Sunday, August 31, 2014

Managing Drought Risk on the Ranch

Grass Repopulation and Reproduction

Importance of Bud Production

Year-to-year replacement of grass tillers primarily depends on the production and survival of vegetative buds on existing plants.

On semiarid rangeland, grass seed production is limited. When viable seed is produced, much of it is consumed by birds, small mammals, or invertebrates such as ants or beetles. The diversity and number of viable perennial grass seeds in the top soil of rangelands are amazingly low.

When growing conditions are favorable and plants are not overgrazed, perennial grasses produce new buds every year. Roots from parent tillers greatly enhance survival and growth of tillers from vegetative buds. They also store the energy needed to maintain dormant buds produced in the current and preceding years.

Generally, only a portion of the buds in perennial grasses break dormancy and develop into tillers in a given growing season. The remaining buds will be viable for several years if the tiller network supplies the energy needed to maintain the relatively small amount of live tissue in dormant buds.

Reduced plant growth under drought conditions or excessive grazing may reduce or eliminate formation of new buds.

Parts of a grass tiller
Each year’s forage crop is produced by a new set of tillers that develop from buds produced in the previous growing seasons. A tiller is the smallest stand-alone unit of a grass. Basic components of a tiller include a shoot, a crown and roots. Tillers originate from seed or from vegetative buds on previously existing plants. Image: http://www.for.gov.bc.ca/hfd/pubs/Docs/Fpb/RMF01/RMf-4024.htm

Vegetative Reproduction - Two Types of Grasses

Bunchgrasses

Little Bluestem buds
Multiple cadres of buds occur on Little Bluestem crowns ranging from 1-year-old (a) to 3-year-old (b) generations.

Bunchgrasses do not have stolons or rhizomes and instead develop dense clusters of tillers. Examples include Little Bluestem, Needleandthread and Prairie Junegrass.

Bunchgrasses repopulate primarily by tillering. New tillers develop in close proximity to current-year and previous-year tillers because buds primarily are located in the crown at the lower most nodes of parent tillers.

Multiple generations of buds often are discernible in bunchgrasses. Density, size and orientation of buds differ among species.

Sod-forming Grasses

Buffalograss buds and stolons 
Buffalograss buds (a) originate at nodes where clusters of tillers develop on stolons (b).

Sod-forming grasses have stolons or rhizomes. Rhizomes are underground stems that produce roots at nodes. Stolons are above-ground horizontal stems that root at the tip or at nodes.

Buffalograss (see diagram, right) reproduces vegetatively by tillering from nodes on stolons.  Buds on the crowns of buffalograss may differentiate into tillers or stolons.

In addition to producing buds for repopulation, nutrients are translocated through rhizomes from non-grazed to grazed tillers and from parent to daughter tillers for several years.

Western Wheatgrass nodes, internodes, buds, roots, rhizome scales 
Western Wheatgrass rhizomes are composed of a relatively long series of nodes (a) and elongated internodes (d). Buds (b,c), roots (e), and rhizome scales (f) originate at nodes. Stage of development varies among buds on the same rhizome ranging from differentiated (b) to non-differentiated dormant (c) buds.

Continuous rhizomes produce aerial shoots from buds located at nodes, and the rhizome continues to grow underground (see Western Wheatgrass diagram, middle right).

In contrast, rhizomes that turn up and emerge as a green shoot are referred to as terminal rhizomes (see Prairie Sandreed diagram, lower right).

Growth of new rhizomes is seasonally most active in the boot stage of development, when tillers begin to elongate. Rhizome growth is dependent on the amount of current year herbage.

Therefore, grasses produce more rhizomes when precipitation is favorable compared to periods of drought, and vigorous grasses produce greater weight and length of rhizomes than overgrazed plants.

Management practices that periodically optimize growth of new rhizomes are likely to increase the spread and productivity of rhizomatous grasses.

  

Prairie Sandreed tillers, rhizomes, buds 
Prairie Sandreed tillers (b) originate from extensive networks of scaly rhizomes (d). Buds occur at crowns of parent tillers (c) and at the end of rhizomes (a) with little or no tiller development from rhizome nodes in this species. Bud development ranges from differentiated (a) to dormant (c). Near the end of the growing season, differentiating prairie sandreed buds elongate to just below the soil surface (a). Tillers produced by these buds do not emerge until the following spring.

 

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