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Statistical Methods for Improving Sensitivity in Dairy Cattle Feeding Experiments¹

Department of Animal Science, University of Nebraska, Lincoln 68503

ABSTRACT

This study compares four methods of statistically analyzing performance data from a dairy cattle feeding experiment. The basic design (modified switch-back) involved a pre-experimental (Period I), an experimental (Period II), and a postexperimental (Period III) period. Periods I and III were standardization, or control periods, during which all cows were fed the same. Thirty-six Holsteins were used to determine the effect of six treatments (2 x 3 factorial) on milk and solids-corrected milk yield, milk fat and solids-not-fat percentage, dry matter and estimated net energy intake, and estimated net energy/solids-corrected milk. The four methods of analysis utilized different data as follows: (A) average performance in Period II, (B) average performance in Period II adjusted by covariance for average performance in Period I, (C) average performance in Period II adjusted by covariance for both average performance in Period I and body weight at the end of Period I and (D) difference in average performance in Period II relative to that in Periods I and III.

The difference method of analysis yielded the smallest error variances and coefficients of variation for each of the traits studied. Compared to Method A, Method D reduced the error variance for milk yield by 94%, estimated net energy intake by 65%, and fat percentage by 52%. Use of covariance adjustment for Period I performance also reduced error variances to almost the same extent without requiring data from Period III. The inclusion of body weight as a second covariable did not result in any appreciable reduction in error variance compared to pre-experimental performance as a single covariable. Consequently, from the standpoint of sensitivity of dairy cattle feeding experiments, the difference method would be the preferred method among those compared; however, analysis using pre-experimental performance as a covariable is relatively sensitive, especially for milk yield.