Of all the biological threats to crops, weeds are the most damaging. In the United States alone, farmers' losses due to weeds are estimated to be 10% of total production. Hence, it is important to determine which way the "balance of power" between crops and weeds will likely shift in a CO2-enriched world.
Probably the best way to make such a projection is to compare the relative responses of various crops and the weeds that typically compete with them, when they are grown together under conditions of atmospheric CO2 enrichment. When this has been done for a C3 crop and its C4 weeds, it has typically been the crop that prevails or benefits the most, due to the greater CO2-induced stimulation of the growth of C3 as opposed to C4 vegetation; while for a C4 crop and its C3 weeds, the weeds have generally fared better, for essentially the same reason.
Fortunately, the former of these situations is the more common of the two, as 80% of the crops most important for feeding the world are of the C3 variety. Hence, on balance, mankind's crops will probably fare just a little better than the weeds that afflict them, as the carbon dioxide content of the atmosphere continues to rise in the years ahead.