BackgroundLeg weakness affects animal welfare and is one of the primary reasons for culling of boars. Low bone mineral density (BMD) is one of the primary factors contributing to leg weakness. Low BMD also appeared to be associated with severe bone pain and has the highest risk of skeletal fragility. Surprisingly, few studies have been performed on the factors influencing BMD in pigs. Therefore, the primary aim of this study was to identify the influencing factors on boar BMD. Herein, the BMD data were determined through the use of ultrasonography from 893 Duroc boars. Logistic regression model was utilized in the analysis of BMD, in which the explanatory variables in the model were lines, ages, body weights, backfat thicknesses and serum mineral element concentrations (Ca, P, Mg, Cu, Fe, Zn, Mn, Se, Pb and Cd).ResultsResults showed that factors significantly influencing BMD included serum Ca, P concentrations, ages and backfat thicknesses (P< 0.05), in which serum Ca concentrations were positively correlated with BMD (P< 0.01), whereas increasing concentrations of serum P decreased BMD (P< 0.01). The serum Ca/P ratio showed significant quadratic effects on BMD (r = 0.28,P< 0.01), and the Ca/P ratio to achieve the best BMD was determined to be 3.7. Furthermore, BMD also changed with age quadratically (r = 0.40,P< 0.01), and reached a peak value around 47 months. Interestingly, a quadratic (r = 0.26,P< 0.01) increase in the BMD was observed as backfat thickness increased, and the inflection point was calculated at around 17 mm.ConclusionIn conclusion, BMD characteristics of boars could be detected by ultrasonic method, and serum Ca, serum P, age, and backfat thickness contributed to the greatest effect on BMD.Supplementary InformationThe online version contains supplementary material available at 10.1186/s40813-023-00318-w