Udder and Teat Morphology in Relation to Milk Yield in Dairy Mares: Farm-Level Confounding in a Multi-Farm Observational Study

Mare’s milk carries cultural and economic importance in Central Asia, yet udder and teat morphology in relation to individual milk yield remain poorly characterized, and farm-level confounding inflates apparent associations in multi-farm data. We analyzed 333 mares with complete morphology and yield records across 12 farms. A linear mixed-effects model with farm as a random intercept estimated ICC = 0.747; approximately three-quarters of yield variance occurred between farms. Leave-one-farm-out ICC values ranged from 0.663 to 0.768, indicating that no single farm drove the result. Pooled Pearson correlations attenuated markedly after within-farm demeaning; udder depth showed near-complete attenuation (raw r = −0.503; demeaned r = 0.098). Among udder and teat traits, within-farm correlations were weak, with the largest being for teat length × udder width (r = 0.207; 95% CI: 0.087–0.348) and teat length (r = 0.178; CI: 0.076–0.318). No morphological fixed effect was statistically significant. After farm adjustment, teat length and teat length × udder width are hypothesis-generating traits that warrant prospective evaluation with parity and lactation-stage control.