Purified condensed tannins (CT) extracted from the legume Calliandra calothyrsus (var. San Ramón CIAT 22310), harvested in the dry and the rainy season and cultivated with low or high level of fertilization were added to soybean meal in a ratio of 600 mg/g of the incubated crude protein (CP). Effects on degradation either in ruminal fluid only, or in ruminal fluid followed by incubation in HCl/pepsin, were evaluated using a modified two-step in vitro method. Season was found to have larger effects on in vitro ruminal and post-ruminal CP degradation than fertilization. Condensed tannins from the rainy season harvest reduced ruminal CP degradation less than that from the dry season harvest. They had also less negative effects on the degradability of rumen escape protein and enhanced the proportion of post-ruminally degraded CP more than CT from the dry season harvest. An increase in level of fertilization reduced ruminal CP degradation in CT from the rainy season plants but this was not associated with effects on post-ruminal degradation. The study demonstrated the importance of environmental factors for the efficiency of CT in modifying ruminal and post-ruminal CP degradation.