Abstract:
Proc. Aust. Soc. Anim. Prod. (1978) 12: 226 THE USE OF CONSTANT SAMPLING FACTORS FOR BUTTERFAT ESTIMATIONS OF COMPOSITE MILK SAMPLES G.K. REASON* and G.W. LEGG* Laboratory testing has replaced field testing of samples for milk composition in many dairy herd production recording services. Samples for testing have previously comprised yield weighted subsamples. A quicker, simpler procedure is necessary for use by unskilled labour in high throughput milking facilities. Separate samples of evening and morning milks were collected and analysed in a trial involving over 9>,200 individual cows in 150 herds over a full year. The volumes of evening and morning milks were in the ratio of 1:1.28. In Queensland, herd recording samples are transported bottles. A sample size of 26 ml is considered optimal for maintenance of sample integrity (Legg - unpublished data). system based on available equipment was devised using 12 ml and 14 ml (morning) - a ratio of 1:1.17. in 30 ml Keil the A sampling (evening) To validate this ratio, and to test its sensitivity, a trial was conducted using data collected from 928 cows in 21 herds in a district not used in the original trial. The true composite fat test calculated from evening and morning milk fat tests and recorded yields was compared with composite tests computed for samples with varying ratios of evening to morning milk about the 1:1.17 ratio (Table 1). The mean composite fat test of all samples was 4.13% and the standard deviation 0.79%. We conclude that 1:1.17 is 'a suitable practical ratio for herd recording sampling. Any small human errors which arise in subsampling, when considered for the population as a whole, are very small compared with the errors due to between cow variation. Obviously no single system will suit all cows , particularly when abnormal milk yield or fat tests occur during recording, but small variations in the sampling . ratios will have comparatively little effect. * Dept. of Primary Industries, Breakfast Creek Road, Newstead, Q. 226. 4006
