Manipulation of photoperiod length during lactation or the dry period affects milk production in dairy cows.'-2 Cows exposed to long days (16 hours light, 8 hours darkness) during lactation produce more milk than cows exposed to short days (8 hours light, 16 hours darkness). In contrast, cows exposed to short days during the dry period produce more milk in the next lactation than cows exposed to long days. Preliminary findings, summarized elsewhere in these proceedings, also suggest that cows exposedto short days during the dry period have enhanced cellular immune responses at calving. Because the severity and outcome of clinical mastitis is largely determined by the cow's response to infection, manipulation of photoperiod length during the dry period might influence the severity or outcome of clinical mastitis in early lactation. The objective of this study was to investigate the clinical, hematological, and milk compositional responses to experimentally induced Streptococcus uberis mastitis in cows exposed to short or long days during the dry period.
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