 vs. 18.5 kg milk per cow, respectively). There were significant interactions between times and periods for milking frequency and number of visits, whereas the daily rumination time was significantly lower (339 vs. 419 min) and the F/P in milk tended to be decreased (1.17 vs. 1.23). These results could be explained by changes in cow behaviour during HS periods.