Effects of heat-stress-induced physiological changes on the productivity of dairy cattle

Dairy cattle exposed to summer heat stress (HS) typically exhibit altered metabolic characteristics, including hyperinsulinemia concurrent with hypoglycemia. The reasons for this change in glycemic status and its consequences are currently unknown. This project aimed to examine the relationship between summer HS, blood glucose concentrations, and milk production in primiparous (n=26) and multiparous (n=27) Holstein cows. Glucose was measured in coccygeal vein blood twice weekly using a hand-held glucometer (Abbott Precision Xtra) from May through July. Rectal temperatures were also collected. The days surrounding sample collections were categorized as having no HS (0 h above 68 Thermal Humidity Index [THI]), moderate HS (>0 to 8 h above 68 THI) or severe HS (>8 h above 68 THI). A subset of cows with ≥5 blood glucose measurements spanning different THI categories were categorized as hypoglycemic (n=8 primiparous; n=9 multiparous) or non-hypoglycemic (n=6 primiparous; n=5 multiparous) based upon blood glucose concentrations on no-HS days and severe-HS days (hypoglycemic if sustained reduction on severe-HS days). When analyzed together, milk production did not differ between those categorized as hypoglycemic and non-hypoglycemic. When separated based upon parity, there was a tendency for primiparous hypoglycemic cows to produce more milk than their non- hypoglycemic counterparts (P<0.15). Variability in milk production measured as standard deviation across no-, moderate- and severe-HS days did not differ based upon glycemic status but was greater in multiparous than primiparous cows (P<0.01). Rectal temperatures were greater on severe-HS days than on no- or moderate-HS days (P<0.01) and were positively correlated with the number of h of HS in a day (r=0.45; P<0.01). Neither milk nor glucose differed based upon the category of HS severity. Interestingly, however, glucose was negatively correlated with milk production on the day of measurement (r=-0.24; P<0.01) and day after measurement (r=-0.23; P<0.01). These results are an initial indication that hypoglycemia during HS is related to milk production, particularly in primiparous cows.