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

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

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

Heat Stress During Lactation

Seasonality of Production in the Southeastern USA Tao & Dahl, J. Dairy Sci. 96:4079-4093

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

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

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

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

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 2000. 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)

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

Cooling Dry Cows Increases Milk Tao & Dahl, J. Dairy Sci. 96:4079-4093

Cooling Increases Calf Birth Weight Treatment effect: P < 0.01

Cooling Increased Apparent efficiency of IgG absorption (AEA*) * AEA = [Serum [IgG] (g/L) * birth weight (kg) * 0.091 / IgG fed (g)] x 100

Weaning Weight P = 0.04

Heifer Body Weight

Milk Production 1st Lactation

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

Thank you! Jepsantos@ufl.edu