CORRELATIONS BETWEEN ADJACENT TEST DAYS FOR FAT PERCENT, FAT YIELD AND MILK YIELD

Abstract

Retest data from the Record of Performance for dairy cattle were used to examine the day-to-day and month-to-month variation in fat percent, fat yield and milk yield and the effect of stage of lactation on this variation. The overall correlation between fat percent on successive days (retest day) was low (0.71) in Holsteins compared to that for fat yield (0.85) and milk yield (0.96). Correlations between values on either test day or retest day with those on the next test day (1 mo later) were 88–90% of those between successive day values for milk yield and fat yield but only 73% for fat percent. Correlations of fat percent on the next test day with those on the original or retest day were particularly low (< 0.40) for the first 60 d in milk. Fat percent is much more variable from day to day and from test day to test day than milk yield or fat yield, particularly in early lactation. Key words: Dairy, test-day, correlations, milk, fat