A Study of Solar Radiation as Related to Physiological and Production Responses of Lactating Holstein and Jersey Cows

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Abstract
Body temperatures, pulse rates, and respiration rates increased during exposure to direct solar radiation throughout a summer day. Body temperatures and respiration rates were higher and pulse rates were lower for individuals of the Holstein breed than for individuals of the Jersey breed. Air temperature variations were the predominant cause of variations in respiration rate and body temperatures, while solar radiation was of considerable importance as a direct cause of increased body temperatures and respiration rates of animals exposed to the sun. For shaded animals, increases in solar radiation caused no appreciable direct influence upon these physiological responses. The influence of the environmental factors upon body temperature and respiration rate was considerable, while the influence upon pulse rate responses was so slight that consistent results could not be obtained. Body temperature and respiration activities of Holstein individuals and individuals located in the sun were influenced to a greater degree by the environmental conditions than were the same measurements of Jerseys and shaded animals. Although exposure to direct solar radiation had considerable effect upon the physiological measurements, it did not cause an appreciable change in milk production. The relationship between the physiological responses and milk production, therefore, may not be as great as is generally assumed.