Quantifying regrowth characteristics of bromegrass species (Bromus)in response to defoliation
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Bromegrass species (Bromus) can produce high forage yields under the short growing season of western Canada and have excellent nutritive value. Smooth bromegrass (Bromus inermis Leyss.) and meadow bromegrass (Bromus riparius Rehm.) are the most commonly cultivated bromegrass species. Hybrid bromegrass (B. riparius X B. inermis) was developed in Canada by hybridizing smooth and meadow bromegrass. Regrowth potential differs among these three bromegrass species, but the morphological and physiological basis for these differences is unclear. Regrowth characteristics of three bromegrass species following defoliation to 5cm at the vegetative and stem elongation stages of growth were studied in the field and greenhouse. Above-and below-ground dry matter production, leaf area index (LAI) development, individual leaf area expansion, leaf-to-stem ratio, photosynthetic rate, tiller and axillary bud development, etiolated regrowth, and nitrogen concentration in stem bases were evaluated. Regrowth was similar among the three species when defoliated at the vegetative stage. Meadow bromegrass consistently produced more (P¡Ü0.05) above-and below-ground dry matter than smooth bromegrass following defoliation at the stem elongation stage, while that of hybrid bromegrass was generally intermediate to the other two species. Individual leaf photosynthetic rates did not differ among the three species. Individual leaf area expansion rate was faster (P¡Ü0.05) in smooth bromegrass than meadow and hybrid bromegrass. LAI of the three bromegrass species increased linearly with days of regrowth (r2¡Ý0.88, P¡Ü0.05), and the increase was greatest in meadow bromegrass, intermediate in hybrid bromegrass, and least in smooth bromegrass in all stages of defoliation. Similarly, the leaf-to-stem ratio was highest in meadow bromegrass, intermediate in hybrid bromegrass, and lowest in smooth bromegrass following all defoliations. Defoliation at the vegetative stage had no effect (P¡Ý0.05) on tiller development relative to the undefoliated treatment, whereas tiller development was negatively affected by defoliation at the stem elongation stage. After 60 days of regrowth, final tiller density was greatest in meadow bromegrass, intermediate in hybrid bromegrass, and least in smooth bromegrass in the field. A lower proportion of tillers in meadow bromegrass reached the reproductive stage compared to the other two species. The final tiller density following defoliation was similar among species in the greenhouse. Total buds tiller-1 and elongated buds tiller-1 were similar (P¡Ý0.05) among three species following defoliation at each growth stage; however, defoliation at stem elongation stage visually delayed bud development. Etiolated regrowth was greater in meadow and hybrid bromegrass (P¡Ü0.05) than smooth bromegrass 10 days after defoliation, but was similar thereafter. Concentration of N in stem bases was similar among species, but decreased with advancing maturity. Rapid regrowth of meadow bromegrass appears to be associated with more tillers, rapid remobilization of organic reserves during early regrowth, and allocation of more biomass to leaf tissue than to stems compared to the other two bromegrasses. Variation in regrowth among the species was not associated with expansion of individual leaf area, photosynthetic rates, total organic reserve remobilization, or nitrogen concentration in stem bases. Based on these characteristics, meadow bromegrass is the most suitable species for grazing, and smooth bromegrass the least suitable.
DegreeDoctor of Philosophy (Ph.D.)
SupervisorCoulman, Bruce E.
CommitteeHucl, Pierre; Knight, Diane; Romo, James T.; Jefferson, Paul; Bai, Yuguang; Johnson, Doug
Copyright DateSeptember 2009