Within Breed Variations in Milk Composition and Estimating the Average Composition of Milk for the Lactation from Partial Records

Abstract

Summary Milk composition using standard procedures was determined at monthly intervals for 4 yr on 178 purebred cows in the University herd to compare within breed variation of the components. The standard errors of estimate ( s y·x ) were 0.26, 0.20, 0.14, and 0.12% for per cent milk fat, solids-not-fat (SNF), protein, and lactose, respectively, when average composition of one record was used to estimate the average composition of the next record (62 pairs); similarly compared, repeatabilities were 0.76, 0.72, 0.67, and 0.65. Linear regression equations were computed comparing average composition of the milk for partial lactations with the completed lactation. The individual breed regressions were significantly different for single test periods 1-8 and for accumulative test periods 2-8 for per cent milk fat and per cent SNF; per cent protein did not differ significantly when six or more consecutive tests were used (accumulative), and the per cent lactose regression equations were similar when at least two consecutive tests were used. Average per cent SNF, protein, and lactose for 305-day lactation periods were satisfactorily estimated from partial records comprising accumulative averages for three or more consecutive tests at monthly intervals.