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Soybean Seed Yield as Impacted by Velvet Leaf Weed Infestations
Ziska, L. 2012. Observed changes in soybean growth and seed yield from Abutilon theophrasti competition as a function of carbon dioxide concentration. Weed Research 53: 140-145.

The author - a research scientist at the USDA's Crop Systems and Global Change Laboratory in Beltsville, Maryland - writes that recent and projected increases in anthropogenic carbon dioxide "are likely to stimulate photosynthesis, growth and reproduction differentially for a wide range of plant species," citing Patterson (1995). And he says that "consequently, differential responses between weeds and crops to rising CO2 levels may, in turn, have significant implications with respect to weed-crop interactions and potential crop production losses," citing one of his own recent papers (Ziska, 2010).

What was done
In studying this most important subject, Ziska grew soybeans (Glycine max) at both ambient and projected levels of atmospheric CO2 (+250 ppm above ambient) over two field seasons with and without the presence of a weed, Abutilon theophrasti, in order to quantify the potential effects of the ongoing rise in atmospheric CO2 concentration on weed-crop interactions and potential yield loss in soybeans.

What was learned
The results of the Research Plant Physiologist are portrayed in the figure below, where we have added a few interpretive observations of our own.

Figure 1. Soybean seed yield as a function of weed biomass. Adapted from Ziska (2012).

First of all, we note that the extra 250 ppm of CO2 enhances the seed yield of soybean by about 55% in the absence of the studied weed (when A. theophrasti biomass = 0), as best we are able to determine from the beginnings of the two seed yield vs. weed biomass linear regressions shown in the figure. From that point on, however, the seed yield of soybeans declines as weed biomass increases in both the ambient-air and the CO2-enriched-air treatments; but the rate of decline is greater in ambient air, with the result that the extra 250 ppm of CO2 enhances the seed yield of soybean by about 110% at a weed biomass of 500 g/m2, while at a weed biomass of 1000 g/m2 it enhances the seed yield of soybean by approximately 350%. And for a weed biomass of 1300 g/m2, an extension of the lower ambient-air relationship to the graph's abscissa suggests that the CO2-enhanced seed yield of soybean skyrockets to infinity, for no soybean seeds are produced in ambient air with that great a presence of weeds; and dividing any amount of seed production by zero gives one that otherworldly (infinite) result.

What it means
In the concluding words of Ziska, "the current study suggests that greater resilience of soybean yield to A. theophrasti limitations may be possible with a future, higher CO2 concentration." In fact, if Ziska's results are anywhere near representative of reality - and we have no reason to believe otherwise - we would say that the "greater resilience" of which he writes is essentially insured.

Patterson, D.T. 1995. Weeds in a changing climate. Weed Science 43: 685-701.

Ziska, L.H. 2010. Global climate change and carbon dioxide: assessing weed biology and management. In: Rosenzweig, C. and Hillel, D. (Eds.), Handbook of Climate Change and Agro-Ecosystems: Impacts, Adaptation and Mitigation. World Scientific Publishing, Hackensack, New Jersey, USA, pp. 191-208.

Reviewed 24 July 2013