The cow environment is a complex mix of natural and constructed factors. Interactions 'between these factors and the cow reaction to them are important in determining the total stress level of the cow. Construction of new facilities which improve cowcomfort have been shown to improve milk production but did not change somatic cell counts (Bewley, et al. 2001). Thermal stress in cattle results in major decreases in dairy production each summer. These decreases have been documented in many studies andreviews (Armstrong, 1994, Collier, et.al., 1982, Ravagnolo, et.al., 2000 and Ray, et.al. 1992). Igono and others (1992) proposed that the Temperature Humidity Index (THI) could be used to evaluate the thermal stress of the environment. This index combines relative humidity and temperature into a single value to estimate the potential environmental heat load. An environment is generally considered stressful for cattle when the THI exceeds 72. When THI is above this level, adverse affects are expected incattle. Others have suggested (Hahn, et al., 1992) that feed intake of cattle will be reduced when temperatures exceed 75°F (24°C). Production losses can be minimized by proper heat abatement measures. However, Harman (1994) summarized the results ofmany heat stress studies and concluded that most did not show any effect of heat stress on milk quality as measured by somatic cell count (SCC). An earlier review by Collier and others (1982) had made similar observations. They suggested that the increase in SCC during the summer months was from the increased development of clinical mastitis and suggested that heat stress either impaired the mastitis defense mechanisms of the cow or cattle were exposed to greater contact with pathogens while attemptingto cool themselves. Elvinger (1992) demonstrated that heat stress reduced the immune response of cows as compared to controls when a chemotactic challenge was administered. Lack of response was attributed to a decreased migration of leukocytes to the udder observed in the heat stressed cattle. A key factor in reducing mastits is reducing the exposure of the teat end to potentially harmful bacteria. During periods of heat stress, cattle will attempt to cool themselves by lying in wet areas, standing in ponds, and seeking shade. If these cooling methods are ineffective, cattle will reduce intake and milk production in an attempt to decrease heat production and thus reduce the need for heat exchange. It is important to note that most of the heat stress studies conducted in environmental chambers are short term and the facilities do not allow the cow to choose cooling methods that may be available on commercial dairies. Thus the combination of a reduced immune system in the presence of increased bacteriachallenge may have not been fully tested in many studies.
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