The aim of this study was to evaluate the impact of heat stress on lying behavior and lameness of lactating dairy cows in northern New York, where episodic heat stress is more typical than prolonged heat stress. Four commercial dairy farms with varying housing conditions and heat abatement systems were selected and monitored for lying behavior and locomotion score from June to October, 2017: Farm A = sand-bedded freestall with natural ventilation; Farm B = sand-bedded freestall with fans over stall beds; Farm C = tiestall with sawdust covered rubber mats enlisting naturally assisted ventilation; Farm D = sawdust covered mattress freestall with fans over stall beds. Environmental conditions of animal housing were monitored in 10-min intervals using a data logger mounted in pen/barn at cow-level. Thirty early to mid-lactation focal animals were selected on each farm (not balanced for DIM) with a locomotion score <3 (not lame, 1 to 5 scale). Lying behavior was monitored continuously using data loggers. Locomotion score was measured at end of study period and classified as Lame or Not Lame. Four days of cool weather (mean THI <65, COOL) and 4 d of hot weather (mean THI >70, HOT) were selected and lying behavior summarized and analyzed by farm using Proc Mixed procedure in SAS to evaluate differences for COOL and HOT days. Lameness status from beginning to end of study period within farms were analyzed by Chi-squared analysis using Proc Freq. Each of the 4 farms, regardless of heat abatement system employed, experienced a decrease in lying time from cool to hot days of 20, 10, 13, and 16% (Farm A, B, C, and D respectively). Lying bouts (n/d) were greater on hot days for cows with only natural ventilation (Farm A; 9.5 vs 8.9 ± 0.53; P = 0.02), whereas cows with heat abatement systems showed no differences. While episodic heat events resulted in reduced lying time across all farms, increased incidence of lameness were significant for only Farm A and D (17 and 14% respectively, P < 0.05). Results are shown in Table 1. Table 1.

Item	COOL	HOT	SE	P-value
THI (mean ± SD)	60.9 ± 3.7	72.3 ± 1.1	—	—
Minutes THI ≥68 (mean ± SD)	74 ± 127	1,279 ± 155	—	—
Lying time (h/d)
Farm A	12.8	10.3	0.3	<0.01
Farm B	11.5	10.3	0.3	<0.01
Farm C	12.8	11.1	0.3	<0.01
Farm D	10.3	8.6	0.3	<0.01