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Correlation between mean panting score and temperature humidity index in lactating dairy cows in a sub-tropical summer
Climate directly influences production and physiological responses of lactating dairy cows. Panting score (PS) can assist in determining the level of heat stress within a herd, but there is little research in this area for dairy cows. Forty lactating Holstein-Friesian cows were used in a 103 d study investigating the influence of summer conditions on animal response to heat load. The cows were managed within a partial mixed ration system and were milked twice daily (0530 h; 1400 h). The cows were observed twice daily (1000 h (AM); 1400 h (PM)) by the same observer. Individual PS were obtained once at each observation. Panting score system uses a 7 point score (0, 1, 2, 2.5, 3, 3.5 and 4) PS0 = no thermal stress; PS4 = severe thermal stress (open mouth, tongue extended, rapid labored breathing). Individual PS were collated and converted into mean PS (MPS) for 40 cows at AM and PM. A MPS of 0.8 to 1.2 = high heat load, MPS > 1.2 = extreme heat load. In both instances milk production and reproduction are adversely affected. Ambient temperature (TA), and relative humidity (RH), were obtained at 10 min intervals from an automated on-site weather station, THI was calculated from these data (THI = 0.8 × TA) + [(RH/100) × (TA - 14.4)] + 46.4). Mean PS were analyzed using repeated measures model. Correlations between MPS and THI at time of observation (0 h; AM/PM) and for 2 and 1 h lags were determined using Pearson correlation analysis. The MPS of cows (0 h) at PM (1.3 ± 0.05; extreme heat load) was greater (P < 0.01) than AM (1.1 ± 0.05; high heat load). Overall, MPS and THI were moderately correlated (P < 0.001) for 0, 1 and 2 h lags for AM and PM. For AM the best correlation between MPS and THI was r2 = 0.50 for 1 h lag. For PM the best correlation between MPS and THI was r2= 0.69 for no lag (0 h). Elevated PS during AM may be an attempt to reduce heat load while there is a positive temperature gradient between the cow and the environment.
Keywords: panting, heat stress, temperature humidity index