The nutritive value determination of feeds induce the knowledge of the 'in vivo' organic matter digestibility. The reference method for the determination of this parameter is based on digestibility balance on animals (cf. service sheet). This procedure is long, expensive and induce the maintenance of many animals. In a aim of simplification, methods based on chemical composition parameters and enzymatic degradability were developed to estimate the 'in vivo' digestibility using multi linear regression equations. But this determination is not sufficient to express all the feeds nutritive potential. The notion of energy and protein synchronisation in the rumen allow not only a better digestive use of the substrate but also reduce the nitrogenous urinary lost. In this way, the study of the degradability kinetics of the most important organic matter components will be helpful to define the animal ration. But the methodological approach of the kinetics studies stay difficult : the enzymatic methods do not permit to tackle this question and the 'in sacco' methods needs fistulated animals and are long and expensive.
Objectives
The aim of this project is, for one part, to define simple chemical parameters to predict as best as possible the 'in vivo' digestibility of the feeds and, for an other part, to develop a methodology combining fermentation of feeds in artificial rumen with NIRS analysis of the incubation residues. This study is included in an European program entitled : “Efficient Utilization of Forage Maize by dairy cattle : key plant parameters, Genetic Determinism, Impact on Milk Production, Animal Behaviour and Environment”. The main aim of this program is the optimal use of forage maize by dairy cattle. This optimal use require : · the identification of the composition parameters having an effect on the valorization by cattle and the understanding of the interactions between them ; · the identification of the genetic determinism of those parameters and their potential interactions ; · the evaluation of the change impact of those features on milk production, nutritious behaviour, animal welfare and environment by modifying allelic composition of the selected loci. Eigh European partners, with complementary abilities, are working in this program.
Results obtained
-) NEW PARAMETERS TO ESTIMATE 'IN VIVO' DIGESTIBILITY The 'in vitro' methods used to estimate feeds digestibility and so to improve the energy value calculation accuracy are all based on enzymatic hydrolysis in buffered solution. The great similarity of the analytical procedures have allowed to study the relation between them. Three enzymatic methods (Aufrère, De Boever and Ensitec), used in routine in the laboratory, were applied to some samples representative of a range of substrates (silage, hay, alfalfa, grass, maize whole plant) and the correlation between these methods computed. So one relation between the results obtained by the Aufrère and Deboever methods was determined for all the substrates excluding maize. For this one, other relations, including the Ensitec method, were obtained (see table of results). The precision of the defined models is compatible with the variability observed between the results of different laboratories working with the same method. All substrates excluding maize DMO Aufrère = 1.086 * DMO Deboever - 12.905 R² = 0.987 MQE = 1.39 DMO Deboever = 0.921 * DMO Aufrère + 11.879 R² = 0.987 MQE = 1.28 Maize DMO Aufrère = 1.011 * DMO Deboever - 4.342 R² = 0.896 MQE = 1.69 DMO Deboever = 0.989 * DMO Aufrère + 4.295 R² = 0.896 MQE = 1.67 DMO Aufrère = 1.169 * DMO Ensitec - 23.564 R² = 0.975 MQE = 1.41 DMO Ensitec = 0.855 * DMO Aufrère + 20.157 R² = 0.975 MQE = 1.21 For maize, a method based on fibre enzymatic degradability was developed. These method applied an enzymatic digestion of the NDF residue according to the Van Soest fibre method. The grouping of cell wall content and cell wall enzymatic degradability allow to develop an 'in vivo' predictive model more accurate that all the arrangement including constitution parameters and global enzymatic degradability. -) RUMINAL DEGRADABILITY A large capacity artificial rumen (picture 1) was developed to study as easy as possible the kinetics parameters of the ruminal degradability. This approach is based on the 'in sacco' nylon bags technique applied in mechanical fermentors filled with buffered rumen juice solution and putted in a hermitic enclosure with saturated atmosphere in CO2 and maintain at 39 °C. This method allow not only to study the dry matter disappearance kinetic by weighting bags removed from the incubator after defined times, but also to study degradability kinetics of organic matter major components by NIRS analysis of each bag residue and on the basis of previously created predictive models. The first results show a very good repeatability between the two mechanical incubators and samples of whole plant maize can be classify according to the variety or to the harvesting date by the use of their theoretical degradability (TD according to Ørskov et Mac Donald model (1979)). This method allow to analyse a lot of samples in standardized conditions. [image] In the project, according to the statistical analysis, a significant difference appears between locations for all tested parameters (Orskov-Mac Donald parameters). Nevertheless this location effect appears too considerable to observe a significant difference between obtentions.
