To investigate the influence of heat stress period and spray-fan cooling system on physiology, milk production and morbidity of dairy cows (in different lactation stages), lactating cows were divided as late perinatal (0-21 d), the early lactation (22-120 d), the mid lactation (121-200 d), and the late lactation (201-305 d) according to days in milk. Ten healthy cows (with similar body condition, ages, lactation and 2-4 parities) were selected in each lactation stage (the total was 40) in this study. All cows were managed as usual without manual intervention, spray-fan cooling system was used during the heat stress period. Temperature, relative humidity, wind speed of the cowshed, rectal temperature, respiratory rates, milk production, milk composition, new cases of hoof disease, ketosis, clinical mastitis and subclinical mastitis were recorded or measured every day during experiment. The results were as follows: Average temperature in cowshed during the heat stress period was nearly 30 ℃ under spray-fan cooling system, relative humidity was 82%, wind speed was 1.35 m·s-1 and temperature humidity index (THI) was within the range of moderate heat stress; The average rectal temperature of cows in each lactation stage increased by 0.9 ℃, 0.7 ℃, 0.6 ℃ and 0.5 ℃, respectively, and respiratory rate increased by an average of 32.9, 29.1, 29.3 and 28.5 times per minute, respectively, which was significantly higher than those in non-heat stress period (P<0.01); Average daily milk yields in late-perinatal, early-lactation and mid-lactation decreased significantly (P<0.01) during heat stress period (with a decrease of 9.17%, 18.97% and 13.23%, respectively), whereas there was no significant change in late lactation stage (P>0.05); Initial milk yield of late-perinatal cows decreased, peak value of milk production decreased in all stages. Milk-fat percentage and milk fat/protein of all cows decreased significantly (P<0.01), milk protein also decreased significantly (P<0.05), the decreases were 0.29 percent, 5.23% and 1.61%, respectively; Incidence of hoof disease, ketosis, clinical mastitis and subclinical mastitis of all cows increased by 5.77, 0.78, 2.60 and 5.77 percent, respectively. In conclusion, spray-fan cooling system could not significantly improve the thermal environment of cowshed. Heat stress greatly impacted the rectal temperature and respiratory rate of late-perinatal cows, early-lactation cows and mid-lactation cow, leading to the decrease of initial milk production, peak milk yield of cows in all lactation stages and milk quality, and the increase of morbidity.
