Competitive effects of redroot pigweed (Amaranthus retroflexus) on the growth indices and yield of corn

The mutual effects of redroot pigweed ( Amaranthus retroflexus ) on corn ( Zea mays ) were evaluated in an experiment conducted in 2005 at the Iranian Plant Protection Research Institute at Qazvin, considering the different densities of redroot pigweed against four different corn densities. Redroot pigweed, at 0, 35, 50, 65, and 80 plants m⁻¹ row⁻¹, was arranged factorially with corn at four, five, six, and seven plants m⁻¹ row⁻¹ in a randomized complete block design. Crop–weed competition resulted in a reduction in the total dry matter, Leaf Area Index, and crop growth rate of corn. Furthermore, an increasing weed density ≤65 plants m⁻¹ row⁻¹ reduced the corn grain yield and biological yield. Overall, six corn plants m⁻¹ row⁻¹ was suggested as the optimum density of this crop in competition with redroot pigweed.     

Quantitative evaluation of wheat against volunteer rye in Iran

Rye infestations have increased in frequency and severity in the wheat fields of Iran and have caused yield reductions in wheat production. In order to study the competition effects of wheat against rye, an experiment was conducted at the research centers of Karaj and Varamin, Iran, during 2001 and 2002. The experimental design was a randomized complete block with 24 treatments and four replications. The treatments included pure stands of wheat at densities of 350, 450, 550, and 650 plants m⁻² and volunteer rye densities of 10, 30, 50, and 70 plants m⁻², and mixed stands of wheat and rye at complete factorial densities. The results indicated that rye was a superior competitor compared to wheat. The biological and economic yields of wheat were mainly affected by interspecific competition in the Karaj region. This was also true during the first year in the Varamin region but, in the second year, the biological and economic yields of wheat were equally affected by interspecific and intraspecific competition because of increasing precipitation. The evaluation of competitive ability, using regression coefficients, showed that in Karaj, the effect of one rye plant on wheat was approximately equivalent to three and two plants of wheat in reducing the grain yield of wheat in the first and second years, respectively, while in Varamin, it was equivalent to three and 1.2 plants of wheat, respectively. In other words, each 0.36 and 0.51 rye plant in Karaj and each 0.36 and 0.84 rye plant in Varamin had the same impact on the reciprocal wheat grain yield as did one wheat plant in the first and second years, respectively.