-The effects of severe underfeeding and N supplementation on nutrient digestion were studied in Bos taurus and Bos indicus cows. Eight non-lactating adult cows, four Bos taurus and four Bos indicus (body weight 173 and 234 kg, respectively) fitted with ruminal cannulas, were used in a 4 × 4 Latin square design within each genotype. They were fed a rice straw based-diet supplemented with cottonseed meal at four levels: HN-, high intake low N; LN-, low intake low N; HN+, high intake high N; LN+, low intake high N. The first diet, HN-, was formulated to supply 100% of net energy maintenance requirements. LN-corresponded to half of HN-and supplied 50 and 63% of the requirements in net energy and in digestible protein in the intestine, respectively. The two other diets, HN+ and LN+, were defined by providing the same amount of additional protein as the previous treatments. They supplied 110 and 60% of net energy requirements, and 165 and 100% of requirements in digestible protein in the intestine, respectively. The variation of digestibility was studied in relation to digesta kinetics and particle size. Apparent OM digestibility decreased with underfeeding, with no effect of protein supplementation (61.7, 53.3, 62.7 and 53.0% in Bos taurus and 59.6, 52.6, 62.1 and 55.7% in Bos indicus, respectively for HN-, LN-, HN+ and LN+). This lower digestibility at the low level of intake was observed despite a longer total tract particle retention time (72.2, 76.9, 68.2 and 82.0 h in Bos taurus and 74.9, 82.2, 67.4 and 85.7 h in Bos indicus, respectively for HN-, LN-, HN+ and LN+). Therefore, our data suggest that the retention time may not be a limiting factor for digestion at low intakes. Rumen microbial activity may thus decrease, although neither DM degradability measured in situ nor ruminal and faecal particle sizes varied with the level of intake. Protein supplementation did not avoid a drop of the digestibility when the level of intake decreased. The presence of a factor limiting microbial activity together with physical modifications of the rumen milieu may be the origin of this phenomenon, which does not support the adaptation of ruminants to severe undernutrition. . En conséquence, ces résultats suggèrent que le temps de rétention n'est pas forcément un facteur limitant de la digestion à faible niveau d'ingestion. L'activité microbienne dans le rumen serait donc réduite ; cependant, ni la dégradabilité in situ, ni la taille moyenne des particules ruminales et fécales n'ont varié avec le niveau d'ingestion. La supplémentation azotée n'a pas empêché cette chute de digestibilité avec le niveau d'ingestion. La présence d'un facteur limitant l'activité microbienne, combinée aux modifications physiques du milieu ruminal, peut être à l'origine de ce phénomène, qui va à l'encontre d'une adaptation des femelles des ruminants à une sous-alimentation drastique.supplémentation azotée / bovins / digestion ruminale / climat tropical / sous-alimentation
