Abstract: More than fifteen provinces in Southern China are characterized as humid tropical and subtropical regions with long period of summer. Heat stress from high ambient temperature and relative humidity is detrimental to dairy production and usually renders dairy farms in the affected regions almost profitless. But so far little information is available on annual heat stress dynamic variation and its interaction with lactating dairy cow performance. Thus,our study objectives were to record the natural period change of heat stress on practical dairy farms from 2012 to 2014,and analyze its impact on the productive performance and milk quality of mid-lactation dairy cows. Temperature and relative humidity on the dairy farm were recorded from July 12 to September 19 of 2012,July 15 to October 12 of 2013 and April 5 to 7 in 2014,respectively. Temperature-humidity index (THI) was calculated using following equation: THI=0.81xT+(0.99xT-14.3)xR+46.3,where T is temperature degree(℃),R is relative humidity(%)(NOAA,1976). According to THI,degree of heat stress can be classified into severe heat stress (THI>88),moderate heat stress (88≥THI>78),Mild heat stress (78≥THI>72) and no heat stress (THI≤72) (Armstrong,1994). During the study period,diet formula and cow management for the experimental cows kept the same as the other cows. Feed samples and milk samples were collected weekly. Rectal temperature and respiration rate were recorded in each heat stress change phase.
The experimental design was to analyze the natural period change of heat stress on the productive performance and milk quality of mid-lactation dairy cows with the data of 2012 (n=10) and 2013 (n=12). The results showed that from July to September of 2012 and July to October of 2013,the degree of heat stress can be marked off into moderate heat stress,mild heat stress and no heat stress. Dairy cows had different reaction to the heat stress cycles. Rectal temperatures and respiration rates increased during moderate heat stress (P<0.05). We found that respiration rate was more sensitive to heat stress than rectal temperature. Long term moderate heat stress significantly reduced milk yield (P<0.05).
We also found that milk protein synthesis was most susceptive to heat stress compared to other milk compositions. In 2012,milk protein content was 2.99% for no heat stress cows,but heat stress decreased it to 2.62% (P<0.05). In 2013,milk protein content was 3.41% for no heat stress cows,but only 2.90% during moderate heat stress (P<0.05). At the same time,urea nitrogen content in milk increased significantly (P<0.05). There was no significant difference for other milk compositions in 2012 and 2013 (P>0.05). A very special phenomenon was that natural period change of heat stress did not reduce dry matter intake and milk yield of the mid-lactation dairy cows in 2012 (P>0.05),but milk protein content decreased and milk urea nitrogen content increased dramatically (P<0.05). These results indicated that natural period change of heat stress may interfere with nitrogen metabolism and cause nutrition repartitioning in mid-lactation dairy cows. Such interference was independent of dry matter intake and milk yield. This unique phenomenon can be defined as "heat-stressed milk protein decrease syndrome" (HS-MPD). Heat-stressed milk protein decrease syndrome can only be revealed through the study of the long-term natural period change of heat stress.
In summary,we successfully delineated the annual natural period dynamic pattern of heat stress in southern China. The main impact of heat stress period change on productive performance and milk quality of mid-lactation Holstein cows was elucidated. Milk protein synthesis was most affected compared to other milk compositions. The synchronous decrease of milk protein content and increase of milk urea nitrogen content implies nutrition repartitioning for heat-stressed lactating dairy cows. This unique phenomenon can be denominated as "heat-stressed milk protein decrease syndrome" (HS-MPD).

Key words: dairy cow; heat stress; natural period; milk protein decrease syndrome