Differences in the biological stress responses of two cattle breeds to walking up to mountain pastures in the Pyrenees.

The aim of this experiment was to study the biological responses of Pirenaica and Parda Alpina cows (2-4 years old, n = 32) and their calves (2-4 months old, n = 32) to the physical exercise of climbing to mountain pastures in spring. These two breeds have a different sensitivity to stress-induced myopathy, the Pirenaica calves being highly sensitive. The animals walked 14 km with an elevation increase of 600 m, in approximately 3 h. Blood samples were obtained the day before, immediately after the move to the mountain pastures and the following day. The plasma cortisol levels in Pirenaica cows were significantly higher than in Parda Alpina cows under basal conditions, but disappeared in later samples. There were also significant differences in the hematological parameters. Under basal conditions, the red blood cell counts, packed cell volume, and hemoglobin content of blood were higher in Parda Alpina than in Pirenaica cows, but calves of both breeds did not differ. On the other hand, the calves of both breeds were more sensitive than their dams to the exercise, as shown by the large increase in white blood cell counts with a reversal of the lymphocytes:neutrophils ratio, a consequence of cortisol secretion. In calves the largest differences were observed in the muscle enzyme activity levels measured in serum (creatin kinase, lactic dehydrogenase, aspartate amino transferase and alanine amino transferase), which were always higher in the Pirenaica calves both under basal conditions and after exercise. These biological changes reflected the sensitivity of Pirenaica calves to muscular problems when they walk up to the pasture. It was concluded that genetic factors and age might play essential roles in the neuroendocrine and metabolic adaptation to this unusual physical exercise.