Elevated CO2 and defoliation effects on a shortgrass steppe: Forage quality versus quantity for ruminants

We assessed the effects of elevated atmospheric CO2 on ruminant forage quality and nutrient yields during 4 years in semiarid shortgrass steppe where grazing by domestic livestock is the primary land-use. A defoliation and a nitrogen fertilization treatment were superimposed on CO2 treatments in large open-top chambers. CO2 effects on forage soluble and fiber (celluloses, lignin) constituents were small, even though mid-growing season yield and end of season production increased. However, large negative effects of elevated CO2 were evident in crude protein concentrations and digestibility of forages. While the effects were more negative mid-growing season than autumn, a reduction in already poor quality autumn forage may be more critical to animals. Crude protein concentrations of autumn forage on the elevated CO2 treatment fell below critical maintenance requirements 3 out of 4 years, compared to 1 of 4 for ambient and control treatments. Forage digestibility declined 14% mid-season and 10% in autumn with elevated CO2. Negative effects of elevated CO2 on animal performance mediated through forage quality are likely to be greater than the positive effects of increased quantity, because quality drops to critically low levels that can inhibit utilization. Further, elevated CO2 shifted the proportional availability of protein and energy to a species of lower overall quality and the species most negatively affected by drought. Current-year defoliation increased both quality and production of protein and energy compared to non-defoliated plots, but no CO2 by defoliation treatment interactions were observed. Nitrogen fertilization increased crude protein concentrations and digestibilities, but not in the least nutritious species that increased with elevated CO2 or in autumn when quality was lowest.