Tow field experiments were carried out during the two successive summer seasons of 2010 and 2011 at the Agric. Res. and Exp. Center of Fac. Of Agric. Moshtohor to study the effect of five plant population densities (20, 22, 24, 26 and 28 thousand maize plants per fed.) and seven periods for weed control (un-weeded control (1), weed control at 20 (2), 20+35 (3), 20+35+50 (4), 20+35+50+65 (5), 20+35+ 65+80 (6) and 20+35+80+95 days after sowing (7)) on growth, yield and its components of maize, associated weeds as well as the net economic return of maize. The obvious results of this investigation can be summarized as follows:
Increasing plant population density from 20 to 28 thousand plants /fed. significantly decreased total number and fresh weight of removed weeds in the first season, total fresh and dry weights of weeds at harvest, number of green leaves / plant, leaf area / plant, stem diameter, ear length, number of grains / ear, ear weight, shelling %, 100-grain weight and grain yield / plant in both seasons. On the other hand, plant height, leaf area index, number of ears /fed., Stover yield, nitrogen up-take and protein yield / fed. were significantly increased. Generally, the greatest grain and biological yields / fed. were results from maize planting by 26000 plants/ fed. This was true in the two growing seasons.
Un-weeded check significantly decreased all these characters except total fresh and dry weights of weeds at harvest compared with weed removal even once after maize emergence. The maximum grain and biological yields/ fed. were produced from planting maize under weed control at 20, 35, 50 and 65 days after sowing. The critical period of weed control (CPWC) in maize crop based on 5% acceptable yield loss was 20 - 35 days after sowing.
The highest values of leaf area index and stover yield / fed. were recorded from planting maize at the highest density under weed control by treatment 5. While, the lowest plant density under the same weed control gave the maximum values for No. of green leaves / plant, leaf area / plant, ear length, No. of grains /ear, 100-grain weight, ear weight and grain yield / plant in the two growing seasons. Meanwhile, the greatest maize grain and biological yields / fed. were resulted from maize planting by 26000 plants / fed. under the same weed control (weed control at 20 +35 + 50 + 65 days after sowing).
The best treatment was that planting 24000 maize plants / fed. under weed control by either treatment 5 or 6 in the first season and planting 26000 maize plants /fed. with weed control by treatments 5 in the second season, where the net farm return valued 1945.2 and 2981.0 L.E. / fed. in the first and second seasons respectively.
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