Comparison of two cooling systems on milk yield of Holstein cows under heat stress conditions

Forty four lactating Holstein cows grouped by age, milk production and stage of lactation were subjected to a misting or fan and spray cooling system under heat stress conditions. Milk yields, grain feed intake, rectal temperatures, and respiratory frequency were measured. Milk yield and concentrate feed intake did not differ among cooling systems. Mean daily milk yield was 28.2 kg per cow for both treatment and control groups through the whole experimental period. Feed intake was slightly higher for cows in the treatment group (10.9 vs 10.7 kg /day) but the difference was not significant. Mean rectal temperature recorded in the morning did not differ among treatments (39.2° C for both treatment and control). Mean afternoon rectal temperatures were significantly higher in the treatment group (39.4 vs 39.2° C). Mean respiratory frequency in the morning was higher for the control group (67.3 vs 64.9 breaths /min.) but the difference was not significant. Afternoon respiratory frequency was significantly higher for the treatment group (76.9 vs 68.6 breaths /min.). There was no significant difference between mean daily milk yield (28.8 vs 27.6 kg for east and west sides respectively). Location effect, however, influenced significantly concentrate feed intake, and rectal temperatures and respiratory frequency. Mean daily feed intake was significantly higher for cows in the west side (11.5 vs 10.1 kg). Animals in the west side had significantly higher morning measurements for rectal temperatures (39.2 vs 39.1° C) and respiratory frequency (68.8 vs 63.3 breaths /min.). Afternoon measurements for rectal temperature (39.6 vs 39.2° C) and respiratory frequency (78.6 vs 66.8 breaths /min.) were also significantly higher for cows in the west side. This effect was due probably to differences on solar radiation incidence between the two sides especially in the first phase of the study when environmental temperatures were higher (25.2° C average temperature). Mild ambient temperatures during the second experimental phase (18.1° C average temperature) produced smaller differences in the effect between locations. The effects of cooling system and location interaction were more accentuated during the first phase of the study when ambient temperatures were higher. Failure to properly adjust the water spray mechanism and large differences in climatic conditions between the two experimental phases may have prevented us from observing the maximum benefits of the fan and spray treatment described in previous studies. Alternatively, the mist cooling system may have been effective and therefore, differences in milk production between the two groups were not detected.