Estimating digestible nutrients in diets for small ruminants fed with tropical forages

Abstract

Equations to estimate energetic content of small ruminants’ diets from chemical composition, in tropical conditions, are still not available. The main objective of this study was to develop equations to predict the digestible content (ether extract (EE), non-fiber carbohydrate (NFC), crude protein (CP) and neutral detergent fiber (NDF)), for different goat and sheep diets. True digestible nutrients and metabolic fecal contribution were the main parameters fitted (Lucas test approach), with exception for NDF, where surface law was used as the main approach. Data from 32 trials, 21 with sheep (n = 881) and 11 with goats (n = 297) were collected. Two approaches where conducted for digestible CP, which considered the nutritional entity test, or bicompartimental estimation for different fractions of CP. Two models were for digestible NDF, which considered the surface law of NDF and lignin interaction with cell wall. Nutritional entity test was also used to estimate digestive content of NFC and EE. There were no effect of species on true digestibility or fecal contribution in estimation of digestible NFC and EE (P>0.05), and only one model was fitted for goat and sheep for each nutrient. The two CP models were fitted without considering the effect of species (P>0.05), as well as NDF. A validation trial was carried from literature data, where data were collected in 50 manuscripts. This step allowed evaluating total digestible nutrients (TDN) prediction, which was made from the sum of digestible nutrients. Also, the estimated metabolic fecal contribution that could be diminished from the main TDN estimate was 4.0. There were conditions to confirm that CP, NFC and EE could be considered homogeneous nutritional entity, and sub-models were adequately applied to study nutrient digestion patterns. Equations to predict digestible CP based on the Lucas test or the surface law theory can be used to estimate the digestible content. Mathematical approaches tested to estimate digestible content of NDF were also indifferent, and both equations based on surface law theory can be used to make the summative procedure to calculate TDN. Then, results of the following equations can be summed to calculate TDN in diets for sheep and goats: dEE (%) = 0.9107 × EE (%) – 0.33; dNFC (%) = 0.9041 × NFC (%) – 3.22; dCP (%) = 0.7934 × CP (%) – 0.44 or dCP (%) = 0.98 × (CP –NDIP)+ 0.7877 × (NDIP-ADIP) - 0.44; and dNDF (%) = {0.7877 × (NDF-Lignin)+[1-(Lignin ÷ NDF) 0.85]}.