Leaf and Stem Forage Quality of Big Bluestem and Switchgrass¹

Abstract

A balanced forage program should include species that provide the highest yields of quality forage throughout the grazing period. In the Northeast, where cool‐season grasses predominate, the short supply of forage during summer limits beef cow‐calf herd size. Warm‐season (C4) perennial grasses are productive in midsummer and may supplement temperate species for grazing and hay. ‘NY 1145’ big bluestem (Andropogon gerardi Vitman) and ‘Blackwell’ switchgrass (Panicum virgatum L.) were grown on soils (Aeric Fragiaquualt and Aquic Hapludalf) low in available P at two locations in Pennsylvania to characterize changes in quality of leaf and stem tissue associated with maturation. Forage was harvested at 10‐day intervals beginning at the 3‐ to 4‐leaf stage in late June and continuing until seed set in early August. Percentage leaf tissue declined similarly with maturation for the two grasses. Leaf dry matter yields were approximately twice those of stems in June but the opposite was true in August. At early head emergence, percentage leaf tissue for big bluestem and switchgrass averaged 34 and 44%, respectively. Averaged over grasses, leaf and stem forage quality estimates at early head emergence, respectively, were: crude protein (CP), 9.7, 4.3% in vitro dry matter disappearance (IVDMD), 60.4, 50.0%; neutral detergent fiber (NDF), 66.0, 75.3%; lignin, 4.7, 7.2%; and phosphorus (P), 0.20, 0.16%. Big bluestem leaves were higher in CP but lower in NDF than switchgrass leaves. Stem tissue of big bluestem was lower in NDF but higher in lignin than that of switchgrass. The decline in leaf and stem CP, IVDMD, and P with maturation was less pronounced in leaf tissue. The increase in NDF and lignin with maturity was greater in stems than in leaves. Fiber accumulation in stem tissue continued after seedheads emerged and was accompanied by decreases in CP and IVDMD. At early vegetative stages, the high percentage of good quality leaf tissue suggest the potential use of big bluestem and switchgrass for ruminants with above maintenance energy requirements. However, at later growth stages (late joint early head), the increase in stem tissue and associated decline in nutritive value suggest using these grasses either for grazing or hay by ruminants to meet only maintenance energy needs.

Leaf Nutritive Value Related to Tiller Development in Warm-Season Grasses

“…Advancing morphological development affects nutritive value by increasing the proportion of low quality stems in the available forage (Griffin and Jung 1983). In this study, IVDMD declined in leaf tissue alone.…”

“…Hutchenson 1984, Perry andBaltensperger 1979). Increases in the proportion of stem tissue with maturity was a major determinate of whole-plant nutritive value in big bluestem (Andropogon gerurdii Vitman) and switchgrass (Panicum virgatum L.) (Griffin andJung 1983, Aman andLindgren 1983). However, IVDMD values between leaves and stems of big bluestem and indiangrass [Sorghastrum nutuns (L.) Nash] were comparable when sampled at the same date (Perry and Baltensperger 1979) indicating leaf maturation, rather than plant development, had the greatest influence on nutritive value.…”

Switchgrass Response to Nitrogen Fertilizer Across Diverse Environments in the USA: a Regional Feedstock Partnership Report

“…Although switchgrass maturity at harvest was not quantified since it was harvested after senescence, greater yields in years subsequent to year 1 at OK, SD, and VA likely indicate a higher proportion of reproductive tillers which may have contributed to the decline in biomass N concentration in year 2. Internodes of reproductive tillers contain less N than the NS not significant leaves which may have led to decreased N concentrations in harvested biomass [7]. Switchgrass biomass from the OK location contained a higher concentration of N than any of the other locations (Fig.…”

Genetic Modification of Herbaceous Plants for Feed and Fuel

“…Plant morphology has a major impact on cell-wall concentration of forages due to differences between leaves and stems. For grasses such as big bluestem (Andropogon gerardii Vitman) and switchgrass, leaves have somewhat lower NDF concentrations (~65 vs. 75%) than stems (Griffin and Jung, 1983;Jung and Vogel, 1992). In a legume such as alfalfa, the leaves have less than half the NDF content of the stems (Jung et al, 1997b).…”

“…Another difference between grasses and legumes is that cell-wall concentration of grass leaves increases at almost the same rate as in stems (Jung and Vogel, 1992), whereas legume leaves remain relatively constant in their leaf composition while stems deposit large amounts of additional cellwall material with maturity (Kalu and Fick, 1983). The proportion of lignin in the cell walls of legume leaves and stems do not differ (Jung et al, 1997c), but cell walls of grass stems do tend to be more lignified than leaves (Griffin and Jung, 1983;Jung and Vogel, 1992). The selection for a change in leaf-to-stem ratio will have dramatic effects on cell wall concentration of the total herbage of legumes and smaller, but significant, effects on grass cell wall concentration.…”