Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/28/2004
Publication Date: 1/26/2005
Citation: Milchunas, D., Mosier, A.R., Morgan, J.A., Lecain, D.R., King, J.Y., Nelson, J.A. 2005. Elevated CO2 and defoliation effects on a shortgrass steppe: forage quality versus quantity for ruminants. Agriculture, Ecosystems and Environment. 111:166-184. Interpretive Summary: By the end of this century the concentration of atmospheric carbon dioxide (CO2) is expected to more than double. This continued increase in CO2 concentration is likely to have profound effects most ecosystems including the grasslands of the Great Plains of the USA. Approximately 40% of the Great Plains is used for livestock production. Since the impact of increasing CO2 concentrations had not been investigated in the short grass prairie an open top chamber study was initiated in 1997 at a shortgrass steppe site in northeastern Colorado, USA. One of the questions to be resolved was the effect of increasing CO2 the quantity and quality of forage produced. This study suggests that the negative effects of elevated CO2 on forage quality are likely to be greater than the positive effects on quality, because quality drops to critically low levels that can inhibit utilization of the quantity that is available.
Technical Abstract: We assessed the effects of elevated atmospheric CO2 on forage quality and nutrient yields during four years in a native shortgrass steppe. Nutritional analyses relevant to ruminants were emphasized, because grazing by domestic livestock is the primary land-use, and the habitat is important for pronghorn antelope and deer populations. Defoliation treatment simulating grazing, and nitrogen fertilization treatment, were superimposed on CO2 treatments in large open-top chambers. Native semiarid grassland is neither productive nor nutritious range, whereby tradeoffs between the potential for elevated CO2 to increase forage quantity while decreasing quality may be accentuated. 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 in senescent autumn forage, 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. The low crude protein and digestibility can slow passage through the rumen and inhibit voluntary intake. Negative effects of elevated CO2 on animal performance mediated through forage quality are likely to be greater than the positive effects through quantity, because quality drops to critically low levels that can inhibit utilization of the increased quantity that is available. 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 two drought years during the study. Current-year defoliation increased both quality and production of protein and energy, but no CO2 by grazing treatment interactions were observed. Similar results were observed for nitrogen fertilization treatment, but conclusions here were limited to dry years when fertilization effects were smaller.