الفهرس 
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Abstract
The present study was carried out using of six Holestien- Friesian bulls differed in their ages from 8-10 years old and their weights from 600-800 kgs at Nobaria Military corporation from the sixth of October. All bulls were used in natural service and adapted to maintain their sexual activity. After three days from the last service semen were collected using the artificial vagina one time/week. The six bulls were assigned into three comparable groups where Group I was injected by PGF2°C. Each bull of this group injected one time every month by lutylase® (5ml intra muscularly) for three months. Group II was injected by gonadotrophine releasing hormone each bull was injected one time by Receptales’. (5mI intramuscularly) every month for three months respectively. Group III was injected using a combined of PGF2°c-GnRH intramuscularly one time every month for three successive injections of (5ml lutylase® and 5 ml Receptales intramuscularly). The sexual activity of bulls was measured by estimating the reaction time per second. The semen was evaluated as follows: Volume (ml), pH, the sperm motility %, the percentage of live/dead” spermatozoa, the sperm concentration xI06/ml, the percentage of primary sperm abnormalities and pregnancy rate for each group. The obtained results may be summarized as follows: * Treatment the bulls with PGF2°C or GnRH and both together decreased the reaction time. The values obtained before treatment were (102.35+6.22;123.75+15.33 and 121.87+6.15 Sec) for the three groups, respectively. These values decreased to 78.95+ 4.58Sec, 47.89+4.49 and 60.38+3.33Sec for PGF2°C& GnRH and both together, respectively. Regarding the effect of treatments. there were a highly significant differences in the reaction time among the treated groups with more prominent effect ofGnRH (47.89+4.49SeC). * There was signific8Jlt increase in the semen volume for bulls treated with PGF2ClC(3.763+0.123ml) compared to that recorded before treatment (2156±O. 124ml) while a non significant variation was observed u*r the effect of GnRH (3.355±O.130ml) compared with (3.89+0.115ml) before treatment and PGF2°CGnRH (4.100+0.120ml) compared with (3.785+0.l0ml)) before treatment. * The present results did not reveals any significant difference in hydrogen ion concentration either before or after treatment with PGF2°C. Mean while a significant variation was observed under the effect of GnRH (6.789+0.025) compared with (6.661+0.028) before treatment and PGF2°c-GnRH (6.787+0.015) compared with (6.698+0.020) before-treatment. * There was a highly significant increase in sperm motility % for PGF2°C (65.00+1.24%) , GnRH (66.32+1.56%) and PGF2°CGnRH (75.75+1.27%) compared with the values obtained (58.24+ 1.86%, 60.25+1.52% and 60.42+1.53%) before treatment for three groups respectively. The present result showed a highly significant increase in sperm motility % of bulls treated with combined of PGF2°CGnRH than the others treated with PGF2°C or GnRH. * Comparing the percentages of live sperm % before and after treatments there were a highly significant increases by treating the bulls with PGF2°c(73.00+1.56%), GnRH (76.210±1.35%) and
PGF2°c-GnRH (81.725+0.768%) while the values obtained before
treatment were, (66.44+2.13%), (69.88+1.25%) and 69.58+
0.847%) for the three groups, respectively. * There were a highly significant increases In sperm cell concentration due to treating the bulls with PGF2°C (1097.3 7+ 25.62 x 106/ml), GnRH (1035.26 + 23.600 x 106/ml) and PGF2°CGnRH (1108.80+32.83 x 106/ml) compared with those values obtained before treatments (727.65+23.20, 752.22+27.37 and 820.63+ 14.98 x 106/ml) for the three groups respectively. * There was a non significant increase in pnrnary sperm abnormalities % in the first and the second ejaculates where the values obtained with PGF2°C were (7.105+0.350%) and (7.105+ 0.358%), respectively. These values before treatment were (6.882+ 0.352%) and (6.353±O.353%), for the same periods respectively. * The present results showed remarkable increase in the rate of pregnancy caused by the bulls treated with PGF2°C,GnRH and PGF2°c-GnRH where the values obtained were 57.24%, 58.99% and 61.70% respectively. These values before treatment were 28.2%,29.45% and 27.9% for the three groups respectively. As a general conclusion we can use the PGF2°C, GnRH and PGF2 oc-GnRH for the improvement of the Libido & semen quality of the bulls.