Abstract

Forage quality is best defined as output per animal and is a function of voluntary intake and digestibility of nutrients when forage is fed alone and ad libitum to a specified animal. The major factors limiting intake and digestibility are those associated with rate and extent of forage degradation by microbial and physical factors in the rumen, primarily the amount of cell wall constituents (CWC) and the extent of lignification. Lignification does not limit digestion of soluble plant constituents (cell contents). There are close relationships between intake and digestibility and between lignification and digestibility when the major difference in quality is due to maturity within a species. Such relationships are not as good among species, suggesting discrepancies in the cause-and-effect relationships between forage structure and quality. These principles apply to both tropical and temperate grasses.

Improved tropical grasses are often of good quality when fed at relatively immature stages of growth, but the maximum attainable intake and digestibility are lower than in immature temperate grasses. This may be related to the higher CWC in tropical grasses. The digestibility of CWC and the digestibility of organic matter (OM) are highly correlated in the tropical grasses. Chemical analyses are less satisfactory as predictors of quality in tropical grasses than in vitro digestion of OM. Discrepancies do exist, however, in the in vivo-in vitro relationship.

Histochemical examination of tropical and temperate grases has revealed that tropical species have lignified sheath cells surrounding vascular bundles. These structures remain undegraded in the rumen. The amount of vascular tissue may be a factor modifying the relationship between lignification and cell wall digestion. Lignin must be considered the primary structural inhibitor of quality in tropical grasses. Determination of its distribution in the plant, especially in the vascular and nonvascular tissues, may be the key to resolving the discrepancies among species in the relationships between plant structure and quality.

Copyright © 1973. . Copyright © 1973 by the Crop Science Society of America, Inc., 5585 Guilford Rd., Madison, WI 53711 USA