Résumé
The objective of the present work was to determine the effect of Ovsynch protocol on pregnancy rate in lactating dairy cattle subjected to synchronization of ovulation during summer season.Induction of ovulation for timed artificial insemination [TAT] with Ovsynch protocol was conducted on forty Holstein Friesian cows from a commercial dairy herd in Alexandria disert road [Dina farm],Egypt during the summer season of 2004. Animals were assigned to four groups according to lactation number: G I 1[st]lactation n=13'',G II"2[nd] lactation n=7",G III"3[rd] lactation n=14" and GIV" 4[th] lactation n=6".The mean number of days postpartum [DM] at the start of treatment was 89.38, 77.28, 90.42and 84.66 for group I, II, III and IV, respectively. The frozen-thawed semen used for AI was of good quality. Pregnancy diagnosis was checked by rectal palpation 45 to 55 d after insemination. The pregnancy rate per AI was defined as the percentage of cows that were confirmed pregnant after one AI. High pregnancy rate [50%] was recorded for animals in G IV followed by G H and G HI [42.85%]. Low pregnancy rate [30.7%] was reported in Group I .The overall pregnancy rate was 40% . r From this investigation it could be concluded that synchronization of ovulation with GnRH and PGF2 a provided an effective way to manage reproduction in lactating dairy cattle exposed to heat stress during summer