Effect of freeze drying on in vitro ruminal fermentation dynamics of three tropical shrub legumes with and without condensed tannins

Extensive comparisons of the effects of tropical shrub legumes rich in condensed tannins (CT) require well-conserved material. It is, however, unclear if the application of even gentle methods like freeze drying (lyophilization) affects the results in comparison to fresh material. Therefore, an experiment with the gas-pressure transducer technique, simulating ruminal fermentation dynamics in vitro, was conducted to investigate the effect of freeze drying on the ruminal nutrient degradability of three tropical multipurpose shrub legumes. Leaves of the CT shrubs Calliandra calothyrsus and Flemingia macrophylla and of the CT-free shrub Cratylia argentea were tested either in fresh form or lyophilized. In order to simulate practical feeding conditions, the legume leaves were incubated together with Brachiaria humidicola (1:2) for 144 h. Additionally, incubations were carried out either with or without polyethylene glycol (PEG) to be able to separate effects either dependent or independent of the CT. Only few differences were found between fresh and lyophilized leaves. These included that the proportion of apparently undegraded nitrogenous compounds was higher in fresh than in lyophilized leaves of Flemingia macrophylla, and freeze drying had a limited influence on volatile fatty acid production in Calliandra calothyrsus. The variables related to degradation dynamics (i.e., total gas production, the time until the point of inflection, apparent dry matter degradability), however, were not influenced. There was also no difference between CT and non-CT plants in that respect, as is also obvious from the lack of interactions of state of the plant material and PEG addition. This indicates that effects of freeze drying of shrub leaves on overall ruminal nutrient degradation in mixed grass-legume diets were minor.