Effect of mild heat stress on faecal cortisol metabolites, blood minerals and energy metabolism of Holstein cows

We investigated the effect of climatic conditions (daily average THI between 60 and 78) and access to shade on faecal cortisol levels, blood minerals and indicators of metabolic alterations in 20 Holstein cows.
From June to September 2011, the ILVO-herd was divided into a group with and without access to shade on pasture. Blood and faecal samples were collected from 10 matched shade-control pairs at respectively the evening and morning milking for11 days with varying climatic conditions. Blood samples of control animals for the two days with lowest versus highest risk for heat stress were compared for indicators of metabolic alterations (glucose, alkaline phosphatase (ALP), triglycerides, cholesterol, creatinine, urea) and blood minerals (Na+, K+, Cl-). All faecal samples were analysed for concentration of 11,17–DOA (a group of cortisol metabolites) to test whether it was associated with various climatic indices (e.g. temperature humidity index, THI) and whether it was affected by access to shade.
The climatic conditions during the 11 days were relatively mild and not exceptional for summers in temperate regions. On the two days of highest heat stress risk (THI = 71 and 78) serum concentrations of ALP (P=0.046), cholesterol (P=0.011), urea (P=0.004) were lower and the creatinine (P<0.001) and Cl- (P<0.001)concentration were higher than during the days of lowest risk for heat stress (THI = 60 and 64). Levels of 11,17–DOA were positively correlated with climatic indices such as the THI (P<0.001) but were not affected by access to shade. Shade-use was limited suggesting that conditions were unlikely to have induced severe heat stress. These results illustrate that blood serum ALP, cholesterol, creatinine, urea and Cl- and faecal 11,17-DOA concentration are good indicators to evaluate mild heat stress in dairy cattle and - more importantly – potential preventative measures.