Prediction of the energy value of cattle diets based on the chemical composition of the feeds under tropical conditions

Determination of the energetic contribution of different chemical components of cattle diets based on the content of total digestible nutrients (TDN) has been suggested by several authors and was adopted by the NRC [National Research Council (NRC), 2001. Nutrient Requirements of Dairy Cattle, seventh ed. Academic Press, Washington]. The main objective of that proposal is to overcome the high costs and the extended time of traditional in vivo digestion assays. Although theoretically based, the summative system adopted by the NRC [National Research Council (NRC), 2001. Nutrient Requirements of Dairy Cattle, seventh ed. Academic Press, Washington] has been shown to be inaccurate in predicting the TDN content of individual feeds or diets under tropical conditions. Therefore, a new model, that also uses a summative approach, was developed but it was based on data obtained in the tropics. The new model consists of equations that predict the digestible fractions of ether extract (n = 108), crude protein (n = 93), non-fibrous carbohydrates (n = 84), and neutral detergent fibre (n = 164). Non-fibrous components were evaluated by the Lucas [Lucas, H.L., 1960. Relations Between Apparent Digestibility and the Composition of Feed and Feces. 1: A Quantitative Theory. North Caroline State College, Raleigh] test to obtain true digestibility coefficients, and by the factorial system to distinguish between truly digested and metabolic fractions. Fibre components were evaluated using an adaptation of the surface law to obtain the indigestible fraction and a meta-analysis approach to obtain digestibility coefficients of the potentially digestible fraction. The estimates obtained with the new model were validated and compared to those produced with the model adopted by the NRC [National Research Council (NRC), 2001. Nutrient Requirements of Dairy Cattle, seventh ed. Academic Press, Washington] for cattle managed under tropical conditions. Data from four trials, two with lactating cows (n = 63) and two with growing and finishing cattle (n = 44) were used. The estimates produced with the two models were compared based on the decomposition of the mean square prediction error. It can be concluded that the new model was more accurate in predicting TDN values than the NRC [National Research Council (NRC), 2001. Nutrient Requirements of Dairy Cattle, seventh ed. Academic Press, Washington] model under tropical conditions. (C) 2007 Published by Elsevier B.V.