{"references": ["Abbaspour, M. and P. R. Moghaddam. 2004. The critical period of weed\ncontrol of corn in Mashhad region, Iran. (In farsi). Iranian J.of\nAgronomic Research. 2: 182-195.", "Dawson, J. W. 1970. Time and duration of weed infestations in relation\nto weed-crop competition. Proc. South. Weed Sci. Soc. 23:13-15.", "Del Pino, A. and G. Covarelli. 1999. Critical period of weed competition\nin maize. In: Proceedings of the 11th EWRS (European Weed Research\nSociety) Symposium, Basel, Switzerland, 68.", "Evans, S. P., S. Z. Knezevic, J. L. Lindquist, C. A. Shapiro, and E. E.\nBlankenship. 2003. Nitrogen application influences the critical period\nfor weed control in corn. Weed Sci. 51:408-417", "Ferrero, A., M. Scanzio and M. Acutis. 1996 Critical period of weed\ninterference in maize. In: Proceedings of the Second International Weed\nControl Congress, Copenhagen, 171-176.", "Halford, C., A. S. Hamill, J. Zhang, and C. Doucet. 2001. Critical period\nof weed control in no-till soybean and corn (Zea mays).Weed Technol.\n15:737-744.", "Hall, M. R., C. J. Swanton, and G. W. Anderson. 1992. The critical\nperiod of weed control in grain corn (Zea mays). Weed Sci. 40:441-447.", "Isik, D., Mannan, H. Bukan, B., Oz, A. and M. Ngouajiro. 2006. The\ncritical period for weed contror in corn in Turkey. Weed Technol. 20:\n867-872.", "Knake, E.L. and F.W. Slife. 1969. Effect of time of giant foxtail removal\nfrom corn and soybeans. Weed Sci. v.17:281-283.\n[10] Knezevic, S. Z., S. P. Evans, E. E. Blankenship, R. C. Van Acker, and J.\nL. Lindquist. 2002. Critical period for weed control: the concept and\ndata analysis. Weed Sci. 50:773-786.\n[11] Knezevic, S. Z., S. P. Evans, and M. Mainz. 2003. Row spacing\ninfluences the critical timing for weed removal in soybean (Glycine\nmax). Weed Technol. 17:666-673.\n[12] Kropff, M.J., Van Keulen, N.C., Van Laar, H.H. and B.J. Schenieders.\n1993. The impact of the environmental and genetic factors. In: Kropff,\nM.J and Van Laar, H.H. (Eds.). Modelling crop-weed interactions.\nWallingford, UK. CAB International. p.137-147.\n[13] Nieto, H. J., M. A. Brondo, and J. T. Gonzalez. 1968. Critical periods of\nthe crop growth cycle for competition from weeds. PANS 14:159-166.\n[14] Norsworthy, J. K. and M. J. Oliveira. 2004. Comparison of the critical\nperiod for weed control in wide- and narrow-row corn. Weed Sci. 52:\n802-807.\n[15] Ritchie, W. S., J. J. Hanway, and G. O. Benson. 1997. How a Corn Plant\nDevelops. Special Report No. 48 (Revised). Ames, IA: Iowa State\nUniversity of Sciences and Technology, Cooperative Extension Service.\n[16] SAS Institute (1999) SyS Ststems, Version 8. Statistical Analysis\nSystems Inst\u2500\u2592tute, Cary, NC, USA.\n[17] Swanton, C. J. and S. F. Weise. 1991. Integrated weed management: the\nrationale and approach. Weed Technol. 5:657-663.\n[18] Tollenaar, M. Daynard, T.B. and R.B. Hunter. 1979. Effect of\ntemperature on rate of leaf appearance and flowering date in maize."]}

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. Field studies were conducted in 2005 and 2006 in the University of Birjand at the south east of Iran to determine CPWC of corn using a randomized complete block design with 14 treatments and four 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 phonological stages of corn development) with a weedy check and a weed-free check. The CPWC was determined with the use of 2.5, 5, 10, 15 and 20% acceptable yield loss levels by non-linear Regression method and fitting Logistic and Gompertz nonlinear equations to relative yield data. The CPWC of corn 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 height and 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 significant positive correlation between yield and LAI of corn at silk stage when competing with weeds (r= 0.97).