Abstract
This study was carried out during the period from February 2016 to February 2017 on 100 Friesian dairy cows aged (3-7 years old) of different parities belonged to private dairy farm which locate at Hehia city at Sharkiya governorate. The aim of the present study was to investigate the effect of age of dairy cows on their ingestive behaviour, suckling disorders, productive performance, milk constituents and serum cortisol level of Friesian dairy cows. The results were revealed that, on ingestive behaviour, the age of 4th years old was the highest significant (P<0.05) feeding time, feed frequency, drinking time, drinking frequency, rumination time and rumination frequency (399.9 min, 87.2, 18.6 min, 142.9, 52.2 min and 37.2); respectively. The mutual suckling time and frequency were at highest significant level (P<0.05) at early life of age (three years), while the tongue rolling, ear and nose suckling were increased with age. On productive traits at different ages studied, the 7 years old age group was the highest significant (P<0.05) at parity (4.66). Meanwhile, the 6 years old age group was the highest significant (P<0.05) at milk production and day in milk (8077.5 kg and 300.0 day; respectively). Dry period showed non significant results between the different ages. The 6th years of age was the highest significant (P<0.05) cortisol level (2.80 ng/mL) than other ages. Correlations between productive and ingestive behaviour traits for all ages studied showed positive high significant estimates (P<0.05) at parity with drinking frequency, milk production with day in milk , feeding time with drinking frequency and feeding time with rumination time. Concerning the effect of age on the milk components, the SNF and protein components of the milk were significantly lower (P<0.05) in young than adult cows (from 3 years to 7 years), however, significant difference was not observed in fat, lactose and pH of the milk between age groups.

Key words: Friesian dairy cow, Age, Ingestive behavior, suckling disorders, productive traits, correlation, milk components.