Reproductive Stage
The third stage is the reproductive stage. Reproductive tillers are characterized by fully elongated stems and differentiation of the shoot apex into the inflorescence, i.e., the flowering parts. If you are not familiar with perennial grasses, you may not be aware that they have flower structures (Exhibit 14). Notice the anthers and pollen production on the inflorescence of big bluestem in Exhibit 15. While the absence of inflorenscences suggests that tillers have yet to reach the reproductive stage, keep in mind, that young, developing inflorescences may be hidden within elongating stems prior to boot stage. In the next section, we will take a closer look at the inflorescence structures.
Exhibit 14. Reproductive tillers of switchgrass (Panicum virgatum L.) on left image. Photo credit: John Guretzky, University of Nebraska-Lincoln
Exhibit 15. Flower structure of big bluestem (Andropogon gerardii Vitman) on right image. Photo credit: John Guretzky, University of Nebraska-Lincoln
Environmental conditions influencing flowering differ among perennial cool-season and warm-season grasses. Most perennial cool-season grasses depend on vernalization and short days in autumn to induce flowering. After floral induction, the apical meristem will initiate inflorescence primordia as temperatures warm and days lengthen in spring. By late spring, inflorescences will emerge. There is some variation among grasses. For instance, smooth bromegrass does not require vernalization but instead is considered a short-day, long-day plant. Tillers initiated during short days in autumn will flower with long days in spring. Many of the candidate biomass energy grasses including switchgrass, big bluestem, and indiangrass do not have vernalization requirements but flower as temperatures warm and days get shorter in summer. Still, there are others that are photoperiod insensitive or day-neutral plants.
The reproductive stage is an important time for management of perennial grasses in traditional pasture and range, as well as expanding biomass energy systems. It is during this stage that carbohydrate and protein reserves are maximized in a stem bases and belowground storage organs. Harvesting or grazing of grasses during this stage will not be detrimental if time for regrowth is provided. Although harvesting and grazing will remove the apical meristem and thus, terminate growth of reproductive tillers, crown or basal buds will be fully developed and capable of producing new vegetative tillers provided favorable environmental conditions.
During management for pasture or range realize that biomass yield will continue to increase but at decreasing rates when perennial grasses reach the reproductive stage. Forage quality, which is highest during vegetative stages, continues to decline during the reproductive stages. Boot stage, the stage where the inflorescence remains enclosed in the sheath of the flag leaf just prior to its emergence, is the first visible reproductive stage and ideal time for harvesting of perennial grasses for hay production. Harvesting during boot stage optimizes forage yield and quality. Although forage quality declines, biomass yield continues to increase as perennial grasses transition from reproductive to seed production stages of development.