Trend for Diurnal Temperature Variation and Relative Humidity and Their Impact on Milk Yield of Dairy Cattle in Tropical Climates

Climate change is expected to adversely affect the agricultural systems principally in dairy farming. Ambient temperature and relative humidity are the main climatic factors that exert influence on dairy production. The diurnal temperature variation in tropical regions (DTV; daily difference between maximum and minimum temperatures) should be considered in conjunction with the relative humidity (RH) due to their large daily variation. The objectives of this study were to characterize the effect of DTV and RH on milk yield of dairy cattle, and their trend in Thailand. The dataset consisted of monthly test-day milk yields (46,444 records) from 5,080 first-lactation cows from 456 farms in Thailand. The climate data consisted of DTV and RH (76,619 daily records) collected from 2002 to 2014 by 17 stations of the Thai Meteorological Department (TMD) located near the dairy farms in this study. The mixed linear model considered herd-year-season of calving (HYS), age at first calving (AFC), days in milk (DIM), days in milk squared (DIM²), DTV and RH as fixed effects, and animal, permanent environment and residual as random effects. All factors (HYS, AFC, DIM, DIM², DTV and RH) had an effect on milk yield (P < 0.01). Milk yield increased as DTV increased (b = 0.029 ± 0.007; P < 0.01), whereas milk yield decreased as RH increased (b = - 0.011 ± 0.002; P < 0.01). The correlation between DTV and RH was high and negative (r = -0.625; P < 0.01). The yearly LSM ranged from 9.30 °C (2011) to 10.29 °C (2004) for DTV and from 72.57 % (2004) to 75.23 % (2011) for RH. However, the DTV (b = -0.005 ± 0.022; P > 0.05) and the RH (b = 0.076 ± 0.056; P > 0.05) trends from 2002 to 2014 were close to zero and not significant. These results confirmed the impact of climate effects (DTV and RH) on milk production of dairy cattle raised under tropical conditions, reiterating the need to account for these factors in both management and genetic selection programs.