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Leaching losses of nitrogen (N) from grazed pasture are driven predominantly by urinary N deposited by cattle. This work aimed to use non-invasive accelerometer-based sensors to capture the timing and duration of urination and defecation events in grazing cattle. To register the back arching motion that accompanies urination and defecation, each cow was fitted with two accelerometer sensors. Experiments took place outdoors under grazing conditions during two seasons—autumn and summer—and captured a total of 160 urination events. Across the majority of these events, the tilt angle was observed to shift within a range of 8 to 22.5 degrees. Using 18 selected features that characterise a cow's back arching, ensemble machine learning classifiers were constructed to recognise urination and defecation. The urination classifier delivered an F1 score of 0.80 (harmonic mean computed from a precision of 0.82 and a sensitivity of 0.78). In contrast, the defecation classifier gave an F1 score of 0.73 (calculated from 408 hours of recordings made between 9 a.m. and 3 p.m. from 19 cows). Predictive models for urination event duration were statistically significant. Still, they underperformed, with low explanatory power, in part because the relationship between the duration of back arching and the duration of urine passage was only moderate (R² values of 0.29 in autumn and 0.47 in summer). The work highlights that non-invasive accelerometer sensing has real promise for capturing individual cows' urination patterns and for generating animal-level information that farm decision support systems (such as those addressing urine-N excretion) and managerial actions (such as selective culling or breeding) could use as input.