ERRORS OF ESTIMATING 300-DAY MILK YIELD IN DAIRY COWS USING VARIOUS SAMPLING INTERVALS

Daily milk records from 367 lactations of 179 Dairy Shorthorns and 147 lactations of 89 Friesians were used to study the errors which arise when milk yield is estimated from sampling at 7-, 14-, 30-, 42-, 60- and 90-day intervals. No consistent bias of practical importance was found except at the 90-day testing interval when biases of +30 gallons in Friesians and +15 gallons in Shorthorns were estimated. Ninety-five percent confidence limits on the errors of estimate varied with month of calving and length of lactation but tended to be in the order of ? 15, ?20, ?34, ?45, ?59, and ?110 gallons for 7-, 14-, 30-, 42-, 60- and 90-day testing intervals.