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Glucose infusion during heat stress restores normoglycemia but does not improve milk production.

J. Stewart




Glucose infusion during heat stress restores normoglycemia but does not improve milk production.
J. Stewart*, H. Newberne, A. Arneson, M. Harrod, V. Negron-Perez, H. Haines, J. Jordan, R. White, A. Ealy, S. El-Kadi, R. Rhoads, M. Rhoads. Virginia Tech Blacksburg, VA.

Mild hypoglycemia is one of a myriad of physiological changes typically experienced by lactating dairy cows exposed to heat stress (HS); the consequences of which are not yet well defined or understood. The objective of this experiment was to infuse dextrose to re-establish normoglycemia during HS, hypothesizing that bringing the cow out of hypoglycemia would improve milk production during HS. Multiparous lactating Holstein cows (n = 6; 63.33 � 2.35 DIM, 3.17 � 0.40 lactations, 316.38 � 13.93 kg of BW) were housed in temperature-controlled rooms and subjected to a thermoneutral period (TN; 10 d; 18 � 4�C) followed by a HS period (15 d; 33 � 4�C). Cows were offered a total mixed ration and water for ad libitum consumption, and feed refusals were recorded once daily. All cows were milked 2 times per day and milk yields were recorded after each milking. Rectal temperatures and respiration rates were measured once daily, and both indices increased during HS (38.18 � 0.07 vs 39.44 � 0.09�C, respectively; P < 0.01 and 28.86 � 1.2 vs 73.80 � 3.00 breaths per min, respectively; P < 0.01). Circulating glucose concentrations were determined during TN and HS conditions with hand-held glucometers using blood collected from indwelling jugular catheters. As expected, blood glucose concentrations declined with HS (61.63 � 1.58 and 55.85 � 1.73 mg/dL for TN and HS, respectively; P < 0.04). Over the last 96 h of the HS period, dextrose was continuously infused at rate that returned each individual cow's circulating glucose concentrations to those observed during TN (100 � 10% of TN values). During TN conditions, both milk production (50.44 � 1.21 kg) and feed intake (49.02 � 1.44) were at their highest (P < 0.01). Milk production decreased under HS conditions (36.49 � 1.23 kg) but did not improve with dextrose infusion (33.99 � 2.02 kg; P = 0.76). Feed intake also decreased under HS conditions (35.58 � 2.25 kg,) and did not improve with dextrose infusion (27.27 � 3.46 kg; P = 0.17). The results of this experiment indicate that, although cows become hypoglycemic during heat stress, this reduction in circulating glucose concentrations is not limiting milk production.

Keywords: heat stress, glucose, milk production.