Ruminal Degradability and Summative Models Evaluation for Total Digestible Nutrients Prediction of Some Forages and Byproducts in Goats

In in vitro true dry matter degradability (IVTDMD), in situ dry matter degradability, and neutral detergent fiber degradability, both in vitro (IVNDFD) and in situ (ISNDFD) techniques were used with crossbred goats to determine dry matter and neutral detergent fiber (NDF) ruminal degradability in eight forages and four industrial byproducts. Total digestible nutrients (TDN) content obtained with five different summative models (summative equations) were studied to compare the precision of estimates. All these models included digestible fractions of crude protein, ether extract, and nonfiber carbohydrates that were calculated from chemical composition, but digestible NDF (dNDF) was obtained from IVNDFD (IVdNDF), ISNDFD (ISdNDF), or by using the Surface Law approach. On the basis of the coefficient of determination ( ) of the simple lineal regression of predicted TDN ( -axes) and observed IVTDMD ( -axes), the precision of models was tested. The predicted TDN by the National Research Council model exclusively based on chemical composition only explains up to 41% of observed IVTDMD values, whereas the model based on IVdNDF had a high precision (96%) to predict TDN from forage and byproducts fiber when used in goats. 1. Introduction Currently, feeding standards for ruminants are based on predictions of total digestible nutrients (TDN), digestible energy (DE), metabolizable energy (ME), and/or net energy (NE) from the in vitro digestibility determinations of organic matter (IVOMD) and/or chemical composition [1–3]. However, there are no energy prediction equations created and validated specifically for goats. The tabulated energy values in the National Research Council (NRC) for small ruminants [4] are based on measurements of TDN in vivo, most of which were carried out some years ago, in sheep. Therefore, the prediction of TDN is justified to increase the accuracy of the values ？？used in ration formulation and ultimately reduce the risks of energy imbalances in dairy goats. There are a number of prediction equations in ruminants, from simple regression equations [5–7] to equations called “multiple” or “summative” [8, 9]. However, it is recommended to evaluate the validity of any prediction equation by taking into account the evaluation of digestibility and chemical composition of local feedstuff [2]. The summative equation developed by Weiss et al. [9] and later adopted by the NRC (2001) of dairy cattle [3] calculated each digestible feed fraction and among these, the potentially digestible NDF (dNDF) by a subecuación which is based on the nonlinear