Genetic variation for in sacco dry matter and fiber degradation kinetics in alfalfa
Bernadette Julier, Jean-Claude Emile, Marc Lila and Christian Huyghe
INRA, Unité de Génétique et d’Amélioration des Plantes Fourragères,
F-86600 Lusignan, France
Energy content is an important criterion for evaluation in alfalfa breeding. The in sacco method was used to investigate the phenotypic variation in the kinetics of degradation of dry matter and fiber for 15 alfalfa cultivars grown in one location, and to compare these kinetic parameters to laboratory analyses of dry matter solubility, fiber content (NDF) and fiber solubility. Among the 15 cultivars, NDF ranged from 42.2 to 48.4% and NDF solubility from 27.5 to 31.0%. Bags containing forage samples were incubated for 2, 4, 8, 12, 24, 48, and 72 h in 3 fistulated Holstein cows. Dry matter and NDF degradation curves were modeled with a sigmoid curve with three parameters. Phenotypic variations were observed for dry matter and NDF disappearance at each time of incubation, and for the parameters of the degradation kinetics.
For dry matter degradation, the rapidly degraded fraction ranged from 32.6 to 40.3%, the slowly degradable fraction from 31.2 to 38.4%, the undegradable fraction from 24.7 to 34.4%, and the rate of degradation from 0.11 to 0.19. For NDF degradation, the slowly degradable fraction ranged from 41.6 to 50.5%, the undegradable fraction from 42.0 to 55.4% and the rate of degradation from 0.07 to 0.16.
NDF and dry matter solubility measured in wet chemistry were correlated with dry matter undegradable fraction. The NDF solubility was correlated to the NDF undegradable fraction. The chemical measurement of digestibility, useful for the investigation of variations in energy value in lucerne were thus validated by the in sacco method. But some parameters of degradation (rapidly degraded fraction, rate of degradation of the slowly degraded fraction), that showed some variations, are not related to chemical analyses. The effect of the variations in the parameters of degradation has to be studied on the ruminant performances. The rate of degradation could influence the voluntary intake (Nozière and Michalet-Doreau, 1999).
Reference
Nozière, P., Michalet-Doreau, B. 1999. In sacco methods. In 'Farm animal metabolism and nutrition: critical reviews' (Ed JPF D'Mello), in press.