Variation in Estimates of Milk Fat, Protein, and Lactose Content Asssociated with Various Bulk Milk Sampling Programs1,2

Abstract

Three milk samples were taken from each bulk shipment from 26 herds for 13 mo; four samples were taken from each bulk shipment from 22 herds for a month. All herds were located in the Fraser Valley in British Columbia. Some samples (5656) were analyzed fresh with duplicate determinations on 955; the remaining 9842 were used to form herd composite samples (both 1-wk and 2-wk collection periods per composite). All samples were analyzed for milk fat, protein, and lactose by Infrared Milk Analyzers. These data were used to estimate the variation associated with various sampling and testing programs for bulk milk. The estimate of monthly herd mean milk constitute percentage from four simple random samples or three stratified random samples is expected to be at least as precise as the estimate from two 2-wk composite samples. Two-week composites underestimated percent milk fat by .045±.004 and overestimated percent protein and lactose by .023±.003 and .010±.002 compared to fresh samples. Thus, random sampling is expected to be cheaper and more accurate than composite sampling. Milk marketing areas considering changing from composite to random sampling may find useful the formula for calculating the number of random samples required to maintain the precision of composite sampling.