C₃ grasses have higher nutritional quality than C₄ grasses under ambient and elevated atmospheric CO₂

Abstract

Grasses with the C 3 photosynthetic pathway are commonly considered to be more nutritious host plants than C 4 grasses, but the nutritional quality of C 3 grasses is also more greatly impacted by elevated atmospheric CO 2 than is that of C 4 grasses; C 3 grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C 4 grasses under elevated CO 2 . Will C 3 grasses remain nutritionally superior to C 4 grasses under elevated CO 2 levels? We addressed this question by determining whether levels of protein in C 3 grasses decline to similar levels as in C 4 grasses, and whether total carbohydrate : protein ratios become similar in C 3 and C 4 grasses under elevated CO 2 . In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C 3 grasses, levels of fructan respond most strongly to elevated CO 2 . Five C 3 and five C 4 grass species were grown from seed in outdoor opentop chambers at ambient (370 ppm) or elevated (740 ppm) CO 2 for 2 months. As expected, a significant increase in sugars, starch and fructan in the C 3 grasses under elevated CO 2 was associated with a significant reduction in their protein levels, while protein levels in most C 4 grasses were little affected by elevated CO 2 . However, this differential response of the two types of grasses was insufficient to reduce protein in C 3 grasses to the levels in C 4 grasses. Although levels of fructan in the C 3 grasses tripled under elevated CO 2 , the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C 3 grasses. We conclude that C 3 grasses will generally remain more nutritious than C 4 grasses at elevated CO 2 concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C 4 grasses.

Large regional-scale variation in C3/C4 distribution pattern of Inner Mongolia steppe is revealed by grazer wool carbon isotope composition

“…Several studies reported that the nutritional quality and/or digestibility of C4 species is inferior to that of C3 species (e.g., Sponheimer et al, 2003b;Barbehenn et al, 2004) although the opposite was also reported (Scheirs et al, 2000). Digestibility decreases strongly with age of plant material (Fick et al, 1994).…”

Modelled effects of rising CO2 concentration and climate change on native perennial grass and sown grass-legume pastures

“…In native pastures this was mostly due to the increased dominance of C 4 grasses, which are nutritionally inferior to C 3 grasses, even at elevated [CO 2 ], due to their lower protein content (Barbehenn et al 2004). Rising [CO 2 ] has also been shown to lead to decreased protein and N contents of a variety of species across a range of temperatures (Zvereva & Kozlov 2006, Taub et al 2008, further reducing pasture quality.…”

Climate change effects on pasture systems in south-eastern Australia

“…3), leading to a substantial change in species composition. Since C 3 species are expected to have higher forage quality than C 4 species at elevated CO 2 concentrations, despite some changes in protein and carbohydrate levels (Barbehenn et al 2004), this shift towards C 4 dominance could reduce forage quality, lowering ruminant animal production and increasing methane emissions (Howden et al 2008).…”

“…In most cases, C 3 species are of higher forage quality than C 4 species, and are expected to remain so under elevated atmospheric CO 2 conditions (Barbehenn et al 2004), so the effect of climatic change on the balance between C 3 and C 4 species could have important implications for animal production. In addition, changes to rainfall patterns may influence natural resource degradation processes such as erosion and salinity through changes in runoff and drainage patterns (van Ittersum et al 2003;Howden et al 2008).…”