The objective of this study was to estimate the impact of heat stress on milk and component yields at the Penn State University research farm. Data included 703,106 daily records from 1156 Holstein cows that calved from 2004 to 2016. Temperature-humidity index (THI) was calculated with daily average temperature (°C) and average relative humidity (%) retrieved from the nearest weather station. Milk, fat, protein, and energy corrected milk (ECM) yields were evaluated with a model that included the fixed effects of lactation, year-month of calving, biweekly days in milk (DIM), age at calving; random effects included cow, date, and random regression on THI within cow. This model assumes that, in the same herd, cows respond differently to the same severity of heat stress, which was termed the heat-resistance coefficient (HRC). For every THI unit above 72, there was a significant (P < 0.05) reduction of 0.282 kg, 0.011 kg, 0.009 kg, and 0.301 kg for milk, fat, protein, and ECM yields, respectfully. Production for the least heat resistant quartile of cows and the most heat resistant quartile of cows based on HRC for ECM was compared. The heat tolerant group did not decline in production as THI rose, whereas it declined steadily for the least heat tolerant group. Production in the most extreme THI groups (not heat stressed = THI <72; highly heat stress = THI >78) are shown in Table 1. Production for intermediate THI levels were in-between the extremes presented in the table. Cow HRC for milk yield was positively correlated with HRC for ECM (0.95), fat (0.15), and protein (0.59) yields. The HRC of fat yield was positively correlated with ECM (0.38) and protein (0.36) yields. In conclusion, there is significant variation among cows within the same herd for reaction to heat stress. Table 1 (Abstr. 156). Production for the top and bottom quartiles of heat-resistant cows for ECM

	Least heat resistant group (n=132)			Most heat resistant group (n=132)
Not heat stressed	Heat-stressed(THI >78)	Not heat stressed	Heat-stressed(THI >78)
Milk, kg	37.08a	31.86b		35.60c	36.52a
Fat, kg	1.40ac	1.34bd		1.42a	1.36cd
Protein, kg	1.15a	1.07b		1.13a	1.10b
ECM, kg	39.17ac	36.02b		38.81a	38.15c

^a–dDifferent letters within the same row indicate a significant difference (P < 0.05).