The objective of this study was to propose and validate a model to estimatethe apparently digestible fraction of crude protein (CP) of cattle diets. Thetheoretical framework for model development was based on true digestibilitycoefficient and metabolic fecal contribution. The estimation of both parameterswas done by using the Lucas test. The dataset for model building was composedby 39 and 54 means from 10 experiments with lactating cows and 23 experimentswith growing and finishing cattle, respectively. For comparisons of animal categoriesa Dummy variable was used. The dataset for model validation came fromtwo trials with lactating cows and two trials with growing and finishing cattle.The true digestibility coefficient, which was estimated as 0.7845 did not differamong animal categories. The greatest metabolic fecal CP was observed on lactatingcows but it was not affected by dietary CP level when expressed as percentageof dietary CP. Thus, for animals fed ad libitum the content of apparentlydigestible CP (CPad%) in the diet can be estimated by the followingequations: CPad% = 0.7845CP% - 0.97 for lactating cows and CPad%= 0.7845CP% - 1.61 for growing and finishing cattle. The validation procedureindicated no differences between observed and estimated values of CPad%.It can be concluded that the proposed models will improve the accuracy of TDNprediction using the chemical composition of the diet.
