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Thirty Friesian bull calves averaged 8.5 ± 0.16 month of age and 195 ± 15 kg body weight were used in this study. All bulls were fed similar diets. The bulls were divided into three similar groups (10 each), according to their weight and age. Bulls in the 1 st group were considered as control (G1). while those in the 2 nd group were received single oral dose from L-tyrosine (50 g/bull) at 8.5 months of age (G2). Bulls in the 3 rd group were received two oral doses from L-tyrosine (50 g/bull) at 8.5 and 10.5 months of age (G3). The results revealed that L-tyrosine treatment increased (P<0.01) the live body weight (LBW) in G3 and G2 compared to G1 at 12 month, first appearance of spermatozoa, sexual puberty, and at 24 months of age. Age at first appearance of spermatozoa and puberty age were (P<0.001) earlier by about 1.2 and 1.5 and 0.9 and 1.7 months, respectively, for bulls in G2 and G3 than the control. Changes in testosterone concentration during the study period were higher (P<0.05) in G3 and G2 than in G1. L-tyrosine increased (P<0.01) percentages of progressive motility and livability of spermatozoa, and sperm cell concentration compared to control at first appearance of spermatozoa, puberty, and at 24 months of age. In general, semen quality improved (P<0.01) at 24 month of age as compared to puberty age. Sperm motility and livability and sperm cell concentration significantly increased (P<0.01) with advancing age in all groups. Morphology defects in the head of spermatozoa were lower (P<0.05) in G2 and G3 than in G1 at first appearance of spermatozoa, sexual puberty and at 24 months of age. Also the proximal droplets were lower (P<0.05) in G2 and G3 than in G1 at puberty and at 24 months of age. There was a significant increase in number of normal sperm cells with age advancement as well as significant decreases in proximal droplets and head defects. The present results indicated that treated bulls by one or two doses of L-tyrosine (50 gm/bull calves) induced an improvement of growth performance, earlier sexual puberty and improved semen quality of Friesian bull.

EFFECT OF L-TYROSINE ON GROWTH PERFORMANCE, AGE OF SEXUAL PUBERTY, SEMEN QUALITY AND TESTOSTERONE LEVEL IN FRIEZIAN BULLS