Evaluation of energy value of grass silage

Abstract

Concentration of net energy for lactation (NEL) of 80 grass silage samples was estimated on the basis of chemical composition and in vitro method, which was based on the gas produced during the incubation of samples with rumen liquor. Values were compared to the estimates obtained by the procedures that are usual for Slovenian laboratories, i.e. on the basis of chemical composition and digestibility coefficients from tables or by the use of regression equations from literature. It was established that recently published tables and equations overestimated NEL concentration. NEL concentrations which were obtained by the use of digestibility coefficients published in DLG (1991) tables overestimated in vitro obtained values for 0.42 ± 0.43 MJ, DLG (1997) tables for 0.50 ± 0.36 MJ, equation according to Menke and Steingass (1987) for 0.36 ± 0.42 MJ and GfE (1998) equations for 0.53±0.40 MJ NEL kg^–1 dry matter (DM). The regression analysis showed that all estimates deviated systematically from in vitro estimated values. For the estimation of low quality samples, which contain less than 5.5 MJ NEL kg^–1 DM, the usage of old German tables (DLG, 1982) can be recommended while for samples that contain more than 5.5 MJ NEL kg^–1 DM recently published tables (DLG, 1997) and official German equations (GfE, 1998) are more suitable. Relatively high average individual deviations from in vitro estimated values (from 0.33 to 0.57 MJ NEL kg^–1 DM) were characteristic for all estimates. Maximal individual deviation reached 1.5 MJ NEL kg^–1 DM. New regression equations for estimation of metabolizable energy concentration and NEL in grass silages were proposed. It was found out that by the use of new equations deviations from the in vitro estimated values can be reduced, however, unexplained deviations of about 0.3 MJ NEL kg^–1 DM cannot be avoided by the means of classical Weende analysis.