Weeds have a greater genetic diversity than crops. Consequently, if a resource (light, water,
nutrients or carbon dioxide) changes within the environment, it is more likely that weeds will
show a greater growth and reproductive response. It can be argued that many weed species
have the C 4 photosynthetic pathway and therefore will show a smaller response to atmospheric
CO 2 relative to C 3 crops. However, this argument does not consider the range of available C 3
and C 4 weeds present in any agronomic environment. That is, at present, the U.S. has a total of
46 major crops; but, over 410 “troublesome” weed species (both C 3 and C 4 ) associated with
those crops (Bridges 1992). Hence, if a C 4 weed species does not respond, it is likely that a C 3
weed species will. In addition, many growers recognize that the worst weeds for a given crop
are similar in growth habit or photosynthetic pathway; indeed, they are often the same
uncultivated or “wild” species, e.g. oat and wild oat, sorghum and shattercane, rice and red rice.
To date, for all weed/crop competition studies where the photosynthetic pathway is the same,
weed growth is favored as CO 2 is increased (Table 1, Ziska and Runion, In Press).