The decrease in the digestibility of forage is linked with an increase in cell walls and lignification of tissues as they mature. All forages are composed of a heterogeneous group of cell types with particular characteristics that determine the availability of polysaccharides to the rumen's microorganisms. In the present paper the degradation of
Bromus auleticus
Trin. ex Nees leaf tissues was studied at different incubation times in the rumen. Samples were taken in two phenological phases: vegetative and pre-flowering, which were submitted to different digestion times
in situ on two dry Holando Argentine cows. Simultaneously, leaf blade sections were stained with phloroglucinol acid stain to detect the presence of lignin. Leaf anatomy influenced the degree of tissue digestion, whether due to lignified cell walls or accessibility. After 24 h incubation in the rumen, only the xylem, sclerenchyma and mestome sheath were undigested. The phloem and clorenchyma were digested after 18 h in the pre-flowering phase and 24 h in the vegetative phase. The epidermis was partially digested, and the abaxial epidermis was better digested. The accessibility of carbohydrates in cell walls to the microflora of the rumen is limited by the chemistry of cell walls, and thus, highly organized tissues with secondary lignified walls, such as sclerenchyma and xylem, have a double barrier (physical and chemical) which prevent their digestion.
