I. Izhaki

Reduced protein and/or energy in the diet may be of a high probability to occur in free-grazing ruminants in Israel. In ruminants, low protein intake limits the utilization of crop residues as a source of energy, due to a low supply of nitrogen to the rumen microbes. Thus, protein-deficient diets in ruminants may, in fact, be caloric-deficient diets with similar metabolic modulations. The aim of this study was to evaluate the differential effect of protein and energy stresses, at the organism, biochemical, and molecular levels, in order to enable the development of specific markers that would reliably monitor the source of stress in free-grazing beef cattle. Heart rate (HR), retained energy, dry matter intake (DM), free fatty acids (NEFA), and beta-hydroxybutyrate (BHBA), and the expression of various proteins in the subcutaneous fat were tested in pregnant and lactating beef cows in response to energy and protein stresses. The results show the following: (1) Energy and protein stresses reflect similar changes in HR and energy balance. (2) NEFA increase precedes the appearance of BHBA in the serum, in response to nutritional stress. (3) Reduction of protein in the diet stimulates a delicate molecular balance between lipid and protein metabolism.