Effects of time and intensity of defoliation on yield and yield components of corn

In order to evaluate the effects of time (20, 40, 60 and 100 days after sowing) and intensity of defoliation (0, 33, 66 and 100%) on a corn cultivar (three way cross 647), a complete randomized block design with split plot arrangement and three replications was conducted in Experimental Field of Faculty of Agriculture Ferdowsi University of Mashhad.The main plots were consisted of defoliation time and the sub plots were consisted of defoliation intensity. For this purpose leaves were defoliated at 33, 66, and 100% and at the end of experiment, different morphologic and physiologic criteria such as number of leaves per plent, total dry matter and plant components dry matter, seed yield and yield components and also harvest index were evaluated. Results indicated that defoliation at the early stage of crop growth (20 and 40 days) and at the end of crop growth (100 days) showed no effects on most of criteria measured. However at the middle of growth period, defoliation reduced these parameters. Intensity of defoliatin up to 66% had no effects, but 100% reduced all those parameters. Therefore, it appears that by defoliation up to more the 50% of plant leaves, plants could recover and appears as a normal plant, but with 100% defoliation this was not happened. Defoliation at early stages had no negative effects, because regrowth of leaves could have compensated for these type of damage. At the end of growing season, plant cycle was completed and so defoliation had no negative effects on growth and development, while at the middle stage (60 and 80 days) the negative effect was due to the fact that plants could not recover completely from defoliation.