The effects of heat stress on protein metabolism in lactating Holstein cows

Abstract Text: Independent of decreased feed intake, heat stress (HS) decreases the synthesis of milk protein but the mechanisms responsible for the decrease are not clear. To research the direct (not mediated by feed intake) effect of HS on the synthesis of milk protein, four multiparous, lactating Holstein cows (101±10 DIM, 574±36 kg of BW, 38±2.4 kg of milk/d) were randomly assigned to four environmental chambers and divided into two groups, for two experimental periods of 18 d (control period 9 d, trail period 9 d). A crossover design was used and period 1 and 2 were separated by 30 d. Cows in control period of both period 1and 2 were exposed to constant thermal neutral (TN) conditions[20°C, 55% humidity; temperature-humidity index (THI) = 65.5; 12 h light and dark cycles] and allowed to eat ad libitum for 9 d. Trial period of both period 1 and 2 included HS (n=2) group and pair-fed TN (PFTN, n=2) group and two groups exposed to HS [0600-1800h 36°C, 1800-0600h 32°C, 40% humidity; THI = 84.5; 12-h light and dark cycles] and TN respectively. The pattern and magnitude of reduced feed intake in the PFTN cows mirrored that of the HS cows. Compared to PFTN, HS decreased milk protein yield (17.7%) and content (4.1%) (P<0.05). HS increased prior to feeding rumen liquid NH₃-N concentration compared to PFTN. MCP (microbial crude protein), estimated by urinary excretion of purine derivatives, absorbed by intestine was not different between HS and PFTN cows. HS decreased plasma AA (total AA and 5 specific of free AA) (P<0.05) and plasma glucose (P<0.1), and tended to increase BUN and increased urea nitrogen in urine (UUN) and decreased NEFA. The decrease in plasma AA could have resulted from an increase in non-mammary AA oxidation or an increase in AA utilization for the synthesis of ligands involved with an acute phase protein response. Regardless, it appears blood AA utilization is reprioritized away from milk protein synthesis during HS.

Keywords: heat stress, milk protein, restricted intake, milk protein precursor, protein metabolism