Abstract
The objective of this study was to compare feed intake and milk production responses in continuous and change-over design dairy cow experiments with a meta-analysis. The dataset included 988 treatment means (354 in continuous and 632 in change-over trials) from 204 studies. Sub-datasets from studies investigating the effects of the amount (n=217) and crude protein (CP) concentration of concentrate (n=336 diets) were analyzed separately. Mixed model regression analysis with a random study effect was used evaluate the effects of experimental design on feed intake and milk production responses. Concentrate dry matter (DM) intake, dietary CP or metabolizable protein, total DM intake index and metabolizable energy intake were used as independent variables in univariate models. In addition, multiple regression models were developed from the continuous and change-over trial data and cross-validated using the other data. Generally the intake and milk production responses did not depend on experimental design, and the P-values for the interactions between the experimental design and independent variables were high. When the data from studies displaying large variation in the intake potential were analyzed separately, change-over designs tended to underestimate milk production responses. However, up to the range of 5kg/day in DM intake potential within a study the feed intake and milk production responses to the changes in nutrient supply were similar irrespective experimental designs. The analysis of five cyclic-change-over studies including 60 diets indicated that there was no relationship between the direct and carry-over effects of dietary treatments. The mean squares of carry-over effects were on average similar to the error mean squares. The probability of the significance of carry-over effects was not different from the random effects. It is concluded that the change-over designs are as accurate as continuous designs in estimating feed intake and milk production responses, excluding studies with extreme treatments resulting in extensive mobilization of body fat and protein.
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