Abstract
Replacement programs and decision-making in dairy herds depends on the production rate. Statistical quality control charts (SQCC) are graphs used to assess the changes occurring in the process of interest over time. The aim of this study was to monitor the milk yield of Holstein-Friesian dairy cows using SQCC. Monthly test dates records of milk yield were collected through dairy com 305 computer system of the farms. A total of 100 random records of cows calved between 2015 and 2021 were incorporated. Subgroups of test dates yields of the first three parities of such cows were analyzed. Mean, standard deviation (SD) and range based control charts were used in this study along with mixed charts of both mean and SD. All mean charts showed central lines of production at 27.9 kg, with control limits ranged between 18.5 kg and 36.4 kg. Variation charts denoted central lines at 8 kg whereas control limits ranged between 1.49 kg and 16 kg. Results showed that milk yield have been found to be out-of-control using all control charts. However, variation charts (SD and range) revealed that the uncontrolled yield was only for data points exceeding the upper control limit (UCL) whereas mean-based charts demonstrated yield beneath the lower control limit (LCL) and other above the UCL. Therefore, the farm breeders and owners could rely on this finding to identify the factors causing deviation under the lower limits and make the required changes according to the cow’s age and parity. Additionally, cows with superior yield above the expectations could be selected for future breeding plans. This study showed the significant importance of SQCC as a guideline for decision-makers at the farm level to monitor milk yield in dairy herds and support quality improvement.

Key words: Holstein-Friesian, Dairy Cows, Test Dates Lactation Records, Statistical Quality Control, Milk Yield, Control Limits