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The study was carried out in summer season (from May to August) 2003 on 380 Italian Friesian cows raised in a free stall barn located in the Po Valley. The barn was equipped with fans plus misting in the feeding area and fans without misters in the resting area.

The temperature and humidity recorded during the trial indicate that the summer was very hot. The minimum daily temperatures (month average) varied from 12.7C in May to 20C in August. The maximum daily temperatures (month average) were of 27.2C in May and 35.5C in August. Even though the summer was very hot, the milk reduction during lactation in primiparous and multiparous cows was a little higher than the reduction due to the effect of lactation phase. The milk yield during the trial has shown, on average, a reduction of 4.0% /month in mid-lactating primiparous cows (90 200 days in milk) and 7.6% /month in late lactation (201 300 DIM). The multiparous cows have shown a reduction of 10.5% /month in middle lactation (90 200 DIM) and 11.6% /month in late lactation (201 300 DIM).

The greater reduction of milk yield has been observed in June in relation to the beginning of the hot period and more in relation with the minimum daily temperature than the maximum daily temperature.

The conditioning system mitigated the negative effect of heat stress on milk yield and the results highlight the importance of carrying out the ventilation and misting in all areas of the barn.