1. SHORT- AND LONG-TERM IMPACTS OF HEAT STRESS
José Eduardo P. Santos
Department of Animal Sciences
University of Florida
July 25 to 27, Port Macquarie, NSW

2. Heat Production + Heat Gain = Heat Loss
metabolites
Feed
Heat Production + Heat Gain = Heat Loss

3. Body Temperature Responses to Ambient Temperature
Sartori et al. J. Dairy Sci. (2003)

5. Heat Stress During Lactation

6. Seasonality of Production in the Southeastern USA
Tao & Dahl, J. Dairy Sci. 96:

7. US Milk Market Milk Composition
Fat
~0.25
Units
True Protein
~0.20
Units

8. Heat Stress + Solar Radiation
Environmental/metabolic Chambers
Heat Stress
Heat Stress + Solar Radiation

9. Effects of Heat Stress on Feed Intake
Heat stress  feed intake by ~30 %
Rhoads et al., 2007

10. Underfeeding  yield by ~19%
Effects of Heat Stress on Milk Yield
Heat stress  yield ~45%
Underfeeding  yield by ~19%
Thus,  feed intake only accounts for 50% of the reductions in milk yield
Rhoads et al., 2007

11. Changes in Pregnancy per AI in Dairy Cows In Northeastern Spain
Cool season
Warm season
Milk yield
Look what happens in Barcelona region of Spain when milk yield goes up from 1991 to In 1991, cows were more fertile in cool weather – the difference in preg rate as about 10%. By 2000, no change in fertility in winter but it declines to 22% in summer and the summer-winter difference is 25%
Lopez-Gatius, Theriogenology 60: 203 (2003)

12. Heat stress during the DRY PERIOD
Direct & Indirect effects of Heat Stress during the dry period
(late-lactation)
“Direct”
“indirect”
Produce 4-5 kg/d less milk throughout lactation than cows cooled with soakers and fans when dry (even when all of the cows are cooled during lactation)
Heat stress during the DRY PERIOD
(ITH>68, late gestation
6-8 weeks prepartum)
Produce 4 kg/d less milk throughout lactation than in-utero cooled heifers (even when there are cooled during their first lactation)
Lower birth BW (-4.5 kg)
Decreased IgG absorption
Compromised immunity
Reduced grain intake
Reduced growth rate (ADG)
Increased culling rate
Tao et al., 2011; Tao and Dahl, 2013; Monteiro et al., 2016; Laporta et al., 2017

13. Cooling Dry Cows Increases Milk
Tao & Dahl, J. Dairy Sci. 96:

14. Cooling Increases Calf Birth Weight
Treatment effect: P < 0.01

15. Cooling Increased Apparent efficiency of IgG absorption (AEA*)
* AEA = [Serum [IgG] (g/L) * birth weight (kg) * / IgG fed (g)] x 100

16. Weaning Weight
P = 0.04

17. Heifer Body Weight

18. Milk Production 1st Lactation

19. Conclusions
Dairy cows are very sensitive to ambient temperatures > 25 oC or THI > 68
Heat stress
Reduces yields of milk and milk components
Impact goes beyond the reduction in dry matter intake
Depresses concentrations of fat and true protein in milk
Marked effects on reproduction
During the dry period
Affects mammary development and subsequent milk yield
Transgenerational effect on the offspring
Reduced growth
Compromised first lactation production

20. Thank you!