ABSTRACT
Summary In the temperate areas of the U.S.A., high temperatures and low soil moisture in summer limit the production of cool-season grasses and, consequently, the size of most beef cow-calf herds. Warm-season grasses, most productive in midsummer, may serve as a supplement to cool-season grasses for grazing and hay. Switchgrass (Panicum virgatum L.), a warm-season species native to the Great Plains of the U.S.A., was grown on soils low in available phosphorus at two locations in Pennsylvania to determine changes in forage dry-matter yields, leaf (leaf blade):stem (sheath + stem) ratios, and quality of leaf and stem fractions associated with increased maturation. Forage was harvested at 10-day intervals at both locations beginning at the three- or four-leaf stage in late June and continuing until seed maturity in early August. Forage yield at early head emergence averaged 4.72 metric tons/ha with no nitrogen fertilizer. Yield variability between locations and years was apparently due to previous stand management. Leaf: stem ratios declined as plants matured, and in early August stems accounted for 63% of the total dry-matter yield. Leaf and stem forage-quality estimates at early head emergence, respectively, were: in-vitro dry-matter disappearance (IVDMD), 58.8% and 47.2%; crude protein (CP), 8.5% and 3.8%; neutral detergent fiber (NDF), 67.1% and 77.7%; and lignin, 4.7% and 8.3%. In concurrent feeding trials conducted using sheep, IVDMD underestimated in-vivo digestibility by 17 percentage points. Percentages of IVDMD and CP generally decreased with leaf and stem maturation. The decline was greatest at the early harvests and less pronounced in leaves than in stems. In contrast, percentages of NDF and lignin generally increased with advancing growth stage, and changes were most rapid for stems at earlier harvests. In conclusion, switchgrass can produce good yields of medium-quality forage at low soil fertility levels from late June to August when production of temperate grasses is inadequate. Grazing switchgrass at more mature growth stages would provide forage of adequate nutritional value for mature brood cows and Stocker cattle and would also increase pasture carrying capacity. In a hay program, harvesting at early head emergence would provide the best alternative when considering yield, quality, and persistence factors.
