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The effects of ventilation regimen on air quality, and on the welfare and production performance of thirty-six Comisana<br />ewes were assessed in a 6-week trial conducted during the summer of 2002. Animals were divided into three groups of<br />12, and subjected to the following treatments: low ventilation regimen providing a mean ventilation rate (VR) of 35 m3/h<br />per ewe, split in 30 min ventilation cycles at an air speed of 2 m/s (LOV-30); moderate ventilation regimen (VR = 70<br />m3/h per ewe) split in 30 min ventilation cycles at an air speed of 4 m/s (MOV-30); moderate ventilation regimen (VR =<br />70 m3/h per ewe) split in 60 min ventilation cycles at an air speed of 2 m/s (MOV-60). Air concentrations of microorganisms,<br />dust, and gaseous pollutants were measured twice weekly. Respiration rate (RR) and rectal temperature (RT)<br />were monitored throughout the trial at 0830 and at 1400. Behavioral traits of ewes were recorded twice per week from<br />0900 to 1200 and from 1500 to 1800. Cell-mediated immune response to phytohemagglutinin (PHA) and humoral<br />immune response to chicken egg albumin were determined. At d 37 ewes were injected with porcine ACTH, and subjected<br />to blood sampling for evaluation of cortisol concentrations immediately before and 1, 2 and 4 h after ACTH injection.<br />Milk yield was recorded daily. Individual milk samples were analyzed for composition, renneting parameters, somatic<br />cell count (SCC), and bacteriological characteristics. Averages of maximum THI were about 3 points higher in the LOV-<br />30 and the MOV-30 than in the MOV-60 room, whereas no differences emerged in the air concentrations of dust, gaseous<br />pollutants and microorganisms. Significant interactions of treatment x time (P < 0.05) were found for respiration rate,<br />and for the time the ewes spent lying, idling and eating in the afternoon during weeks 2 and 3 of the study period.<br />Significant effects of ventilation regimen x time (P < 0.05) were also observed for milk yield and milk renneting parameters,<br />the LOV-30 ewes giving smaller volumes of milk with a deteriorated coagulating behavior than those of the MOV-<br />60 group during the second half of the trial. No significant differences emerged in ewe immune and endocrine responses.<br />Results show that ventilation regimen had a moderate impact on ewe behavior, physiology and production performance.<br />This experiment suggests that the length of ventilation cycles and air speed, together with ventilation rate, are<br />critical for efficient ventilation regimens.