Cooling cows with sprinklers: Timing strategy affects physiological responses to heat load

After shade, sprayed water is the most common heat abatement resource provided in dairy farms in the western United States, but little is known about how to manage this resource to improve cow cooling and water-use efficiency. Our objective was to evaluate the cooling effectiveness of 4 spray strategies, using 2 water volumes (approximately 74 or 44 L/nozzle) over 45 min. Strategies varied based on spray frequency (using the same water volume) and the time that water was on and off (using different water volumes). In a crossover design, 20 Holstein cows (milk yield: 38.9 ± 4.2 kg/d) were restrained in shaded head gates and tested twice for each control (shade only) and 4 spray treatments (minutes water on | off, frequency: number of cycles/45 min): 1.5 on | 3 off, 10 cycles; 1.5 on | 6 off, 6 cycles; 3 on | 6 off, 5 cycles; and 3 on | 12 off, 3 cycles (water temperature average ± standard deviation: 26 ± 2°C). Air temperature and humidity averaged 29 ± 5°C and 26 ± 13%, respectively, during testing periods. Body temperature (BT), respiration rate (RR), skin temperature of the leg and shoulder, and air temperature surrounding the cow were measured. Compared with shade alone, all water treatments reduced heat load in cattle. Body temperature, for example, was at least 0.3°C lower (maximum reduction: 0.5°C) for sprayed cows after 45 min (39.0 vs. ≤38.7°C). The only change associated with spraying cows more often using the same water volume (thus manipulating both times on and off) was that applying water more frequently tended to reduce RR by 7 breaths/min. On the other hand, manipulating either time on or off (thus, water volume) affected most responses. Increasing the time on from 1.5 to 3 min (time off: 6 min) or shortening the time off from 12 to 6 min (time on: 3 min) or from 6 to 3 min (time on: 1.5 min) reduced BT by at least 0.1°C (maximum reduction: 0.2°C) and leg temperature by ≥0.2°C after 45 min. Shortening the time off also tended to reduce RR (7 breaths/min). Similarly, shoulder and surrounding air temperatures were, respectively, 0.5 and 0.4°C lower when reducing the time off from 6 to 3 min. In conclusion, applying the same water volume more often had minimal effects on responses to heat load on restrained cattle over a 45-min period. In contrast, spraying cows for longer or reducing the time off (thus, using more water) improved cow cooling compared with strategies that used less water.