Studies for searching factors relating rumination time around the heat season in Holstein milking cows

To identify factors affecting rumination time (RT), factorial analyses in a dairy farm were performed. In univariate analyses, differences in distribution were observed between low (< 45 kg/d) and high ([&ge;] 45 kg/d) milk yield (MY) before the heat season, early and peak lactation periods and mid and late at the start of the heat season, and 1-2 and [&ge;] 3 parity at the start of the heat season. Multiple regression analysis confirmed that RT was affected by low MY, early to peak and 1-2 parity cows, low MY, mid to late and 1-2 parity cows, and low MY, early to peak and [&ge;] 3 parity cows (coefficients = -37.039, -25.353, -44.805 respectively, P < 0.05). When the cows were classified by MY before the heat season, the RT and MY of the high-MY group remained higher than the low-MY group until before the severe heat (THI [&ge;] 84 continued). However, when cows were classified by RT before the heat season, there was no difference in MY between the low-RT (< 485 min/d) and the high-RT group (> 485 min/d). In conclusions, MY is a factor affecting RT up to moderate heat but RT is not a sufficient condition affecting MY.