Impact of thermal-humidity index on milk yield under conditions of different dairy management

The study aimed to evaluate the effect of elevated temperature on milk production of dairy cows in southern Slovakia in the year 2003. The hypotheses that milk yield is influenced by the stage (peak, midlactation and late lactation), cooling (fogger+fan or fan), and breed (Slovakian Pied cattle and Blackand-White Holstein were tested. Production data included 47,600 test-day records belonging to 16 herds situated in lowlands. During the period from May to September, 113 summer and 59 tropical days were recorded, eighty days were with a mean termal humidity index (THI) value above 72.0, on twelve days we recorded mean values above 78.0. Differences between peak stage (S1) and late lactation stage (S3) were significant in all months, between S1 and S2 in March, May, August and November. S2 and S3 differed in April, May, June, September and November. Dairy cows cooled by fans with water foggers produced significantly more milk in June than cows cooled only with forced ventilation (823.5 kg vs 733.20 kg; P<0.05). In all of the months the Black-and-White Holstein breed exhibited higher performance. For whole-year evaluation, all of the differences between months and stages (843.88, 738.51 and 612.39 kg), between coolings (793.38 vs 697.50 kg), and breeds (655.31 kg vs 811.59 kg) were very highly significant. For the period with high temperatures, we found differences among months (P<0.01) and stages (P<0.001), between cooling methods (P<0.01) and breeds (P<0.001). May and August differed statistically (782.06 kg vs 714.00; P<0.01), as did May and September (782.06 kg vs 694.88 kg; P<0.05). The average values of all lactation stages were significantly different (P<0.001). Evaporative-cooled cows produced more milk than non-cooled (794.60 kg vs 709.20 kg) ones, and the Black-and-White Holstein breed more than Slovakian Pied cows (814.88 kg vs 667.58 kg). A higher milk yield was recorded in April than in October (794.44 kg vs 687.75 kg; P<0.001) and in Black-and-White Holsteins than Slovakian Pied (811.88 kg vs 670.31 kg; P<0.001). Similar results were obtained in the comparison of May with September (782.06 kg vs 694.88 kg; P<0.01)