Variables climáticas asociadas a la producción de leche en vacas Holstein criadas bajo condiciones de estrés por calor del Valle del Yaqui, México

Resumen

Milk yield of Holstein cattle is challenged during summer because the regulars conditions of high temperature and humidity common in northwest Mexico; then, a cooling system is recommended to alleviate heat stress in milking cows. Thus, the objective of this study was to evaluate association among climatic factors, milk yield and physiological markers indicative of heat stress in dairy cattle managed under two artificial cooling systems during summer in the Yaqui Valley. Twenty-eight multiparous Holstein cows with ~150 d in milk were randomly assigned to one of two experimental treatments as climatic scenarios: cooling prior milking (EPO, n=14) and additional cooling (EA, n=14). From May 7 to July 23 of 2011, the cows from EPO group were cooled only before milking (0700 and 1700) and cows in EA received an intensive cooling program in the waiting parlor. Milk production (PL) was recorded daily using an electronic system. Rectal temperature (TR) and respiratory frequency (FR) were measured twice per week. Ambient temperature (TA), relative humidity (HR), wind speed (VV) and solar radiation (RS) records were collected (d=day and n= night). All statistical analysis were performed in SAS (2004). The CORR procedure was performed to correlate TR, FR and PL with all climatic variables. To measure the units of change in those associations, a regression model was used through the procedure PROC REG. All climatic variables were correlated with the physiological markers of heat stress or PL in both climatic scenarios, but HR was the main climatic variable that explained the PL during the day (EPO: r= -0.53; P<0.05 and EA: r= -0.28; P<0.05) and night (EPO: r= 0.54; P<0.001 and EA: r= 0.29; P<0.05). The PL model include HRd (R2=0.92; P<0.001) in EPO and FR (R2=0.92; P<0.001) in EA. According to the climatic conditions of southern Sonora, it is important to consider all climatic variables for a cooling system setting, but it should be mainly configured to the humidity removal with forced ventilation and soaking with large droplets of water in order to avoid the intensification the moisture gradient of the cow.