Effect of Frequency and Spacing of Sampling on Accuracy and Precision of Estimating Total Lactation Milk Yield and Characteristics of the Lactation Curve

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Abstract
Daily milk weights of 255 cows in seven herds were sampled using different frequencies and spacing of samples to investigate accuracy and precision of estimating total yield. There were four sampling methods with equal intervals and six methods with unequal intervals. Unequally-spaced samples were placed according to varying degrees of relative emphasis on peak production period of a lactation, which was defined as 15 to 90 d postpartum. For each sampling method, total lactation yield was estimated by linear interpolation and by fitting a nonlinear function to the lactation curve. Fixed factors affecting biases in estimators of total yield and of parameters of the lactation curve were analyzed by a multivariate linear model. Sampling methods differed significantly in ability to predict total yield accurately, and nearly all methods tended to overestimate actual yield. However, sample biases were insignificant for all equally spaced methods. Significant biases most often occurred, in some season of calving and parity classes, when 10 samples were taken and they were spaced unequally. The largest biases occurred when the postpeak period was sampled infrequently. Therefore, the convenient and conventional 30-d equal interval sampling procedure should give acceptable estimates of total lactation milk yield in practice. Both accuracy and precision in estimating characteristic parameters of the lactation curve increased with frequency of sampling, as expected. As for linear interpolation, sampling emphasis on the postpeak period was important in determining the magnitude of biases in the nonlinear procedure.