Impact of crop and weed densities on competition between wheat and Silybum marianum Gaertn.

Field trials were conducted at Peshawar, Pakistan during two crop seasons i.e. 2003-04 and 2004-05 using a Randomized Complete Block (RCB) design with split-plot arrangement. The main plots consisted of four seed rates of wheat ( Triticum aestivum L.) i.e. 100, 120, 140 and 160 kg ha 1 , while sub-plots had 7 densities of the weed holy thistle (Silybum marianum Gaertn.) viz., 0, 3, 6, 9, 12, 15 and 18 plants m -2 . Holy thistle was more aggressive during second year as compared to first year, mainly due to unusual rainfall (140 & 317 mm in year 1 & 2, respectively), which in return affected the yield and yield related traits in wheat. Crop yield losses with increasing weed density were greater with lowest crop populations. The results of grain yield losses caused by the weed at different densities in two trials indicated that both species caused a density-dependent yield loss which is better explained by weed dry weight. Percent light interception increased with increasing wheat or weed density, however, weed density beyond 6 m -2 had no significant effect on percent light interception. Higher fresh and dry biomass of wheat was recorded in medium sowing rates (120 &140 kg ha -1 ). As a result, higher fresh and dry biomass of the weed was recorded in low wheat density rather than high seeding density. All of the agronomic characters decreased with increasing the density of either species. Higher values of reproductive and vegetative characters of the weed were recorded in the second year as compared to that in the first year, and highest grain yield was obtained with wheat seeding rate of 120 kg ha -1 . Maximum yield losses at seed rate 100, 120, 140 and 160 kg ha -1 were 26, 18, 15 and 7% during 2003-04 and 37, 31, 28 and 29 % during 2004-05. Weed seed production was related with weed biomass which was dependent on wheat density; the higher the wheat density, the lower was the weed biomass. However, there was still a large seed production even at the highest wheat density. Thus, crop density alone could not suppress the weed below threshold level. The weed reduced wheat yield chiefly by the indirect effect of decreasing wheat tillers, the earliest formed yield component. The weed density, which resulted in yield losses varied greatly with density and season.