258 Investigations regarding competitive effects of five Galium aparine density levels (0, 18, 36, 54 and 72 m -2 ) on the yield and yield related traits of wheat planted at three sowing times were made in field trials conducted at the Agronomic Research Area, University of Agriculture, Faisalabad, Pakistan during winter 2005/06 and 2006/07. Maximum grain yield of 5480 and 5395 kg ha -1 was obtained in 2005/06 and 2006/07, respectively from weed free wheat. While G. aparine density of 18 plants m -2 did not cause significant yield reduction over control. Wheat growth and yield determinants like number of spike bearing tillers, grains per spike, 1000-grain weight and grain yield were significantly reduced by increasing G. aparine density above 18 m -2 . Wheat grain yield loss varied between 4 to 32% with weed density ranging from 18 to 72 plants m -2 in each year. Sowing of wheat on 7 th November proved to be helpful in reducing crop yield losses due to G. aparine infestation. G. aparine per plant dry weight and seed production were suppressed with delayed sowing and increasing G. aparine densities in the presence of crop. On the basis of these results we can propose that wheat should be planted in early November to get bumper yields and the option of chemical herbicides to control G. aparine at its density beyond 18 plants m -2 should be exploited. Keywords: Galium aparine, density, growth, yield, Triticum aestivum, response surface INTRODUCTION Evolution of short statured, fertilizer responsive, disease resistant and high yielding varieties have brought the real breakthrough in wheat production in Pakistan. However, there is still a wide yield gap between the potential yield (6 tons ha -1 ) and the actual achieved yield at farmer’s field i.e. 2.49 tons ha -1 (Anonymous, 2002). Among the factors responsible for obtaining higher yield, optimum planting time and weed control are of prime importance. Measures commonly used by wheat growers in controlling weeds include delayed planting and tillage. By delaying seeding, growers are able to control early emerging weeds via mechanical or chemical measures. Unfortunately, under lower weed densities, this practice may result in reduced yields and test weight (Humphreys et al., 1994). Early sowing (Before 20 th November) always produces greater yields than late sowing (After 20 th November), mainly due to higher utilization of resources (Arain et al., 1999). A yield reduction of 33% and 31% was observed in wheat genotypes SD-1200/11 and SD- 1200/51, respectively when sowing was delayed from 18th November to 11th December (Sial et al., 2005). Weeds reduce crop yield either by competing for light, nutrients, water and carbon-dioxide (Anderson, 1983) or interfering with crops by releasing secondary substances in the rhizosphere of the crop (Rice,1984). Wheat yield losses due to weeds in Pakistan were estimated to be in the range of 25-30% (Nayyar et al., 1994). Increased labour costs and limited energy resources have hampered the use of physical and mechanical methods, so chemical methods remains the only option for weed control as roughly 60% of wheat area is treated with herbicides in Pakistan. The economic threshold (ET), the density of pest at which the benefit of taking action is greater than the cost of taking action, is an important concept in integrated weed management (Daxl et al.,1994). Below the ETL, a certain amount of damage and the presence of weeds can be tolerated. In order to measure the ET of weeds in wheat we have to realize that bio-ecological relationships in weed crop-competition are complicated ones. In crop production our major concern is that a crop plant should be the dominant competitor. We need to understand weed crop ecology that will lead to more effective weed prevention, management and control based on critical weed density which seriously limit crop yield. It has been observed that weed flora has also changed and some new weed species like Galium aparine (cleavers) are emerging as threats to wheat yields in the near future (Nayyar et al., 2001) as approximately 10% of wheat fields are infested with this weed in Pak. J. Agri. Sci., Vol. 46(4), 2009 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 http://www.pakjas.com.pk DENSITY DEPENDENT INTERACTIONS BETWEEN CLEAVERS (Galium aparine) AND WHEAT (Triticum aestivum) PLANTED AT DIFFERENT TIMES Ahsan Aziz 1* , Asif Tanveer, Asghar Ali and Muhammad Yaseen 2 1 Department of Agronomy, University of Agriculture, Faisalabad, 38040 2 Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad Corresponding author’s e-mail: drasiftanveeruaf@hotmail.com