Temporal dynamics of sperm morphology in broiler breeders: Evidence of rapid non-linear patterns of decline

Broilers are the cornerstone of poultry meat production. However, recent trends have raised concerns about broiler breeder fertility/ hatchability, which is predicted to decline to 60% by 2050. Data from the National Agricultural Statistics Service (NASS), in 2024, confirms that broiler breeder fertility continues to decline, now at 79%. The general consensus is that these declines are a result of selection focusing on 'production traits', but the exact mechanisms and causative factors are unknown. The challenge of improving broiler breeder fertility is also compounded by the relative lack of knowledge surrounding male broiler breeder reproduction traits and the relationship between male reproduction and growth. These knowledge gaps limit our ability to address and solve the ongoing fertility and hatchability crisis in the poultry industry. In this study, we characterized changes in broiler sperm quality in Ross broiler breeders (Yield Plus males) using a longitudinal analysis of sperm morphology features. We collected semen from the male breeders on a weekly basis, then categorized 100 sperm cells per sample. Cells were identified based on a standard set of classifications developed from the literature, and the changes in the relative proportions of sperm morphologies were analyzed to determine the best-fitting model. Sperm with normal morphology were the most prevalent across samples, while the proportion of different defect types increased, decreased, or varied with age. Using Bayesian analyses, the derivatives (rates of change) of normal sperm proportions and the proportions of defects were analyzed to estimate the time of peak utility. Our peak utility function identified an age of 9.48 months (CI 95% = 9.02-10.42, about 38 weeks), at which male broiler breeders are at their peak in terms of sperm quality and fertility. This finding indicates that the peak breeding performance of males occurs significantly earlier and for a shorter duration than previously documented. This work bridges the knowledge gap in broiler reproduction and provides tools for incorporating sperm phenotypes into future poultry and livestock breeding models.