African Journal of Agricultural Research Vol. 3 (8), pp. 523-530, August 2008 Available online at http://www.academicjournals.org/AJAR ISSN 1991-637X © 2008 Academic Journals Full Length Research Paper Critical period of weed control in winter lentil under non-irrigated conditions in Turkey Murat Erman 1 , Iık Tepe 2 , Bekir Bükün 3 , Reyyan Yergin 4 and Mehtap Takesen 5 1 YÙzüncÙ Yil University, Faculty of Agriculture, Department of Field Crops 65080 Van, Turkey. 2, 4, 5 YÙzüncÙ Yil University, Faculty of Agriculture, Department of Plant Protection 65080 Van, Turkey 3 Harran University, Faculty of Agriculture, Department of Plant Protection 63040 anlıurfa, Turkey. Accepted 26 August, 2008 This study was conducted during the growing seasons of 1998 - 1999 and 2003-2004 to determine the critical period of weed control (CPWC) in winter-lentil (cv. Sazak-91). The experiments were laid out in a randomised block design with four replications. The beginning and end of CPWC were based on 5% acceptable yield loss levels which were determined by fitting logistic and Gompertz equations to relative yield data, representing increasing duration of weed-interference and weed-free period, estimated as growing degree days (GDD). The results indicated that CPWC for seed yield was between 237 and 846 GDD in the first year and between 123 and 414 GDD in the second year, while CPWC for biomass was between 216 and 820 GDD in the first year and between 212 and 374 GDD in the second year. Thus weeds should be controlled from the first week after the onset of regrowing stage of the crop in spring up to 7 th week for winter-lentil to avoid losses above 5%. Key words: Lentil, critical period, weed competition, weed interference. INTRODUCTION Turkey, one of the three countries in terms of lentil (Lens culinaris Medik.) production and sowing area in the world, follows after India and Canada. However, Turkey is the tenth in terms of lentil yield in the world. Lentil is the second leading grain legume crop after chickpea in Turkey. According to the latest statistics from FAO (2006), 440 000 hectares were used for lentil production and 555 000 tonnes of production were obtained in 2005 in Turkey. Lentil is commonly grown as a rotation crop in east and southeast of Turkey. It is an important crop because of its high protein content of seed and straw for human and animal nutrition. Traditionally, lentil is grown in spring in Turkey; however, winter-lentil cultivars are grown in recent years. Weeds are known to be the most important factor affecting lentil yield. Halila (1995) reported that the average loss in lentil yield caused by weeds was 60% and that, at the highest density of weeds, loss could *Corresponding author. E-mail: merman56@hotmail.com. Phone and Fax: +90-432-2251794. amount to 100%. Weeds compete with crop for nutrients, soil moisture and sunlight. The extend of weed compe- tition depends upon type of weed species, severity of weed infestation, duration of infestation, climatic condi- tions which affect weed and crop growth. Reduction in crop yield has a direct correlation with weed competition. Generally an increase in one kilogram of weed growth corresponds to a reduction in one kilogram of crop growth. Weeds remove plant nutrients more efficiently than crops. In a drought situation, they thrive better than crop. When left undisturbed, some weeds can grow faster and taller than crop and inhibit tillering and branching. They can curtail sunlight and adversely affect photosynthesis and plant productivity (Rao, 2000). Therefore, weeds are of crucial importance. The loss caused by weeds in lentil production is considerable for two reasons: first, the lentil has a slow rate of develop- ment and, thus, is overwhelmed by weeds in the early stages of its development. Weeds are easily compatible with the lentil and grow without difficulty, because weeds utilize soil moisture and plant nutrients in prevailing environmental conditions efficiently better than crops in their first development stages (Basler, 1981; Bukun and