| Summary: | The study aimed to estimate the components of rumination time (RT) variability recorded by a neck collar sensor and the relationship between RT and milk composition. Milk test day (TD) and RT data were collected from 691 cows in three farms. Daily RT data of each animal were averaged for 3, 7, and 10 days preceding the TD date (RT<sub>D</sub>). Variance component analysis of RT<sub>D</sub>, considering the effects of farm, cow, parity, TD date, and lactation phase, showed that a farm, followed by a cow, had major contributions to the total variability. The RT<sub>10</sub> variable best performed on TD milk yield and quality records across models by a multi-model inference approach and was adopted to study its relationship with milk traits, by linear mixed models, through a 3-level stratification: low (LRT<sub>10</sub> ≤ 8 h/day), medium (8 h/day < MRT<sub>10</sub> ≤ 9 h/day), and high (HRT<sub>10</sub> > 9 h/day) RT. Cows with HRT<sub>10</sub> had greater milk, fat, protein, casein, and lactose daily yield, and lower fat, protein, casein contents, and fat to protein ratio compared to MRT<sub>10</sub> and LRT<sub>10</sub>. Higher percentages of saturated fatty acid and lower unsaturated and monounsaturated fatty acid were found in HRT<sub>10</sub>, with respect to LRT<sub>10</sub> and MRT<sub>10</sub> observations.
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