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Two experiments were conducted on a commercial dairy farm to describe the effects of a reduction in Korral Kool (KK; Korral Kool Inc., Mesa, AZ) system operating time on core body temperature (CBT) of primiparous and multiparous cows. In the first experiment, KK systems were operated for 18, 21, or 24 h/d while CBT of 63 multiparous Holstein dairy cows was monitored. All treatments started at 0600 h, and KK systems were turned off at 0000 h and 0300 h for the 18-h and 21-h treatments, respectively. Animals were housed in 9 pens and assigned randomly to treatment sequences in a 3 x 3 Latin square design. In the second experiment, 21 multiparous and 21 primiparous cows were housed in 6 pens and assigned randomly to treatment sequences (KK operated for 21 or 24 h/d) in a switchback design. All treatments started at 0600 h, and KK systems were turned off at 0300 h for the 21-h treatments. In experiment 1, cows in the 24-h treatment had a lower mean CBT than cows in the 18- and 21-h treatments (38.97, 39.08, and 39.03 +/- 0.04 degrees C, respectively). The significant treatment by time interaction showed that the greatest treatment effects occurred at 0600 h; treatment means at this time were 39.43, 39.37, and 38.88 +/- 0.1.8 degrees C for 18-, 21-, and 24-h treatments, respectively. These results demonstrate that a reduction in KK system running time of >= 3 h/d will increase CBT. In experiment 2, a significant parity by treatment interaction was found. Multiparous cows On the 24-h treatment had lower mean CBT than cows on the 21-h treatment (39.23 and 39.45 +/- 0.17 degrees C, respectively), but treatment had no effect on mean CBT of primiparous cows (39.50 and 39.63 +/- 0.20 degrees C for 21- and 24-h treatments, respectively). A significant treatment by time interaction was observed, with the greatest treatment effects occurring at 0500 h; treatment means at this time were 39.57, 39.23, 39.89, and 39.04 +/- 0.24 degrees C for 21-h primiparous, 24-h primiparous, 21-h multiparous, and 24-h multiparous cows, respectively. These results demonstrate that multiparous and primiparous cows respond differently when KK system running time decreases from 24 to 21 h. We conclude that in desert climates, the KK system should be operated continuously to decrease heat stress of multiparous dairy cows, but that operating time could be recduced from 24 to 21 h for primiparous cows. Reducing system operation time should be done carefully, however, because CBT was elevated in all treatments.