The aim of this study was to assess the effect of the ventila tion system on the environment and the performance of beef cattle during the early fattening period. It was conducted on one group (n\ubc106) of young Charolais bulls from the first of August to the 9th of October. Animals were raised in a roofed, loose housing facility with straw bedding. The day after their arrival bulls (429 \ub1 24 kg) were grouped according to weight in 10 pens. Five pens were equipped with 2 ceiling fans (CF), whereas the other 5 had 2 horizontal fans (HF). The pens with different ventilation were separated by an empty pen with a windbreak. The trial lasted 70 days and it was divided into 3 periods. In the first and in the third periods all the fans were in operation, whereas in the second period they were switched off to see the effect of the ventila tion on the animals and on the environment. In order to measure the average daily weight gain (ADG) and the effect of ventilation on the cleanliness of animals, at the beginning and at the end of the trial the animals were weighed and evaluated for their body surface covered in manure: from 1 (75%). The temperature-humidity index (THI) was continuously measured in CF and HF pens and outside the facility. Each pen was periodically checked for the dry matter of the bedding (DMB). Data on ADG were subjected to ANOVA using the ventilation system as a fixed effect, whereas cleanliness data were subjected to the nonparametric Mann-Whitney U test. Data on THI and on DMB were subjected to ANOVA using the combined effect of period and ventilation system. Period significantly affected THI (74.4 vs 75.2 vs 65.5, p<.001) that was the lowest in period 3, whereas
THI was not significantly different between CF and HF. CF maintained a higher DMB (30.6 vs 45.1% ww; p<.001 and 22.2 vs 31.3% ww; p\ubc.002) in periods 1 and 3, respectively. Bulls raised with the CF ventilation system not only remained cleaner, increasing their degree of dirtiness of only 1.16 points compared to 3 points of the others (p<.001), but they showed also a significantly higher ADG (1.20 vs 1.36 kg/day; p\ubc.039) compared to animals raised with HF. These results indicate that CF help to improve the environmental conditions and the animal performance in hot weather conditions com pared to the horizontal ventilation systems, and that the THI is not always an appropriate index to predict the effect of
temperature and humidity on animal heat stress
