Cotton is an important crop in Turkey. Johnsongrass is one of the worst weeds in cotton fields worldwide. Field experiments were carried out in 1992, 1997 and 2002 to show the effect of johnsongrass densities on cotton yield. Johnsongrass densities were 0, 1, 2, 4, 8, 16 and 32 plants per 8 m and 0, 3, 6, 8, 11, 14 and 22 plants per 4 m of cotton row. Data were fitted to equations Y = 1/(a+b*X8) in 1997 and 2002 for the number of johnsongrass in 8 m of row length and Y = 1/(a+b*X4(-1/2)) in 1992 and 1997 for the number of johnsongrass in 4 m of row length. Cotton yield was affected by all johnsongrass densities. One johnsongrass plant in 8 m of cotton row decreased cotton yield 4.82 and 9.42% in 1997 and 2002, respectively.

Weeds are one of the main obstacles in organic farming as well as other farming systems. In the most agricultural systems such as conventional farming, herbicides are the main tool to solve weed problems in spite of problems bearing from excess use of chemicals. Organic farmers rely on mechanical techniques which include tillage that can make soil open to erosion if it becomes only technique to be used. Allelopathy is both inhibitory and stimulatory biochemical interactions between plants. Allelopathic relations have been used to establish proper farming systems, control directly weeds and other pests, and develop chemicals less harmful to the environment. There have been researches on allelopathy in Turkey since 1980's. However, it is not easy to say that results of these studies have had a chance to be implemented. Allelopathy studies in Turkey can be integrated with organic agriculture; but, research and implementation needs elaborated.