Relationship among phenotypic growth traits, yield and weed suppression in spring wheat landraces and modern cultivars

Economic costs and the potential for unintended environmental effects of herbicides have led to an increasing emphasis by crop breeders on selecting for weed suppression ability (WSA). Genetic variation for traits conducive to crop competitiveness against weeds may be concentrated in historical cultivars and landraces that were selected before the widespread use of crop protection chemicals. We evaluated 63 spring wheat landraces and modern cultivars for 5 potential competition traits, including plant height, leaf area index (LAI), juvenile growth habit (JGH), coleoptile length and 1000 kernel weight (TKW), and for the ability of these cultivars to achieve high yields, suppress weeds and withstand mechanical cultivation. Differences were found among the 63 spring wheat cultivars for grain yield, weed weight and all 5 growth habit traits (P < 0.001). Yield was negatively correlated with coleoptile length (P < 0.001), and positively correlated with plant height, JGH, TKW and LAI (P < 0.01). Negative associations were found between weed weight and plant height (P < 0.001) and weed weight and coleoptile length (P < 0.05). Multiple regression analyses showed that plant height was responsible for 7% of the variation in weed weight, while coleoptile length, JGH, TKW and LAI had no direct effect on weed weight. Plant height, TKW and LAI were responsible for 38% of the variation in yield. There was no genotype × treatment effect for mechanical cultivation on yield; however, a genotype × treatment interaction existed for effect of mechanical cultivation on weed weight (P < 0.05). Ten cultivars showed reduced yield, six cultivars showed improved WSA, and three cultivars showed reduced WSA under the mechanical tillage treatment. Regression analysis showed an increase in yield (r2 = 0.52) and a slight decrease in WSA (r2 = 0.10) over the past 150 years. The top five cultivars for WSA reduced weed weight by 573% over the bottom five cultivars. Our results indicate that direct selection would be most effective for tolerance to mechanical tillage, while indirect selection on plant height may be effective in improving WSA.