Forage Quality Measurements and Forage Research—a Review, Critique and Interpretation

Abstract

Forage makes up a major portion of the rations of cattle and rate of animal gains varies widely within and among forage species grown. Forage production research has been highly productive, but research on increasing forage intake and digestibility by cattle has not been as successful. The literature suggests that resistance to change in specific gravity increases with the age of the plant, particularly with stems, and may increase time required for passage through the rumen and gut, reducing rate of intake. Leaves and leaf sheaths of grasses live for only part of a season and when they die soluble contents are translocated out to young tissue, leached by rainfall and metabolized by bacteria or by the plant leaf itself. Digestibility of dead leaves ranges from 40 to 60% of that of green leaves and when leaves die 15 to 40% of total production may be lost. Plant parts, leaf, leaf sheath, and stem, differ significantly in digestibility and rate of consumption. Variations in quality among forage cultivars may represent a basic difference in the amount of each component that is produced or is present when being grazed. Cultivars high in leaf content will be higher in quality than cultivars which produce more stem. Pasture or range management which maintains green leaves should produce more total forage and more total digestible nutrients than management which allow stems and dead leaves to accumulate. It is hypothesized that by understanding shoot growth in grasses and managing to prevent stem production and leaf death, forage quality over a season can be stabilized at a digestibility of 65% or higher. When processing forage samples, data on green leaves, leaf sheaths and stems, forage quality and digestibility, need to be collected to be descriptive of the species. On pastures or ranges, species with reproductive tillers tend to become stemmy in the summer, requiring defoliation to remove the stems and N application to activate new shoot growth. This is different from those species producing vegetative tillers where, over a season, dead leaves tend to accumulate. On perennial grass species with vegetative tillers, rate, and time of N application should be adjusted to stimulate tillering and green leaf growth. Under range conditions spring N application stimulates the growth of annuals which compete with the perennials. Thus, effective pasture management is expected to reflect manipulation of green leaf producton and utilization in the predominant tiller type present, usually the perennial.