High diurnal fluctuations of ambient temperature do not improve the adaptation of dairy cows to heat stress
Résumé
Climate change should increase the frequency of heat stress periods in temperate conditions. This study was designed to compare the effects of 2 types of heat stress (constant or variable within day) to thermoneutral conditions, on 8 cows either in early (73 d in milk) or mid (155 d in milk) lactation. The patterns of ambient temperature were a constant temperature of 18.0°C (thermoneutrality, TN), a constant temperature of 29.0°C (High Temperature Constant, HTConst), a variable temperature of 32.2°C between 6:00 and 17:00 and 21.5°C between 18:00 and 4:00 with a daily average of 28.4°C (High Temperature Variable, HTVar). Patterns were compared according to a crossover design, with 2 climatic rooms containing each, 2 early and 2 mid lactation cows during 3 periods of 15 d. Recovery periods of 15 d at a constant temperature of 18°C were included between measurement periods. Daily average of temperature-humidity index (THI) was 63 for TN, 75 for HTConst and 76 for HTVar with THI exceeding 78 between 6:00 and 17:00. Between 6:00 and 17:00, cows increased their respiration rate from 29.0 respirations/s at TN to 53.5 at HTConst and 66.5 at HTVar (P < 0.001) and their vaginal temperature from 38.5°C at TN to 39.2°C at HTConst and 39.5°C at HTVar (P < 0.001). Daily average of vaginal temperature increased from 38.6 to 39.3 C° from TN to HT (P < 0.001) with no differences between HTConst and HTVar. Dry matter intake decreased from 21.8 kg at TN to 18.4 kg at HT (P < 0.01). Daily milk yield averaged 28.6 kg and was not significantly affected by the temperature pattern even though morning milk yield decreased from 18.7 kg at TN to 16.3 kg at HT (P < 0.05). Milk fat and protein contents decreased from 42.2 and 30.5 g/kg respectively at TN to 40.0 and 28.7 g/kg at HT (P < 0.001) with no significant differences between HTConst and HTVar. Proportion of eating time between 6:00 and 17:00 decreased at HTVar compared with TN and HT Const (0.41 vs. 0.55, P < 0.001). In conclusion, high diurnal fluctuations of ambient temperature did not improve cow performance compared with a constant temperature with a similar daily average.