When the supply of sugars from the leaves exceeds the demand for new leaf development and growth, certain grass species may develop a rhizome. A rhizome is a segmented, subterranean, modified stem arising from an adventitious bud in the crown zone. Rhizomes may occur on cool and warm season grasses and may be determinate or indeterminate. Determinate rhizomes are short and turn upward to form a new aerial shoot (rhizome daughter plant). The rhizome growth has three phases: downward, lateral, and upward.
Examples of forages having short, determinate rhizomes, which tend to produce sod in more or less isolated patches rather than dense mats are: big and little bluestem, red top, creeping red fescue, Kentucky bluegrass, and side-oats grama. These species differ in their rhizome vigor. For example: Kentucky bluegrass has rhizomes that bore into the soil and is therefore invasive even in compacted soil. In contrast, a rhizome may be indeterminate where the rhizome extends to a greater distance, branches at the nodes, and is therefore more invasive. Examples of forages having indeterminate rhizomes are: bermudagrass, quackgrass, Johnsongrass, reed canarygrass, bromegrass, and Western wheatgrass. (Bermudagrass has rhizomes that develop and remain close to the soil surface and therefore some call them stolons.)
A stolon is a segmented, horizontal stem which remains chiefly above ground. Stolons also arise from an adventitious bud in the crown zone. The segmented stem is covered with scales (modified leaves). Adventitious roots and aerial shoots arise from nodes. The terminal bud nested in the apex of the stolon may turn upward to form a new shoot or continue horizontal gowth. Most stoloniferous grasses are considered invasive species. Lateral spreading via stolons is characteristic of bermudagrass, zoysia, and buffalo grass. As with bermudagrass, grasses may have both rhizomes and stolons.
Grasses which spread laterally via rhizomes or stolons are often characterized as invasive species. Rhizomes, being subterranean, are somewhat protected from livestock trampling, and may serve as storage tissue for vegetative propagation. These features are much to the plant's advantage.
To prevent rhizomatous grasses from crowding out other desireable species it is necessary to implement a grazing or mowing scheme to defoliate them in an untimely manner. Defoliate when stem internodes of flowering shoots have begun internode elongation, thereby raising the shoot growing point (apical meristem) to a vulnerable height. At this stage (early transition), sheath elongation has also occurred. As a consequence, intensive defoliation removes a large proportion of the above-ground meristematic regrowth mechanisms; the shoot growing point and the intercalary merisitem at the collar zone of leaf blades.
The above scheme destroys the above-ground regrowth mechanism(s) before the the below-ground regrowth mechanism (new shoot initials arising from basal buds and rhizomes) is capable of producing rapid, competitive regrowth. Slow recovery allows companion species to flourish. Whether or not companion grasses flourish varies with their stage of develoment when defoliated.
Bluegrass / bromegrass mixture
The following case also appears among other scenarios under the management portion of this project.
A mixture of bluegrass and bromegrass was intended for sale as certified sod. Certification required elimination of the bromegrass. Both grass species are rhizomatous, so it seemed improbable that the grower could achieve certification. However, a mowing scheme could eliminate the bromegrass and salvage the bluegrass. The first step was to mow low when bromegrass commenced to produce an erect stem (jointing or transition stage).
The second step took place in late spring with ample fertility and moisture. By splitting a bromegrass shoot longitudinally with a sharp blade, it was easy to locate the growing point. Mowing low again removed the growing points of the bromegrass. The bluegrass was not adversely affected because it produces many non-flowering shoots, whose growing points remain at the soil surface. Bromegrass recovered slowly from weakly developed shoot initials. These new shoots were allowed to develop for about 5 weeks before further mowing. During the resting phase, the shoots developed a jointed culm thereby raising the growing point to a vulnerable height once again. Low mowing removed the growing point of many shoots. This killed the grass because another cycle of basal (adventitious) buds had not produced new shoot initials. The bromegrass was eliminated and the sod was certified and sold.
This mowing scheme is quite similar to defoliation under rotational grazing. Livestock can act the mowers and graze the grasses for elimination or persistence if they are moved at appropriate times.
Livestock grazing grass-legume mixtures are in jeopardy if grasses are reduced in the pasture and legumes flourish due to the risk of bloat. This risk can be diminished by monitoring the growth stage of grasses, taking precautions to avoid severe defoliation when desireable grasses are approaching the transition stage (early culm development).