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

Abstract

Summary The influence of summer weather conditions in central Texas upon lactating Holstein and Jersey dairy cattle was studied. Analysis of variance and multiple regression methods were used to study the effects of weather factors upon milk production and body temperature, respiration rate, and pulse rate responses of these animals. Weather factors studied were ambient temperature, solar radiation, and vapor pressure. Breed comparisons and comparisons between animals exposed to direct solar radiation and those shaded from direct solar radiation were made. Parts of this study were further subdivided into high and low ambient temperature groups. 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. Although slight increases in body temperature and respiration seemed to be associated with increases in vapor pressure, this influence, under the conditions of this test, was slight in comparison with that of the other environmental factors. 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. The body temperatures and respiratory 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, and merits further study.