The Critical Period of Weed Control in Black Seed (Nigella sativa L.) at Mashhad

In order to determine the critical period of weed control in black seed, a field experiment was conducted at the Agricultural Research Station of Ferdowsi University of Mashhad, during 2009-2010 growing season. The experiment was laid out in a randomized complete block design (RCBD) with 12 treatments and three replications. In this study, onset and end of Critical Period of Weed Control (CPWC) were determined based on 2.5, 5 and 10% acceptable grain loss using Gampertz and Logestic equations. Two set of weed-free and weed-infested treatments were used to determine the CPWC. At the first set of treatments, weeds competed with black seed until 0, 14, 28, 42, 56 and 70 days after emerging (weed- infested periods). At the second ones, plots were kept free of weeds until the mentioned stage (weed-free periods). The goal of the first and second set of treatment was to determine the onset and end of critical period, respectively. In this study, Stellaria graminea at the onset of growing season and Amaranthus retroflexus, Chenopodium album and Echinochloa cruss-galli at the mid and end of growing season were main dominant species. The results indicated that at 2.5, 5 and 10% yield loss levels, the maximum weed-infested periods (onset of CPWC) were 10/5, 13 and 17/3 days after emergence (86, 108 and 149 growing degree days), respectively. Also based on those 3 levels of grain yield loss, minimum weed-free period (end of the CPWC) were 76/8, 74/8 and 71/1 days after emergence (1005, 960 and 883 growing degree days), from whole black seed growing season (equal to 84 days or 1188 growing degree days).