Endocrine Changes in Two Behavioral Types of Cows, Exposed to Heat

1 Institute of Animal Science, BG-2232 Kostinbrod, Bulgaria
2 University of Missouri, Columbia, USA


GUDEV, D., S. RALCHEVA, P. MONEVA and H. D. JOHNSON, 2004. Endocrine changes in two behavioral types of cows, exposed to heat. Bulg. J. Agric. Sci., 10: 629-636

Cows at the University farm were divided into 3 behavioral groups according to their emotional response to the experimenter as follows: 1st behavioral type - cows, expressing steadfast food response; 2nd behavioral type - equally expressed food and defense responses; 3rd behavioral type - timid, with predominance of the defense response.
Holstein cows from 1st (n=3) and 3rd (n=3) behavioral types at 1st lactation were transported to the Missouri climatic laboratory (20 miles) and exposed to heat (35°) for 7 hours (from 9.30 to 16.30). Blood and milk cortisol, epinephrine, norepinephrine, growth hormone and prolactin were measured before transportation (7.30 h) and during exposure to heat (9.30 h, 12.30 h, 13.30 h, 15.30 h and 16.30 h,). Physical and emotional responses raised by the transport and the novelty in the climatic laboratory resulted in a significant elevation in plasma and milk cortisol levels immediately after placement of the cows in the climatic laboratory (9.30 h). The third type of cows tended to have higher cortisol level.
Heat exposure caused a significant decline in plasma and milk cortisol levels. The enumerated environmental stressors resulted in plasma norepinephrine elevation in 3rd type of cows (P>0.05) In contrast norepinephrine in 1st type of cows remained unchanged. Plasma norepinephrine level in 3rd type of cows declined significantly under heat but tended to be higher compared to that in 1st type of cows. Milk norepinephrine levels during heat exposure were under sensitivity of the method. Basal plasma epinephrine levels were higher (P>0.05) in 3rd type of cows. Plasma epinephrine levels under heat were under sensitivity of the method. The emotional arousal caused a significant rise (P<0.01) in plasma prolactin levels, which remained high under heat in both behavioral types of cows, but prolactin levels tended to be higher in 3rd type of cows. The stressors did not influence milk prolactin levels, in both types of cows.
Plasma and milk growth hormone levels were not significantly influenced in 1st type of cows by the exposure to heat. However plasma GH level in 3rd type of cows was significantly higher at 15.30 h and 16.30h, as compared to that at 7.30 h. The presented results suggest that the specific response to heat stimuli has overwhelmed the non-specific "psychological" response in order to reduce the level of calorigenic hormones and to counteract heat-induced inhibition of the immune response. Besides, these results show that plasma cortisol, epinephrine and norepinephrine are related to the behavioral type.

Key words: turkey broilers, grid floor, live weight, feed intake, growth