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Data of 3251 lactation records for the first five lactations of 1022 Holstein-Friesian dairy cows at Alex-Kobenhagen farm during the period from 2000 to 2010 were used to determine the associations between levels of peak yield and peak day with some productive and reproductive traits. The data of peak yield (PY) was categorized as {PY1 (<33.4 kg), PY2 (33.4-<43.03 kg), PY3 (43.03-<52.6 kg) and PY4 (≥52.6 kg)} and the peak day (PD) was expressed as {PD1 (<23.8 d), PD2 (23.8-<77 d), PD3 (77-<131 d) and PD4 (≥131 d)}. The reproductive and productive traits studied were; service per conception (SC), days open (DO), calving interval (CI), total milk yield (TMY), 305-d milk yield (305-MY), lactation period (LP), dry period (DP), fat yield (FatY) and protein yield (ProY). The results revealed that the cows with intermediate peak yield in the first lactation (PY3) tended to stay longer in the herd, on close level of TMY and in intermediate DO value during the five tested lactations, compared with other peak milk yield groups. Moreover, it could be noticed that the PD3 group obtained high TMY and intermediate DO values by the five tested lactations. Insignificant differences have been detected among groups PD1, PD2 and PD3 for all tested traits, except that for SC of group PD3 which was the lowest value. Group PD3 was significantly higher (p<0.05) than group PD4 in TMY, 305-MY, FatY and ProY, while insignificant differences for the reproductive tested traits were detected. The results indicated that regardless of high productive performance, the reproductive performance of cows had early peak milk and delayed DO (PD < DO) was significantly lower (p<0.05) than those had early DO and delayed peak milk. Generally, the present study revealed that cows maintaining peak milk from 77 to <131 day and intermediate peak yield from 43.03 to <52.6 kg could be considered as the most profitable cow. keywords: peak yield, peak day, reproductive and productive performance

Associations between Peak Yield and Peak Day with Productive and Reproductive Traits of Holstein-Friesian Dairy Cows in Egypt