Nadeem et al., J. Anim. Plant Sci. 23(1):2013 216 DETERMINING CRITICAL WEED COMPETITION PERIODS FOR BLACK SEED M. A. Nadeem, A. Tanveer, T. Naqqash, * A. J. Jhala and ** K. Mubeen Department of Agronomy, University of Agriculture Faisalabad * Department of Agronomy and Horticulture, University of Nebraska-Lincoln, USA. ** Department of Agronomy, The University of Poonch Rawalakot (AJ & K, Pakistan) Corresponding Author Email: drman@uaf.edu.pk ABSTRACT Effect of weed competition periods on weeds and yield of black seed was studied through field experiments conducted at Agronomic Research Area, University of Agriculture, Faisalabad during winter 2006-07 and 2007-08. Weeds were allowed to compete with the crop for 40, 50, 60, 70, 80 days after sowing and throughout the growth season along with a weed free treatment. The data on weed density and biomass; yield components and yield of black seed were documented. Weed density and biomass increased with the increase in weed competition periods and the maximum weed density and biomass was recorded in weedy check during both the years of study. Outcome of the differences between the weed free and competition for 40 days after emergence was statistically similar for number of capsules per plant, number of branches and number of seeds per capsule during first year of study, however, all yield components showed a significant decline when weeds were allowed to compete compared with weed free treatments during second year. The maximum seed yield was recorded in weed free treatment and it showed a linear decline as the weed competition period was increased during both the years of study. Weeds must be controlled within 40 days after emergence of black seed to avoid the risk of economic yield loss as it has been found to be the critical period of weed crop competition in black seed. Key words: Black seed, Weed-crop competition, Weeds, Yield, Pakistan. INTRODUCTION There is a period during the life cycle of a crop when it is most sensitive to presence of weeds. This period is known as critical period of competition (Mubeen et al., 2009).There are three relationships which exist in critical period studies: a) the crop must be kept weed-free for the same duration that a weed infestation can be tolerated. In this situation, yield loss will be avoided if weed control is performed at this one critical time; b) when the critical timing of weed removal is longer than the critical weed-free period. In this case, yield loss will not occur if weeds are controlled at any point between these critical stages; c) When the critical weed free period is of no longer duration than the critical timing of weed removal, the crop must be kept weed free between these timings to prevent yield loss. Black seed (Nigella sativa L.) is an annual herbaceous plant belongs to family Ranunculaceae, also known as black cumin. It is widely cultivated throughout South Europe, Syria, Egypt, Saudi Arabia, Iran, Pakistan, India and Turkey (Riaz et al., 1996). It is a spicy herb and is a crop of great medicinal importance. The principle active ingredient isolated from the volatile oil of black seed is thymoquinone (Mahfouz and El-Dakhakhany, 1960). In Turkish folk medicine, black seed was used as a natural stimulant of immunity, antidiabetic, digestive disorders, carminative, anthelmintic and appetitive (Baytop, 1984; Hussain and Tobji, 1997). In spite of its great importance, little attention has been paid to this minor crop in Pakistan. It is grown at a small scale level in Khyber Pakhtunkhwa region with an annual production of about 200-250 tons (Ahmad and Gafoor, 2004). The yield of black seed is lower than its potential. Among different factors responsible for lower yields, weed interference is of supreme importance. Weed interference in black seed can cause 60-85% reduction in crop yield (Ahmad and Gafoor, 2004). The critical weed crop competition period is very important for planning efficient weed control strategy. Critical weed competition period varies with season and nature of crops and weeds. For example in wheat, critical period of weed competition is 30 to 60 days after sowing (DAS) (Ahmad and Sheikh, 2003), chickpea 40 to 45 DAS (Chopra et al., 2003), and in soybean 50 to 60 DAS (Juan and Cendoya, 2002). However, there is no information available about the critical weed competition period in black seed. To realize the full economical and yield potential of black seed, production practices will have to be optimized by improving weed control efficiency in this crop. Being a low acreage crop, the choice of herbicide in black seed is limited. In order to develop efficient herbicide use and provide a logical basis for the development of an integrated weed management system, information on critical period of weed control is essential. This information may lead to less reliance on the use of residual herbicides and to more reliance on well-timed post-emergence herbicides. Reductions in quantities of The Journal of Animal & Plant Sciences, 23(1): 2013, Page: 216-221 ISSN: 1018-7081