- Agronomic: wheat
- Crop Production: application rate management
- Education and Training: extension, networking, participatory research
- Production Systems: organic agriculture
The purpose of this project was to screen locally adapted winter wheat lines for weed suppressive ability against Italian ryegrass, test the relative contribution of allelopathy and competition to weed suppression outcomes in the field, and identify morphological traits that could be utilized for indirect selection for weed suppressive ability in winter wheat. While allelopathy did not affect weed suppression, morphological traits including high vigor during tillering and heading, erect growth habit, early heading date, and height throughout the growing season were correlated with weed suppressive ability. Several high-yielding commercially available cultivars with strong weed suppressive ability were identified.
Organic grain production is increasing in North Carolina, and with initiatives to develop local bread wheat production for artisanal baking and the expansion of organic dairy operations, market opportunities for organic wheat are rapidly expanding. An informal survey estimates that organic corn, wheat, and soybean production increased from 950 acres in 2006 to over 12,000 acres in 2011 in North Carolina. Breeding specific to the needs of organic producers could help close the yield gap between organic and conventional production and strengthen the genetic foundation for organic cropping systems.
Members of the North Carolina Organic Farm Advisory Board (OFAB) identified Italian ryegrass infestations as a major limitation to organic wheat production in the southeast and suggested that breeders select for varieties that compete more effectively against weeds. Italian ryegrass, a winter annual, is a major weed in small-grain crops in the Southeastern Unites States (Everman and Jordan, 2011) where it reduces grain yield by competing for nutrients and light, decreases the number of productive tillers, and promotes lodging (Appleby et al., 1976). Without the option of synthetic herbicides, organic farmers must achieve near total weed control before planting through mechanical cultivation with a rotary hoe or tine weeder (Weisz and Van Duyn, 2007), narrow row spacing (Everman and Jordan, 2011), and high density planting (Paynter and Hills, 2009). Severely infested fields are often taken out of organic wheat production for many years, as few management options are available for Italian ryegrass control (Weisz and Van Duyn, 2007).
Public sector wheat breeders in the southeast devote significant time and resources to breeding for improved disease resistance; but herbicide use is routine in breeding trials, essentially precluding selection for weed competitive ability. In fact, research has suggested that historical varieties often have better weed suppressive ability than modern cultivars (Mason et al., 2007). Current protocols for screening wheat cultivars for weed suppressive ability are expensive and unlikely to be continued in the long-term without sustained interest and funding. The development of more efficient breeding protocols and identification of gross morphological traits, such as early vigor or height, that breeders can select for in the absence of weed pressure will ensure that continual progress is made in organic breeding. By developing new breeding methods and identifying wheat cultivars with superior competitive ability against Italian ryegrass, we can complement cultural methods of control in maintaining acceptable yields and suppressing weed populations.
Combinations of morphological traits that enable the crop to effectively exploit limited resources including light, water, and nutrients (Zimdahl, 2004) and allelopathic activity (Rice, 1984) will likely contribute to the competitive ability of such crop cultivars. Allelopathic cultivars suppress weed germination and growth through the release of chemical exudates. Different cultivars of the same crop species can vary widely in their allelopathic activity (Wu et al., 2000a). Researchers have therefore suggested that the allelopathic properties of cultivars could be significantly improved through plant breeding (Bertholdsson, 2010; Rice, 1984). The equal compartment agar method (ECAM) is a laboratory bioassay developed to screen wheat cultivars for allelopathic activity against annual ryegrass, and control for the effects of competition for light, nutrients, and water between crops and weeds (Wu et al., 2000b). Although researchers have used the ECAM bioassay to test diverse wheat germplasm for allelopathy (Wu et al., 2000a), the performance of allelopathic wheat cultivars identified in the laboratory remains untested in the field. Allelochemicals may behave differently in complex agroecosystems than they do in controlled laboratory settings; thus, the consistency and strength of allelopathic effects must be proven in replicated field trials.
Morphological traits such as cultivar height at the end of the growing season, high tillering capacity, early biomass accumulation, leaf habit, and time to maturity (Lemerle et al., 2001; Lemerle et al., 1996; Mason et al., 2007) have been shown to contribute to weed suppressive ability in spring wheat. Fewer studies have been conducted on weed suppressive ability of winter wheat cultivars, and it is unclear whether the same traits confer a competitive advantage to wheat in the long period of slow growth typical of southeastern winters. By collecting information on the early vigor, growth habit, tillering capacity, leaf area index, heading date, and height throughout the growing season, we can identify traits that breeders can use to indirectly select for cultivars with improved weed suppressive ability. If wheat breeders can select for morphological traits conferring superior competitive ability in the absence of weed pressure, they will be more likely to sustain breeding efforts for weed suppressive ability in the long-term.
- Evaluate whether allelopathic wheat cultivars that inhibit Italian ryegrass root growth in laboratory bioassays also demonstrate superior weed suppressive ability in replicated field trials.
- Identify morphological traits conferring strong weed suppressive ability and yield tolerance to winter wheat cultivars that can be used by breeders making indirect selection in the absence of heavy weed pressure.
- Recommend commercially available varieties with good weed suppressive ability and high yields in organic variety tests to producers.