Mechanisms of corn tolerance and velvetleaf suppressive ability

Crops that better tolerate weed competition or preempt resources from needs may reduce dependence on herbicides for weed control and benefit an integr ated weed management program. Irrigated field experiments were conducted to identify morphological traits of corn (Zea mays L.) that enhance its toler ance and weed suppressive ability. Two old (circa 1940) and two modern corn hybrids were grown in mixture with five target densities of velvetleaf (Ab utilon theophrasti Medik,) in two years. Corn height and leaf area index (L AI) were measured periodically throughout the growing season, Vertical Leaf area distribution was measured at corn anthesis, Quantitative estimates of these traits were obtained for each experimental unit using linear and non linear regression analysis, Magnitude of the correlation between these trai t estimates and corn yield reduction in mixture with velvetleaf or velvetle af seed capsule production was used as an indicator of the importance of ea ch trait to the enhancement of corn tolerance or velvetleaf suppressive abi lity, Canopy traits having highest correlation with corn relative yield in 1995 were maximum LAI (r = -0.38) and crop density (r = -0.24). Those with highest correlation in 1996 were crop density (r = -0.36), degree days accu mulated between emergence and 50% maximum height (r = 0.34) and LAI (r = 0. 19), and vertical leaf area distribution (0.49). Corn canopy traits were no t correlated with velvetleaf seed production in 1995, However, vertical lea f area distribution (r = 0.60), maximum LAI (r = -0.32), and corn density ( r = -0.26) were correlated with velvetleaf seed production in 1996. Results suggest that increasing maximum corn LAI, rate of canopy closure, or heigh t at which leaf area occurs in the canopy can improve corn tolerance and ve lvetleaf suppressive ability. These traits can be modified by varying cultu ral practices such as row spacing and population density, or through breedi ng.