Abstract: [Objective] This study was to explore and quantify the differences in fat and protein percentages between milk samples collected on test day milk recordings and those obtained from daily bulk collections in commercial dairy herds in Ireland. [Method] The dataset comprised 5 742 milk recordings from 2 841 herds in 2021,where daily bulk collections were taken on the same day as the milk recording. The predicted fat and protein percentages for test day milk were estimated using either the AM or PM milk samples and were incorporated into prediction equations approved by ICAR. [Result] The Pearson correlation between fat and protein percentages from test day milk recording and daily bulk samples was 0.82 and 0.96,respectively. These correlations correspond to mean differences of 0.1% and 0.01% for fat and protein percentages,with the average test day milk recording showing lower values for both measurements. Fat percentage correlations for herds using EDIY （electronic do it yourself meters which take a test sample automatically） and Non-EDIY （manual recordings where a milk recorder visits and takes a test sample） recording devices were 0.87 and 0.72, respectively. Herds with a higher average test day cow yield demonstrated a lower fat percentage correlation. Similarly,samples taken during peak milk production season exhibited larger differences in fat percentages compared to those taken during the off-peak milk production season. [Conclusion] The findings of this study clearly indicate that the largest variation is observed in the fat percentage reported in test day milk recordings when compared to the corresponding daily bulk samples. The extent of this variation is affected by yield,season,recording type,or a combination of all three factors.

Key words: milk recording, Irish, dairy farm, daily cow