• Ismail Mohamed Elfagir
• Abdulmoniem M. Abu Nikhaila
• Mohamed Khair Abdaalla
• Adil Mousa Younis Waniss

Abstract The aim of this research was to study the age at which Holstein Friesian heifers were first serviced under the factors of year , and season of birth, class of weight at weaning , class of age at weaning and their interactions under the prevailing environmental and management conditions in a tropical country, Sudan. A total number of 505 records of age at first service of Holstein heifers born in the northern Gezira dairy project was used.The data covered the period from the year 1992 to 2000.They were analyzed by the Method of least Mean Squares (Steel, Torrie and Dickey,1997). The overall mean of age at first service was 21.82 ± 0.28 months . The effect of year of birth on this trait was highly significant (P ≤ 0.001) while the effect of season of birth was not significant (P≥0.05). The weight at which the heifers were weaned significantly affected the age at first breeding (P≤ 0.05), while the age at which the heifer was weaned exerted no significant effect on the trait (P ≥ 0.05). the interactions of the factors with each other did not exert any significance on age at first service (P≥ 0.05). It is concluded that the results of this study have added to the information about the age at first service of Holstein Friesian heifers born and reared in a tropical environment without using any means to mitigate the effects of heat stress .

Key words: year, season ,age at first service , weight at weaning , age at weaning, interactions.

"Age at First Service and Factors Affecting it in Holstein Friesian Heifers in Northern Gezira Dairy Project", International Journal of Emerging Technologies and Innovative Research (www.jetir.org), ISSN:2349-5162, Vol.11, Issue 11, page no.c324-c334, November-2024, Available :http://www.jetir.org/papers/JETIR2411241.pdf

"Age at First Service and Factors Affecting it in Holstein Friesian Heifers in Northern Gezira Dairy Project", International Journal of Emerging Technologies and Innovative Research (www.jetir.org | UGC and issn Approved), ISSN:2349-5162, Vol.11, Issue 11, page no. ppc324-c334, November-2024, Available at : http://www.jetir.org/papers/JETIR2411241.pdf