Establishment and Seedling Growth of Big Bluestem and Switchgrass Populations Divergently Selected for Seedling Tiller Number

Selection at the seedling stage in grass breeding would be extremely useful if seedling traits are correlated to desired agronomic traits. The objective of this study was to evaluate seedling morphological development, plant growth, and field establishment of big bluestem (Andropogon gerardii Vitman) and switchgrass (Panicum virgatum L.) populations that were developed by divergent selection for seed- ling tiller number while selecting for high shoot weight. Six populations were evaluated: (i) ‘Pawnee’ big bluestem base, (ii) ‘Pathfinder’ switchgrass base, (iii) big bluestem multiple tiller, (iv) big bluestem single tiller, (v) switchgrass multiple tiller, and (vi) switchgrass single tiller. Field plots were seeded in spring 1999 and 2000 in a Kennebec silt loam (fine-silty, mixed, superactive, mesic Cumulic Hapludolls). Plants were excavated and evaluated during the growing season for shoot weight, root weight, and morphological stage of shoot and root systems. There were no major population differences in shoot weight, root weight, and morphological root stage. Shoot stage was higher for multiple tiller populations than single tiller populations 6 wk after emergence. Stand counts for all populations exceeded 10 seedlings per linear m of row, which is considered an acceptable stand density, and there were no consistent differences among populations. Populations divergently selected for seedling tiller number did not differ in ability to become established under field conditions because root systems apparently were not altered by the selection for seedling tiller number and weight. These results suggest that selection for high shoot weight did not improve seedling vigor in the field

Seasonal Changes in Dry Matter Partitioning, Yield, and Crude Protein of Intermediate Wheatgrass and Smooth Bromegrass

Seasonal patterns of dry matter partitioning and nutritive value of leaf and stem components of grass species is important for selecting species for planting, planning grazing strategies, and making management decisions. Our objective was to compare dry matter yield and crude protein (CP) yield of blade, sheath, and stem fractions of intermediate wheatgrass [Elytrigia intermedia (Host) Nevski] and smooth bromegrass (Bromus inermis Leyss.) during the growing season. Intermediate wheatgrass and smooth bromegrass were sampled on a weekly basis from 14 May to 25 June in 1997 and from 5 May to 23 June in 1998 at Lincoln, NE. Samples were separated into blade, sheath, stem, and inflorescence components and yield and concentration of CP were determined. Smooth bromegrass tended to have greater blade and stem yield than intermediate wheatgrass during the 1997 and 1998 sampling periods. Yield of sheath was similar between species. Crude protein yield of blade and stem was also greater for smooth bromegrass than intermediate wheatgrass in both years. Sheath CP yield was greater for intermediate wheatgrass in 1998. Both species followed a similar pattern of dry matter accumulation; however, intermediate wheatgrass dry matter accumulation, especially stem, tended to be 1 to 2 wk behind smooth bromegrass. Differences in dry matter partitioning, yield, and CP in these two species illustrates the advantages having a complement of forage species. Choosing a diversity of species with differing growth habits would be beneficial for improving the distribution of forage yield and quality to match the seasonal demand of grazing livestock

Effects of Summer Grazing Strategies on Organic Reserves and Root Characteristics of Big Bluestem

Quantifying root structure response to multiple defoliation events in a grazing situation is critical in developing management plans for warm-season tall grasses. A pasture experiment was conducted in 1999, 2000, and 2001 near Mead, NE. The objective of the experiment was to determine the effect of timing and frequency of grazing on big blue- stem (Andropogon gerardii Vitman) etiolated tiller growth and root and rhizome structure. Paddocks were grazed at a stocking rate of 9.9 Animal Unit Month (AUM) ha-1 in two to four cycles from mid-May to early-September. In April 2002, five 6.6- 132-cm soil cores were extracted from each paddock. Soil cores were subsampled at 30-cm depth increments for estimates of root mass, root surface area, and root volume. Etiolated tiller tents were used to estimate organic reserves of big bluestem in each paddock in spring 2002. Mean number and weight of etiolated tillers were reduced by up to 40% and 50%, respectively, in paddocks grazed in a sequence of June after internode elongation, early August, and early September. Root structure in the top 30 cm of the soil profile was affected most by multiple defoliation events with \u3c40 d of recovery between grazing periods. Root mass decreased by 25%, while mean surface area and volume of roots declined 10 and 15%, respectively, in the upper 30 cm of the soil profile in paddocks grazed in the sequence of post-internode elongation in June, early- August, and early-September. To maintain vigorous big blue-stem pastures, grazing management should concentrate on the elongation and post-elongation periods. Grazing at the elongation stage should be rotated among paddocks in successive years and the recovery period following grazing at internode elongation should be \u3e40 d