Breeding Winter Wheat for Increased Weed Suppressive Ability Against Italian Ryegrass.

The utilization of weed suppressive wheat cultivars has been suggested as a complement to chemical and cultural methods of weed control. The objective of this study was to develop improved methods for breeding winter wheat cultivars with superior weed suppressive ability against Italian ryegrass (Lolium perenne L. ssp. multiflorum (Lam.) Husnot) by (1) determining the relative contribution of allelopathy and competition to weed suppression and tolerance in the field and (2) identifying morphological traits and molecular markers associated with weed suppressive ability in winter wheat. The allelopathic activity of lines from the 2011 North Carolina Official Variety Test (NC OVT) was assessed using a seedling bioassay. The allelopathic potential of tested lines varied significantly, ranging from 18 to 73 % Italian ryegrass seedling root length suppression compared to a nil wheat control. Eight lines with extremely high or low allelopathic activity and varying final height were chosen for inclusion in a strip plot experiment comparing their yield tolerance and Italian ryegrass biomass suppression. Neither the allelopathic potential nor final height of the lines contributed to weed suppressive ability or tolerance in this study. Sixty lines from the 2012 NC OVT were then over-seeded with Italian ryegrass, tested for various morphological traits throughout the growing season, and evaluated for weed suppressive ability.  The weed suppression of the lines differed significantly; end of season Italian ryegrass seed head density varied from 76 to 173 heads m^-2. Bayesian Information Criterion (BIC) indicated that an erect growth habit during tillering (Zadoks 29), high leaf area index (LAI) at stem extension (Zadoks 31), tall stature at heading (Zadoks 50-55), and early heading date contributed to weed suppression.  Forty-one of the 60 lines were screened for reduced height, vernalization, and photoperiod genes.  Short vernalization alleles at VrnA1 and VrnB1 were associated with superior weed suppressive ability.