Short communication: comparison of the effects of heat stress on milk and component yields and somatic cell score in Holstein and Jersey cows.

The objectives of this retrospective study were to (1) investigate the effects of heat stress (HS) climatic conditions and breed on milk and component yield for Holstein and Jersey cows on the same farm and (2) determine the effects of breed on udder health as measured by somatic cell score during HS climatic conditions. Data were collected from Dairy Herd Improvement Association records of 142 Jersey and 586 Holstein cows from the Bearden Dairy Research Center at Mississippi State University (Mississippi State). Heat stress climatic conditions were determined using a temperature-humidity index (THI) to combine dry bulb temperature and relative humidity into one measure. Two analyses were conducted to determine the effects of HS. Heat stress was defined as THI ≥ 72, and reported as HS+ for the first analysis and HS for the second analysis. The first analysis compared breeds during HS+ and non-heat-stress (HS-) conditions. Holstein milk yield decreased during HS+, whereas Jersey milk yield increased. Milk fat percentage for Holstein and Jersey cows declined during HS+. Holstein fat-corrected milk yield decreased during HS+, whereas Jersey fat-corrected milk yield during HS+ did not differ from that during HS-. During HS+, somatic cell score increased in milk from Holstein and Jersey cows compared with HS-. In the second analysis, HS was categorized as mild, moderate, or severe. The corresponding THI values were THI ≥ 72 but <79, THI ≥ 79 but <90, and THI ≥ 90. Holstein milk yield declined during moderate and severe HS, whereas Jersey milk yield declined during severe HS. Holstein milk fat percentage was less during moderate and severe HS compared with milk fat percentage during mild HS. Jersey milk fat percentage did not differ with regard to HS category. Jersey cows appeared to be more heat tolerant than Holstein cows; however, Holstein cows still produced larger volumes of milk.