to the skin surface are major forms of evaporative cooling used to dissipate excess heat. In this study it was found that heifers with access to active cooling during the last eight weeks of gestation had significantly lower rectal temperature, skin temperature, respiratory rate and sweating rate com- pared to their heat-stressed counterparts. To assess patterns of thermoregulatory responses, see graphs below. Vaginal temperature was also lower with the cooled heifers com- pared to the heat-stressed heifers. Respiratory rates and sweating rates had bigger weekly differences compared to other thermoregulatory responses. The researchers stated that although their findings accord with results from dry and lactating cows under heat stress, the magnitude of the physiological differences in heifers is less. This was explained by the fact that heifers generate less metabolic heat per unit of surface area and have less heat to dissipate at this stage of life. The loss of minerals was also assessed in this study. Potassi- um is the primary cation lost in sweat, while bicarbonate (through panting) and sodium (through urine) excretion also increases in hot climatic conditions, exposing cows to higher risk of respiratory alkalosis. In the current study, heat-stressed heifers had reduced levels of sodium, potassium and chloride compared to cooled heifers. These reductions coupled with increased thermoregulatory responses indicate that heat stress results in loss of minerals.
Growth performance and milk yield Providing cooling during the last eight weeks of gestation did not affect growth performance of heifers. Fat yield tended to be higher in the cooled heifers compared with heat-stressed heifers (1.30 vs. 1.18kg/d). Protein yield was higher during weeks 3, 4 and 7–15 in the cooled heifers compared to the heat-stressed heifers. However, overall fat and protein percentage were similar. In the present study, heifers cooled during the last 60 days of gestation average a 3.9kg/day increase in milk in the first 15 weeks of lactation relative to heat-stressed heifers, indicat- ing that heifers in the cooling group benefited from the ac- tive cooling system. Studies show that heat stress can de- crease milk production by up to 0.59kg/cow/day with each unit increase of THI above 68.
Magnitude of heat stress The researchers concluded that their study demonstrates a strong and positive thermoregulatory response of dairy heifers to active cooling with fans and water soakers in late gestation. “Although the magnitude of those responses is less pronounced than that of mature multiparous dry cows, these alterations translate into production improvements post- calving. It is clear that more studies are needed to con- firm the production responses, perhaps with larger numbers of animals, and to unravel the molecular and metabolic
Figure 1 - Respiration rate and sweating rate measurements from cooled or heat stressed nulliparous heifers during the last 8 weeks of gestation.
80 70 60 50 40 30 –8
50 40 30
20 10 0
–8 –7 –6 –5 –4 Weeks relative to calving –3 –2 –1 –7 –6 –5 –4 Weeks relative to calving ** ** ** ** ** ** ** ** –3 –2 –1 ** ** ** ** ** ** **
**
Figure 2 - Milk yield measurements from cooled or heat stressed nulliparous heifers during their first lactation.
35 40 45
15 20 25 30
10 1 2 3 4 5 6 7 8 9 Weeks in Milk 10 11 12 13 14 15 Cooled ** Heat Stressed ** # ** ** ** ** ** ** ** ** ** **
implications of the observed responses resulting in higher yields,” they remarked.
This article is based on the original article by Davidson B.D., Dado-Senn B., Rosa Padilla N., et al. 2021. Late-gestation heat stress abatement in dairy heifers promotes thermoregulation and improves productivity. Journal of Dairy Science, Vol 104, No. 2, 2021. 10.3168/jds.2020-18998.
▶ DAIRY GLOBAL | Volume 8, No. 2, 2021 33
Milk Yield (kg/d)
Sweating Rate – Unshaved (g/m2h)
Respiration Rate (bpm)
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