The critical period of weed control in forage corn in Birjand region.

The critical period for weed control (CPWC) is the period in the crop growth cycle during which weeds must be controlled to prevent unacceptable yield losses. A field study was conducted in 2006 in the University of Birjand to determine CPWC in Birjand region using a randomized complete design with 14 treatments and three replications. The treatments consisted of two different periods of weed interference، a critical weed-free period and a critical time of weed removal were imposed at V3، V6، V9، V12، V15، and R1 (based on phenological stages of corn development) with a weedy check and a weed-free check. The CPWC was determined by fitting non-linear regression models of Logistic and Gompertz equations to the data in range of 2. 5، 5، 10 and 20% of acceptable yield loss levels. The CPWC was from 5- to 15-leaf stage (19-55 DAE) to prevent yield losses of 5%. This period to prevent yield losses of 2. 5، 10 and 20% was 4- to 17-leaf stage (14-59 DAE)، 6- to 12-leaf stage (25-47 DAE) and 8- to 9-leaf stage (31-36 DAE) respectively. The leaf area index of corn were significantly decreased by weed competition in both weed free and weed infested treatments (P<0. 01). Results also showed that there was a positive correlation between forage yield and LAI of corn at silking stage when competing with weeds (r= 0. 95) and could be considered as an index of weed effect on corn yield.