A negative correlation between growth and reproductive traits has declined fertility in the second half of the flock's reproductive life. Excessive weight gain in males is one of the crucial reasons for the reduction of fertility rate in broiler breeder flocks. Field data from commercial flocks were collected at different ages to determine the excess weight of roosters. Forty-four 17-week-old breeder roosters were weighed individually and divided into two groups (Control and Overweight). With the difference in the amount of feed distribution, a 26 % difference in weight was observed between the two experimental groups after six weeks (23rd week). Sperm and blood samples were collected every four weeks from the age of 24 to 64 weeks to evaluate sperm parameters and testosterone levels. Testis weight and testicular histomorphology were evaluated on weeks 24, 44, and 64
and artificial insemination was performed at the end of the experiment. Results showed that the number of active roosters and semen volume did not differ between the two groups (P >
0.05). Sperm concentration and total sperm motility at 24, 28, 32, and 36 weeks in the overweight group were higher than control group (P <
0.05). In other weeks, there was no significant difference between the two groups for sperm concentration (P >
0.05). Semen lipids oxidation was not significantly different between the two groups at any age (P >
0.05). Testosterone levels in the overweight group up to 32 weeks and in the control group at 40 to 56 weeks of age were high (P <
0.05). The number of seminiferous tubules and Sertoli cells only at 24 weeks in the overweight group showed a higher value than the control group (P <
0.05). Artificial insemination showed no significant difference between the two groups in fertility and hatching rates (P <
0.05). Finally, being overweight during the initial stage of the experiment resulted in early testicular hyperplasia and improved sperm quality, followed by a decline with age. In contrast, the control group showed a pattern of slow increase followed by a decrease.