Prediction of the in vivo digestible energy value of barley for the growing pig on the basis of physical and chemical characteristics and in vitro digestible energy : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science at Massey University
The study aimed to develop statistical relationships to allow the prediction of apparent digetible [sic] energy in barley based on simple physical and chemical measures. A second aim was to evaluate a recently developed in vitro energy digestibility assay. Seventeen barley samples representing nine varieties were obtained throughout New Zealand during the 1995 harvest. The samples were subjected to chemical analysis and several physical attributes were determined. Ten barley samples were selected on the basis of their crude protein and fibre contents to cover the range in gross chemical composition and digestible energy contents were determined after sampling faecal contents from 30 kg liveweight pigs, given barley as the sole source of energy. In vitro dry matter digestibility of the barley samples was determined using a multi-enzyme assay. The physical characteristics of the barley samples were variable, especially the level of screenings (ranging from 1 to 11.6%) and to a lesser extent the moisture content (ranging from 12 to 16.2%) and 1000 seed weight. The chemical composition of the barley samples differed with the crude protein content ranging from 7.8 to 11.7%. The mean levels of Neutral Detergent Fibre (NDF), Acid Detergent Fibre (ANF) and lignin were 16.4 %, 4.2%, and 1.1%, respectively. Total β-glucan and Gl extracted β-glucan contents were also determined with mean values of 4.5% and 1.4%, respectively. The in vivo apparent digestibility of energy (DE) ranged from 72.5% to 78.4% with a mean digestibility of 75.8%. Among the physical and chemical characteristics, only the seeding rate was significantly correlated with in vivo energy digestibility (r = 0.73, P< 0.05). The gross energy (GE) content was significantly correlated with apparent digestible energy content (r = 0.78). When the gross energy value of a sample is known, an approximation of the apparent digestible energy (ADE) content can be made using a simple prediction equation: ADE MJ/kg dry matter = -10.48 ± 1.33 GE MJ/kg dry matter. Repeatability of the in vitro digestibility of dry matter (DDM) was high (r =0.68) but the correlation coefficient between in vivo DE and in vitro DDM for the barley samples (r = 0.29) was not statistically significant. However, when combined with results for several wheat milling by-products, the in vitro DDM was significantly (p<0.01) correlated to the in vivo DE (r = 0.96) indicating that in vitro DDM is a good predictor for in vivo DE across feedstuffs but not within a feedstuff.