S. Ribarski1*, H. Lukanov2, I. Penchev3, A. Genchev2, E. Lyutskanova2 1Department of Morphology, Physiology and Nutrition, Faculty of Agriculture, Trakia University, 6000 Stara Zagora, Bulgaria 2Department of Animal Science, Faculty of Agriculture, Trakia University, 6000 Stara Zagora