Breeding Winter Wheat for Increased Weed Suppressive Ability Against Italian Ryegrass (Lolium perenne).

Infestations of Italian ryegrass are problematic in both conventional and organic wheat production systems, and with the efficacy of herbicidal modes of action at risk and market opportunities for organic grain expanding, alternative or supplementary methods for control are worth exploring. The development of wheat cultivars with superior competitive ability against Italian ryegrass could play a role in maintaining acceptable yields and suppressing weed populations.  The objective of this study was to develop breeding methods to improve the competitive ability of winter wheat cultivars by comparing indirect methods of selection for weed suppressive ability. Three wheat lines with different morphological traits were over-seeded with varying amounts of Italian ryegrass in a four replicate, factorial treatment structure and used to test potential methods of screening wheat cultivars for ryegrass competitiveness.  The experiment was conducted at two locations in each of two years in North Carolina, USA.  Counts of ryegrass seed heads m^-2 during grain fill were strongly associated with the weed to crop biomass ratio, the generally accepted measure of weed suppressive ability.  Visual ratings of weed biomass in the plots taken at all growth stages, especially during and after anthesis, were also strongly correlated with weed suppressive ability.  Measurements from non-imaging spectrophotometers and overhead photographs taken during tillering to early dough development were unreliable estimates of weed to crop biomass.  These measurements failed to account for wheat cultivar differences in biomass, color, and growth habit.  We concluded that visual ratings and counts of ryegrass seed heads m^-2 during grain fill are indirect methods of selection for weed suppressive ability appropriate for use in large breeding programs.  Morphological traits associated with weed suppressive ability in a set of 60 cultivars and experimental lines from the 2012 North Carolina Official Variety Test will be described.