Bu çalışma, postpartum 40. günden sonraki dönemde bulunan, sağlıklı 120 baş safkan Holstein inek, spontan östrus gösterenler (Grup A; n=60) ve prostaglandin (Cloprostenol, 2ml: 526 ig Cloprostenol sodium; Estrumate) hormonu kullanılarak senkronize edilenler (Grup b; n=60®) olmak üzere iki gruba ayrıldı. Bu her iki grupta GnRH (Gonadorelin; 2 mi: 10 $\mu$ g: Ovarelin®) hormonu uygulamalarının zamanları dikkate alınarak ve her birinde 20 baş inek sayısı olmak üzere, alt gruplara (Grup Al; Ali: AIII-Grup BI: Bil: Bill) bölündü. Grup Al ve grup Bl'deki hayvanlara tohumlamadan 6 saat önce, Grup Ali ve BU'deki hayvanlara da tohumlama esnasında GnRH enjeksiyonu uygulandı. Grup Alil ve grup BHI'deki (Kontrol) hayvanlara GnRH enjeksiyonu yerine serum fizyolojik enjekte edildi. Sunulan çalışmada, aktif bir corpus luteum belirlenerek $PGF2\alpha$ uygulanan ineklerin tümünün östruslannın uygulamayı izleyen 72 saate toplandığı gözlendi (Grup B). Çalışmada buzağılama-ilk tohumlama aralığı, grup Al'de 65.2 ±11.21, grup Ali'de 65.41±10.39, grup Bl'de 60.0±9.08 ve grup BlI'de 64.7±2.99 olarak belirlendi. Ortalama buzağılama ve ilk tohumlama aralığı grup BI, grup Bil ve grup BI, grup Bil arasında belirgin bir farklılık gösterdi (p

The experiment was performed on 120 Holstein cows 40 days after parturation. Firstly, these cows were divided into two groups as cows that were shown spontanaeous oestrus (group A, n=60) and cows that were synchronised with PGF2a when a mature corpus luteum was found (group B, n=60). Cows that were shown the spontaneous oestrus were alloted separately to three subgroups and given 100 ig gonadorelin at 6 hours before insemination (group Al, n=20), 100 ig gonadorelin at time of insemination (group All, n=20) and a saline (control, group III, n=20). Cows in group B were assigned equally to three subgroups as like group A and given 100 ig gonadorelin at 6 hours before insemination (group BI, n=20), 100 ig gonadorelin at time of insemination (group BII, n=20) and control (group Bill, n=20). In this study, all cows that were injected when a mature corpus luteum was found (group B, n=60) were synchronised their oestrus at approximately 100 percent within 72 hours. The interval from calving to first service day were 65.2±11.21 in group AI, 65.41±10.39 in group All, 60.0±9.08 in group BI and 64.712.99 in group BII. The mean interval to the first service showed significant diffirences between group BI and group BII (p<0.05).The mean calving to conception interval was short in group BI and BII (group BI, 86.33±5.69 days; group BII, 70.0±25.76 days) compared with group AI, group All and each control groups (100±28.77, 100.33±28.76, 100.9±5.92 and 103.6±1.21 respectively). Cows that were synchronised with $PGF2\alpha$ and received 100 ig gonadorelin at the time of insemination (group BII) had significant effect on the mean calving to conception interval in comparison with other groups (p<0.05). The mean pregnancy rates, to first insemination were 40 percent iagroup BI, 80 percent in group BII and 60 percent in group BUI, while the pregnancy rates in group AI, II and III were 49%, 45% and 35%, respectively. Cows in group BII had a significantly higher pregnancy rate than other groups but not among group AI, AII and AIII. It is suggested that the $PGF2\alpha$ injections when a functional corpus luteum was found 40 days after parturation and GnRH administration at the time of insemination could improve the pregnancy rate to first service and the calving conception intervals.