Longitudinal evaluation of environmental and physiological factors associated with semen production in Kankrej breeding bulls using a mixed-effects model.

Reliable prediction of semen production in breeding bulls requires an understanding of the interactions among developmental history, physiological maturity, and environmental conditions, yet these relationships remain poorly characterized in indigenous Bos indicus breeds. This study quantified the independent and interactive effects of developmental, physiological, and environmental factors on longitudinal semen production records of Kankrej breeding bulls using a Restricted Maximum Likelihood (REML)-based mixed-effects model. A total of 4012 ejaculates collected from 24 breeding bulls during 2011-2020 were analysed. Bull was included as a random effect to account for repeated observations, whereas season and period of birth, age at first semen collection, age at semen collection, season and period of semen collection, ejaculate number, and biologically relevant interaction terms were fitted as fixed effects. Mixed-model analysis revealed significant associations between semen production traits and several non-genetic factors, including biologically relevant interactions. Age at semen collection and ejaculate number consistently influenced semen production, while seasonal conditions modified these responses. The findings highlight the value of longitudinal mixed-effects modelling for evaluating non-genetic variation in semen production and may support evidence-based reproductive management, optimization of semen collection strategies, and improved utilization of elite indigenous breeding bulls in artificial insemination programmes.