To evaluate the effect of rye population on reproductive characteristics and grain yield in wheat, an experiment was conducted in Roch Cool Season Cereals Research Station in Torbat Haydarieye, Khorasan Agricultural Research Center, in 1998. In this experiment, 5 levels of rye contamination (0% as control, 10% = 25 p/m2, 20%= 50 p/m2, 30%= 75 p/m2, and 40%=100 p/m2 ) as well as two levels of wheat cultivars (Alamoot and C-73-5 ) were studied in a factorial randomized complete block design with 4 replications. The results indicated showed that with increase in rye population (rye contamination) the number of seeds/per secondary spike, 1000 seed weight per spike in main stem as well as secondary ones, grain yield and harvest index decreases significantly but the number of seeds per main spike did not vary. It was found that the decrease in biological wheat yield was due to a decrease in grain yield. Straw and grain yield in wheat as compared to control decreased by 21% and 52%, respectively. The presence of 25 rye plants per m2 was considered as the threshold line .
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