The uncertainty of results when estimating daily milk records

Abstract

Two studies were conducted to evaluate the uncertainty of measurements (daily milk yield, fat, protein and lactose content) on Slovenian milk recording data, respecting the ISO 5725 definition of precision i.e. as repeatability (sr) and reproducibility (sR) standard deviation. The aim of study 1 was to evaluate the within operator (repeatability) variability of daily measurements and to compare the precision of AT (alternating morning and evening milking records at subsequent monthly visits) and the ICAR standard reference A4 recording scheme (monthly records of two daily milkings). In study 2 we were interested in the reproducibility of milk recording results and the contribution of factors associated with the operator (official representative of the recording organization) and herd or residual variation. In addition, the precision of daily milk yield measurements was studied in relation to herd size and cow productivity. Study 1 was conducted on 332 cows from 14 herds with 14 operators. Study 2 was made on 57639 records gathered from the Slovenian central milk recording database. The results on two consecutive days were obtained by the same operator (study 1) or by operator and supervisor (study 2). Statistical analysis was performed on the differences between two consecutive days. The lower sr obtained for the AT than A4 recording method (1.7 kg vs. 1.2 kg, respectively) indicates a loss of precision in daily milk recording results due to the passage to the AT recording scheme. In current Slovenian situation (AT recording scheme), the reproducibility standard deviation obtained on two consecutive days was about 1.7 kg, 0.40, 0.11 and 0.09 for milk yield, fat, protein and lactose percentage indicating that we can normally expect the difference between regular control and supervision up to approximately 5 kg, 1.2%, 0.3% and 0.3% for daily milk yield, fat, protein and lactose, respectively. Similar estimates were obtained for repeatability and reproducibility standard deviation. Uncertainty in milk yield results explained by the factors related to the operator pair was minor (0.4-2.5%) compared to herd related factors (11.0-15.1%), while the main part (82.4-88.6%) was due to biological variability and other uncontrolled factors. Lower reproducibility of daily milk yield was associated with small herds and/or lower productivity. © INRA, EDP Sciences, 2006.