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.
Study 1: Relative Contributions of Allelopathy and Competitive Traits to the Weed Suppressive Ability of Winter Wheat Lines against Italian Ryegrass
Before this study was initiated, we tested the allelopathic activity of 75 breeding lines and cultivars from the 2011 North Carolina Official Variety Test using the ECAM bioassay (Wu et al., 2000b). According to this bioassay protocol, germinated wheat seedlings were grown with ryegrass seedlings in 500 mL beakers with nutrient free agar for 20 days. A sheet of sterilized paperboard was suspended above the agar to control for varietal differences in competition for light. The varieties tested in our study differed significantly in their allelopathic activity; Italian ryegrass root growth suppression was 12-56% compared to a nil wheat control. Based on these results, we selected eight cultivars with consistently high and low allelopathic activity in the ECAM bioassay and dissimilar morphological characteristics to screen in intensive replicated field trials in order to evaluate whether the effects of allelopathy in the field justify further efforts to breed winter wheat for enhanced allelopathy.
Selected varieties were planted in replicated strip plot experiments at Caswell Research Station, in Kinston, NC; Piedmont Research Station, in Salisbury, NC; and Tidewater Research Station, in Plymouth, NC. These locations are the primary research sites used by the NCSU small grains breeding program and are representative of the major organic wheat production regions in NC. All varieties were planted in seven rows in 6 m long plots at a rate of 35 seeds m-2, a seeding rate typical for organic production. Four of the eight blocks planted in each location over-seeded with a commercial turf variety of Italian ryegrass. We sowed the Italian ryegrass at a rate of 300 seeds m-2 perpendicular to the direction wheat was planted. This selected seeding rate maximized varietal differences in weed suppressive ability in a pilot study conducted during the 2010-2011 and 2011-2012 growing seasons.
Approximately six to eight weeks after planting, we counted the number of Italian ryegrass seedlings that emerge in square meterquadrats in all plots in the weedy blocks to test whether allelopathic activity impacts weed seedling germination and establishment. We then tested whether allelopathic wheat cultivars suppressed Italian ryegrass growth and reproduction by counting the number of ryegrass seed heads m-2. Wheat grain yield (kg ha-1) was harvested in weedy and weed-free plots with a combine at maturity (GS 92) and adjusted to 14% moisture. Grain yield tolerance to weed interference was calculated as the percent reduction of wheat grain yield in weedy plots compared to weed-free plots in each block.Information on morphological traits was also recorded throughout the course of the growing season in the four weed-free control blocks (see study 2) to control for the effects of resource competition.
Statistical analyses were conducted using SAS 9.2 (SAS Institute Inc., Cary, NC). Plots of model-predicted values versus residual errors showed that all measurements of yield, weed suppressive ability, and potentially correlated wheat morphological traits met the assumption of normal error distribution. The combined experiment was evaluated in the MIXED procedure with genotype treated as a fixed effect and site, block nested within site, and the interaction of genotype and site treated as random effects. Pearson’s correlation coefficient was used to test the significance of correlations between the genotypic LS means for allelopathic potential and wheat morphological traits potentially affecting competitive ability with Italian ryegrass seed head density and wheat grain yield tolerance.
Study 2: Morphological Traits Associated with Superior Weed Suppressive Ability of Winter Wheat against Italian Ryegrass
Fifty-one entries from the 2012 North Carolina Official Variety Test (NC OVT) and two hard winter wheat cultivars developed by the USDA-ARS at North Carolina State University (Appalachian White and Nu East) were evaluated for weed suppressive over two years in the same field sites used in study 1 (Table 1). The test was organized as a randomized complete block design (RCBD) with three replications per site. Wheat was planted in 3-m-long plots using a calibrated cone drill with seven rows at 17.1 cm spacing with depth set at 2.5 cm. Gulf Italian ryegrass, a commercial turf cultivar, was then sown in a 1-m wide swath across the center of each plot using the same planter driving perpendicular to the direction in which wheat was planted with depth set at 1 to 5 mm. This experiment was planted alongside the parallel study focused on assessing the relative contributions of allelopathy and competitive traits to the weed suppressive ability of winter wheat lines (study 1).
Data on wheat morphological traits potentially correlated with weed suppressive ability were collected in the 1-m long weed-free area at the edges of each plot when Pioneer 26R12, a cultivar with intermediate heading date, reached early tillering (Zadoks growth stage 25; GS 25), advanced tillering (GS 29), stem extension (GS 31), heading (GS 55), and grain fill (GS 70 to 80) (Zadoks et al., 1974). The dates when measurements were made varied widely due to differences in growing conditions across sites. Consequently, not all genotypes had attained the exact same growth stage when measurements were collected. Such ‘snapshot’ comparisons of wheat lines at specific instances throughout the growing season were considered appropriate given that the primary objective of this experiment was to identify measurements or traits that breeders could use to indirectly select for lines with high weed suppressive ability in their weed-free nurseries.
Measurements of normalized difference vegetation index (NDVI) and visual ratings of early vigor were made during early and late tillering (GS 25 and 29). Readings of NDVI were taken using a Crop Circle ACS-210 Plant Canopy Reflectance Sensor (Holland Scientific, Inc., Lincoln,NE). Visual ratings of early vigor, based on a combination of percent ground cover and height, were made on a 1 to 9 scale with the most vigorous genotypes rated as 1 following Zhao et al. (2006). Growth habit was also rated on a 1 to 9 scale during late tillering (GS 29) with the most erect genotypes rated as 1 and the most prostrate genotypes rated as 9. Leaf area index (LAI) was estimated during stem extension (GS 31) and heading (GS 55) using an LAI-2000 sensor (LI-COR Environmental, Lincoln, NE) in overcast conditions. Plant vigor was also visually estimated on a 1 to 9 scale during heading (GS 55), with the fullest canopies rated as 1 and the sparsest canopies rated as 9. The heading date of each experimental entry was measured in single 1.2-m row plots at Lake Wheeler Road Field Laboratory in Raleigh, NC during each growing season.
Plant height was estimated as the distance from ground level to the top of the canopy before heading (GS 29, 31). During and after heading, plant height was estimated as the distance from ground level to the tip of the average head, excluding awns (GS 55, 70 to 80). An area under height progress curve (AUHPC) index was created to describe height accumulation over the course of the growing season modeled after the area under disease progress curve (AUDPC) developed by Shaner and Finney (1977):
Where Hi = height at the ith observation, Xi = time (days) at the ith observation, and n = the total number of observations. All genotypes were assumed to have equal height on January 1 (DOY 0) in each site. The date of the final height score for all sites was set to May 17 (DOY 138), the date when all genotypes had reached final height in Tidewater 2013. Weed suppressive ability was measured by counts of Italian ryegrass seed heads in a 0.5 m2 quadrat placed in the center of the weedy area in each plot during grain fill (GS 70-80).
Statistical analyses were conducted using ASREML (VSN International LTD., Hemel Hempstead, UK) and SAS 9.2 (SAS Institute Inc., Cary, NC). The combined experiment was evaluated in ASREML with genotype treated as a fixed effect; site, block nested within site, and the interaction of genotype and site treated as random effects; and spatially correlated error structure included for each location as determined in the preliminary analysis. Mean separation was performed using Fisher’s protected LSD (P ≤ 0.05). Pearson’s correlation coefficient was used to test the significance of correlations between LS means of wheat morphological traits potential affecting weed suppression and Italian ryegrass heads m-2 in SAS 9.2 (SAS Institute Inc., Cary, NC). Stepwise multiple linear regression was conducted to identify wheat morphological traits that strongly influenced weed suppression.
Publications based on results from studies 1 and 2 are currently ‘in press’ at Crop Science. I have attached the accepted versions of each paper. All relevant statistical charts and tables are included in the attached manuscripts. I will be happy to provide Microsoft Word versions of the manuscripts as well as tables and figures if so desired.
Growing conditions were similar in three of the four study sites, but one site (Tidewater 2013) was planted late and had particularly cool winter and early spring conditions. Thus, results from Tidewater 2013 were excluded from the pooled analysis and presented separately.
Study 1: Relative Contributions of Allelopathy and Competitive Traits to the Weed Suppressive Ability of Winter Wheat Lines against Italian Ryegrass
The highly allelopathic lines chosen for field evaluation were Coker 9553, Pioneer 25R32, SS 560, and Oakes. The low allelopathy lines chosen for field evaluation were NC05-19896, Pioneer 26R22, NC-Neuse, and Pioneer 26R12.
Weed suppressive ability and grain yield tolerance
There were no significant genotypic differences in initial Italian ryegrass seedling density (GS 29) found in the pooled sites or Tidewater 2013, indicating that the tested lines did not differ in their ability to suppress Italian ryegrass germination or establishment. Significant genotypic differences in Italian ryegrass seed head density (GS 70) were observed in the pooled sites, but not Tidewater 2013. Growth and tillering of Italian ryegrass was more extensive in Tidewater 2013 than the pooled sites. Least square means of Italian ryegrass seed heads m-2 were 303 in the pooled sites and 509 in Tidewater 2013. Thus, it is possible that genotypic difference in weed suppressive ability were obscured under very heavy weed interference in Tidewater 2013.
Wheat grain yields adjusted to 14% moisture in weedy and weed-free conditions were much lower in Tidewater 2013 than the pooled sites; average yields in weed-free and weedy conditions were 6190 and 3690 kg ha-1 in the pooled sites and 3400 and 1540 kg ha-1 in Tidewater 2013. The low grain yields recorded at the Tidewater site can be partially attributed to its late planting date and cool temperatures, which contributed to poor tiller development and reduced wheat biomass accumulation compared to other sites. The interaction between genotype and weed treatment (weedy vs. weed-free) for grain yield was significant in both the pooled sites and Tidewater 2013. Genotypic differences in wheat grain yield under weedy conditions were observed in both the pooled sites and Tidewater 2013. However, under weed-free conditions no differences in grain yield were detected in the pooled sites.
Wheat grain yield was lower under weedy conditions than weed-free conditions in both the pooled sites and Tidewater 2013. Significant differences in grain yield tolerance to weed pressure among genotypes were found in both analyses. However, genotypes yielded inconsistently across sites; the grain yield tolerance of genotypes in the pooled sites and Tidewater 2013 was not correlated (r = -0.22). While Pioneer 25R32 was the least tolerant genotype in the pooled sites, SS 560 was the least tolerant genotype in Tidewater 2013. Such genotype by environment interactions are common in studies of weed suppressive ability and tolerance (Coleman et al., 2001; Mokhtari et al., 2002); therefore, field screenings for weed suppressive ability should be conducted in multiple growing environments and years. Grain yield tolerance was correlated (r = 0.81) with weed suppressive ability in the pooled sites, indicating that weed suppressive winter wheat genotypes will likely also be tolerant of weed interference in North Carolina.
Allelopathy and Weed Suppressive Ability
Although significant variation in allelopathic activity was found among the 58 genotypes tested in this study, allelopathic activity measured using the ECAM bioassay was not positively correlated with wheat grain yield tolerance or Italian ryegrass seed head density. Strongly allelopathic lines did not have better weed suppressive ability than weakly allelopathic lines in the pooled sites or Tidewater 2013. These findings are inconsistent with the important role of allelopathy in weed suppression trials conducted with spring and winter wheat lines in Sweden (Bertholdsson, 2005, 2010, 2011).
The average root length suppression of Italian ryegrass seedlings grown with the wheat lines tested in this study ranged from 12 to 63%. In a more extensive bioassay of 453 wheat cultivars from around the world, Wu et al. (2000a) found a normal distribution of allelopathic activity across the tested genotypes, with the average root length suppression of rigid ryegrass (Lolium rigidum Guadin) seedings grown with wheat lines ranging from 10 to 91%. Thus, it is possible that none of the lines screened in this study had allelopathic potential on the highest end of the spectrum. However, Wu et al. (2000a) used a different species as a receiver, and it is plausible that rigid ryegrass is more or less responsive to wheat seedling allelopathy than Italian ryegrass.
Soil or climate conditions in North Carolina may also have affected the expression of allelopathy in this environment. Bertholdsson (2010) found that highly allelopathic spring wheat lines derived from a cross between allelopathic and non-allelopathic parents suppressed weed biomass 24% more than the non-allelopathic parent in a dry year and only 12% more in a wet year. Dilday et al. (1998) also found that year to year variation, soil type, weed density, crop density, and root density all affected the expression of allelopathic activity in rice lines tested in the field.
Competitive Traits and Weed Suppressive Ability
While allelopathic activity was not associated with weed suppressive ability in this study, wheat morphological traits commonly associated with competitive ability were positively correlated with weed suppressive ability and grain yield tolerance. Several wheat morphological traits including early vigor and erect growth habit during tillering (GS 29), high leaf area index (LAI) at stem extension (GS 31), and plant height at tillering and stem extension (GS 29, 31) were correlated with Italian ryegrass seed head density and grain yield tolerance in the pooled sites. Meanwhile, grain yield tolerance at Tidewater 2013 was only correlated with vigor at heading (GS 55) and final plant height (GS 70-80).
Some wheat morphological traits that have been implicated in weed suppressive ability in other studies were not associated with improved weed suppression in North Carolina. Though prostrate growth habit was correlated with high weed suppressive ability in spring wheat grown in Saskatchewan and Australia (Huel and Hucl, 1996; Lemerle et al., 1996), erect growth habit during tillering was strongly associated with weed suppressive ability in this study. Final cultivar height was also an important determinant of competitive ability in many studies (Huel and Hucl, 1996; Lemerle et al., 1996; Coleman et al., 2001; Vandeleur and Gill, 2004; Mason et al., 2007; Murphy et al., 2008) and was associated with weed suppressive ability in a preliminary study conducted in North Carolina (Worthington et al., 2013). However, final height was only associated with grain yield tolerance at Tidewater 2013. The competitive advantage gained by rapid early growth from tillering to stem extension (GS 29-55) was far more important than final cultivar height in determining weed suppressive ability in the pooled sites.
Researchers have suggested that elite allelopathic wheat lines from exotic sources could be crossed with locally adapted lines to achieve gains in weed suppressive ability (Belz, 2007; Bertholdsson, 2010). Bertholdsson (2010) crossed Mohan 73, a highly allelopathic Tunisian cultivar, to an adapted but weakly allelopathic Swedish cultivar and evaluated the agronomic performance and weed suppressive ability of highly and weakly allelopathic F2:3 lines derived from the cross. Although early weed biomass was significantly lower in the highly allelopathic lines, the highly allelopathic lines were also significantly lower yielding (Bertholdsson, 2010). This yield loss was likely a result of linkage drag from the poorly adapted allelopathic parent used in the cross. Allelopathy is controlled by the action of multiple small effect QTLs (Niemeyer and Jerez, 1997; Wu et al., 2003), so it is unlikely that allelopathy could be recovered without some loss of adaptation in wide crosses.
Significant variation in weed suppressive ability was found among the small group of lines tested in this study in the pooled sites and variation in grain yield tolerance was found in all sites. The lack of correlation between the grain yield tolerance of genotypes in Tidewater 2013 and the pooled sites and lack of significant genotypic differences in weed suppressive ability observed in Tidewater 2013 indicate that selection for weed suppressive ability may not be equally efficient in all environments and that the performance genotypes identified as highly weed suppressive may be affected by planting date and environmental conditions. Still, wheat breeders in the southeastern United States should be able to improve weed suppressive ability and grain yield tolerance within locally adapted material by selecting for yield in weed-free environments as well as competitive traits including early vigor (GS 29), erect growth habit (GS 29), and height at tillering, stem extension, and heading (GS 29, 31, 55). The effects of competitive morphological traits on weed suppressive ability will likely vary from year to year but yields should not be compromised by selection for improved competitive ability. Researchers have suggested that breeders should strive to improve allelopathic and competitive ability simultaneously to achieve maximum weed suppression (Lemerle et al., 2001; Olofsdotter et al., 2002; Belz, 2007). However, the results of this study suggest that wheat breeders in the southeastern United States would make greater gains in weed suppressive ability against Italian ryegrass and grain yield tolerance by focusing their time and resources on improving competitive traits within adapted germplasm.
Study 2: Morphological Traits Associated with Superior Weed SuppressiveAbility of Winter Wheat against Italian Ryegrass
A large amount of variation in weed suppressive ability was observed among elite adapted germplasm from the southeastern US. Differences in end of season Italian ryegrass seed heads m-2 (P ≤ 0.05) were detected among the wheat genotypes tested in both the pooled sites and Tidewater 2013. Least square means of Italian ryegrass seed heads m-2 ranged from 269 to 483 in the pooled analysis and 381 to 634 in Tidewater 2013. Despite a non-significant Pearson correlation between the weed suppressive ability rankings of genotypes in the pooled sites and Tidewater 2013 (r = 0.09), several lines performed consistently in all environments. Five lines (Dyna-Gro Baldwin, NC-Cape Fear, Featherstone VA258, SY 9978, and USG 3120) performed as well as the most weed suppressive line in both analyses. All four lines that performed as poorly as the least suppressive line in the pooled analysis (AGS 2056, Appalachian White, SS 8700, and USG 3438) were also among the least weed suppressive group in Tidewater 2013.
The precision of genotypic weed suppressive ability estimates was greater in the pooled sites than Tidewater 2013, as evidenced by a smaller genotype LSD and higher F statistic for genotype. The late planting date and cool early spring conditions at Tidewater 2013 influenced wheat development and delayed the onset of stem extension (GS 31) by 36 days compared to the next latest site. While these growing conditions set Tidewater 2013 apart from other study sites, growers in the Tidewater region of North Carolina often plant wheat in mid-November if wet conditions postpone the harvest of spring-sown crops and delay field preparation. These findings suggest that selection for weed suppressive ability may not be equally efficient in all environments and that weed suppressive genotypes may be affected by planting date and other environmental conditions.
Morphological Traits Associated with Weed Suppressive Ability
Reduced Italian ryegrass seed head density was correlated (P ≤ 0.05) with high vigor during tillering (GS 25, 29) and heading (GS 55), erect growth habit (GS 29), low NDVI (GS 29), high LAI at stem extension (GS 31), early heading date, tall height throughout the growing season (GS 29, 31, 55, 70 to 80), and high AUHPC in the pooled analysis of Caswell 2012, Piedmont 2012, and Caswell 2013. In contrast, only early vigor and high NDVI during tillering (GS 25) were correlated (P ≤ 0.05) with weed suppressive ability in Tidewater 2013.
End of season height was associated with competitive ability of wheat lines in many studies (Huel and Hucl, 1996; Lemerle et al., 1996; Coleman et al., 2001; Vandeleur and Gill, 2004; Mason et al., 2007; Murphy et al., 2008). However, while final height was correlated with weed suppressive ability in the pooled sites, correlations between height and weed suppressive ability were much stronger earlier in the growing season. The competitive advantage gained by rapid early growth and the accumulation of height throughout the season was far more important than final cultivar height in determining weed suppression (Ogg and Seefeldt, 1999). A plot of AUPHC constructed with the mean height of the genotypes that performed as well as the most weed suppressive line or as poorly as the least weed suppressive line shows that height accumulated over the course of the growing season influenced competitive ability against weeds in the pooled sites.
A recent review of breeding for improved weed suppression in organically grown cereals stated that crop ground cover was the most influential characteristic affecting weed suppressive ability (Hoad et al., 2012). Prostrate growth habit was correlated with high weed suppressive ability of spring wheat in at least two studies (Huel and Hucl, 1996; Lemerle et al., 1996). However, erect growth habit at tillering (GS 29) was strongly associated with weed suppressive ability in the pooled sites in this study. Erect growth habit during tillering was also correlated with improved weed suppression in a study of winter wheat in the UK (Korres and Froud-Williams, 2002).
Early vigor ratings during tillering (GS 25, 29) were also correlated with improved weed suppressive ability. Many other studies have also found that early vigor was correlated with weed suppressive ability in wheat (Huel and Hucl, 1996; Lemerle et al., 1996; Mason et al., 2007). Interestingly, high NDVI during early tillering (GS 25) was associated with reduced Italian ryegrass seed head density in Tidewater 2013, while high NDVI during late tillering (GS 29) was associated with increased Italian ryegrass seed head density in the pooled analysis of other sites. NDVI was correlated (P ≤ 0.05, data not shown) with prostrate growth habit at GS 29 in the pooled sites, possibly confounding results.
Early heading (Huel and Hucl, 1996) and maturing (Mason et al., 2007) lines were associated with high weed suppressive ability in wheat in some studies, while other studies found no significant association between maturity and competitive ability (Bertholdsson, 2005) or found that later maturing lines were better weed suppressors (Coleman et al., 2001; Wicks et al., 2004). Early heading date was correlated with reduced Italian ryegrass seed head density in the analysis of pooled sites. However, early heading date is associated with increased susceptibility to late spring freeze (Worland, 1996), and is not considered a desirable breeding trait. Competitive traits such as tall height, erect growth habit, and vigor during tillering (GS 29) were correlated (P ≤ 0.05) with early heading date in the pooled sites. However, several vigorous, erect lines (DG 9053, DG Baldwin, and SY 9978) had later than average heading dates. Thus, it should be possible to breed for improved early vigor and erect growth habit without impacting local adaptation and increasing susceptibility to late spring freeze.
The wheat morphological traits measured in this study were included in multiple regression models to determine which characteristics most influenced weed suppression. Early vigor (GS 25) was the only variable chosen as influential in weed suppression in Tidewater 2013, explaining just 23% of variation in weed suppressive ability. Vigor during tillering (GS 29), height at heading (GS 55), and NDVI during tillering (GS 29) were selected as the most influential traits affecting Italian ryegrass seed head density in the pooled sites, together accounting for 77% of observed variation in the weed suppressive ability of genotypes . Many of the morphological traits associated with competitive ability were also correlated with one another, so some model terms could be substituted without losing much of goodness of fit. Six models involving combinations of vigor, growth habit, or height at tillering (GS 29) with either height at heading (GS 55) or final height (GS 70 to 80) explained at least 70% of the observed variation in the weed suppressive ability of genotypes in the pooled sites .
Researchers have suggested that weed suppressive ability may be negatively correlated with grain yield under weed free conditions (Donald and Hamblin, 1976; Olofsdotter et al., 2002). However, many of the weed suppressive lines identified in this experiment yielded competitively in separate trials conducted in conventional and organic conditions in North Carolina. Featherstone VA258 and USG 3120 were both ranked in the top 10% of lines screened in the NC OVT in 2012 and 2013 (http://www.ncovt.com/) and Featherstone VA258 had the highest two year rank of 20 lines screened in the 2011-2013 Organic Wheat OVT (RAFI-USA, 2013). Breeding for improved weed suppressive ability in North Carolina is, therefore, not expected to negatively impact grain yield.
The lack of correlation between the weed suppressive ability of genotypes in Tidewater 2013 and the other study sites and the weak association between morphological traits and weed suppression ability in Tidewater 2013 suggest that selection for weed suppressive ability may not be equally efficient in all environments. Furthermore, highly weed suppressive genotypes may not perform reliably if planting date is delayed or cool spring conditions inhibit plant development. Still, correlations between the weed suppressive ability of adapted genotypes and many wheat morphological traits in the pooled sites suggest that indirect selection for weed suppressive ability in weed-free environments is likely to be generally effective.
Multiple regression models in the pooled sites indicated that 59% of variation in the weed suppressive ability of tested genotypes was explained by either visual ratings of plant vigor or measurements of canopy height during tillering (GS 29), while 71% of variation in weed suppressive ability was accounted for when final genotype height (GS 70 to 80) was added to either model. Thus, breeders should select weed suppressive winter wheat lines in weed-free breeding nurseries by imposing a weighted selection index for genotypes that are either tall or vigorous during late tillering (GS 29) and tall at the end of the growing season (GS 70 to 80). Given their stronger correlation with weed suppressive ability, genotype height or vigor rating at tillering (GS 29) should be given more weight than final height in the selection index. Breeders should also discard lines that reach heading (GS 55) before early checks in their breeding nurseries to ensure that selected lines are well-adapted and not susceptible to late spring freeze. Final genotype height and heading date are routinely measured in winter wheat breeding programs (NC OVT 2012). Thus, the proposed selection index would require only one additional phenotyping step (either a visual rating of plant vigor or measurement of canopy height) by the breeder when average-heading lines have reached late tillering (GS 29) in the early spring.
Educational & Outreach Activities
The results of this project have been published in a doctoral dissertation and four peer reviewed publications. We have also presented our findings to other breeders and extension workers at meetings of the National Association of Plant Breeders, the Crop Science Society of America, and the Eastern Wheat Workers and Southern Small Grains Workers. The research also has been directly presented to growers at the NCSU CEFS field day and the North Carolina Joint Commodities Conference. Furthermore, we have communicated our results to Michael Sligh and Kelli Dale of the Breeding for Organic Production Systems project at RAFI, who coordinate on farm trials with organic producers across NC.
- Worthington, M. 2014. Breeding Winter Wheat for Improved Powdery Mildew Resistance and Weed Suppressive Ability against Italian Ryegrass. Doctoral dissertation. NCSU Dept. of Crop Science.
Peer reviewed publications:
- Worthington, M., S.C. Reberg-Horton, G. Brown-Guedira, D. Jordan, R. Weisz, and J. P. Murphy. 2015. Morphological Traits Associated with Superior Weed Suppressive Ability of Winter Wheat against Italian Ryegrass. Crop Science. In press doi:10.2135/cropsci2014.02.0149
- Worthington, M., S.C. Reberg-Horton, G. Brown-Guedira, D. Jordan, R. Weisz, and J. P. Murphy. 2015. Relative Contributions of Allelopathy and Competitive Traits to the Weed Suppressive Ability of Winter Wheat Lines against Italian Ryegrass. Crop Science. In press doi:10.2135/cropsci2014.02.0150
- Worthington, M. and S.C. Reberg-Horton. 2013. Breeding Cereal Crops for Enhanced Weed Suppression: Optimizing Allelopathy and Competitive Ability. Journal of Chemical Ecology. 39:213-231. doi:10.1007/s10886-013-0247-6
- Worthington, M., S.C. Reberg-Horton, D. Jordan, and J.P. Murphy. 2013. A Comparison of Methods for Evaluating the Suppressive Ability of Winter Wheat Cultivars against Italian Ryegrass (Lolium perenne). Weed Science. 61:491-499. doi:10.1614/WS-D-12-00167.1
Presentations at field days and meetings:
- Breeding winter wheat for increased weed suppressive ability against Italian ryegrass. Worthington M., C. Reberg-Horton, D. Jordan, and P. Murphy. Poster at the 2014 North Carolina Joint Commodities Conference.
- Breeding winter wheat for increased weed suppressive ability against Italian ryegrass. Worthington M., C. Reberg-Horton, D. Jordan, and P. Murphy. Oral presentation at the 2013 Annual Meetings of the Crop Science Society of America.
- Breeding winter wheat for increased weed suppressive ability against Italian ryegrass. Worthington M., C. Reberg-Horton, D. Jordan, and P. Murphy. Poster at the 2013 North Carolina State University Center for Environmental Farming Systems Organic Grain Field Day.
- Breeding winter wheat for increased weed suppressive ability against Italian ryegrass. Worthington M., C. Reberg-Horton, G. Brown-Guedira, D. Jordan, and P. Murphy. Oral presentation at the 2013 joint meeting of the Eastern Wheat Workers, Southern Small Grains Workers, and Great Lakes Wheat Workers.
- Breeding winter wheat for increased suppressive ability against Italian ryegrass. Worthington M., C. Reberg-Horton, D. Jordan, and P. Murphy. Poster at the 2012 Annual Meetings of the Crop Science Society of America.
- Breeding winter wheat for increased suppressive ability against Italian ryegrass. Worthington M., C. Reberg-Horton, D. Jordan, and P. Murphy. Poster at the 2012 Annual Meeting of the National Association of Plant Breeders.
- Breeding winter wheat for increased suppressive ability against Italian ryegrass. Worthington M., C. Reberg-Horton, D. Jordan, and P. Murphy. Poster at the 2012 NCSU Center for Environmental Farming Systems 2012 Field Day.
- . A Comparison of Methods for Evaluating the Suppressive Ability of Winter Wheat Cultivars against Italian Ryegrass (Lolium perenne)
- Relative Contributions of Allelopathy and Competitive Traits to the Weed Suppressive Ability of Winter Wheat Lines against Italian Ryegrass
- Morphological Traits Associated with Superior Weed Suppressive Ability of Winter Wheat against Italian Ryegrass
- Breeding Cereal Crops for Enhanced Weed Suppression: Optimizing Allelopathy and Competitive Ability
The most important short term benefit of our project is the identification of commercially available varieties adapted to winter wheat production in the Southeast that compete effectively against Italian ryegrass based on their performance in replicated field trials. Dyna-Gro Baldwin, NC-Cape Fear, Featherstone VA258, Syngenta 9978, and USG 3120 all yield competitively in conventional environments and have superior weed suppressive ability. These cultivars are good options for organic producers and conventional growers struggling with herbicide resistant weed populations.
The long-term benefit of our research is the development of breeding methods suitable for improving the weed suppressive ability of winter wheat cultivars in the southeastern US. We have identified several easily measured morphological traits that confer weed suppressive ability to winter wheat. We have also proposed a simple selection index (visual rating of vigor during tillering and final height) that breeders can use to pre-select high performing lines with good weed suppressive potential for inclusion in on farm trials and the organic OVT. Because this selection index is so simple and cost-effective, it is likely to be adopted by other breeders in the region.
We have reached dozens of farmers with extension presentations at CEFS field days and the NC Joint Commodities Meeting. However, the majority of farmer outreach activities are yet to come. Chris Reberg-Horton (NCSU Organic Cropping Systems Specialist) will be testing weed suppressive cultivars in organically managed official variety tests while Michael Sleigh and Kelli Dale from RAFI will be conducting on farm trials with organic growers and communicating our results directly to organic wheat producers in NC. We suggest that organic producers struggling with Italian ryegrass infestations should combine competitive cultivars (e.g. Featherstone VA 258, USG 3120, Dyna-gro Baldwin) with cultural practices to reduce weed pressure (e.g. narrow row spacing, high seeding rate, early-medium planting date, good fertility). These practices will also be useful to conventional growers with herbicide resistant Italian ryegrass populations.
In addition to farmer adoption, we are also interested in encouraging “breeder adoption.” The development of simple, low-cost breeding methodologies that public sector wheat breeders across the US can use to select weed suppressive lines in their nurseries was a major aim of this study. We have presented our results to most breeders in the region at the Eastern Wheat Workers annual meeting and in field days of SUN grains cooperative breeders. Paul Murphy will continue to communicate with other breeders in the region and encourage them to submit high yielding advanced lines with weed suppressive ideotypes (well-adapted, vigorous at tillering and stem extension, with tall final height) to the NC Organic OVT.
Areas needing additional study
While some researchers hoping to screen cultivars or breeding lines for weed suppressive ability have used natural weed populations, we chose to over-seed plots with a selected weed species. Although natural weed populations may be more relevant to conditions in farmers’ fields and provide evidence of suppressive activity against several important weed species, it is difficult to achieve uniform weed densities and therefore to obtain good estimates of weed suppressive ability. Even in experiments where weeds are directly seeded, obtaining uniform weed densities is sometimes problematic. However, now that we have identified material with superior weed suppressive ability against a commercial cultivar of Italian ryegrass, we need to test the performance of these cultivars against natural populations of Italian ryegrass and in on-farm trials.
Marker assisted selection for weed suppressive ability may also be a useful tool. Various researchers have suggested that dwarfing, vernalization, and photoperiod genes may play a role in weed suppression, but few studies have investigated this issue. Further research is needed to determine the importance of these genes and other possible quantitative trait loci (QTL) to weed suppressive ability in different growing environments.
A new graduate student is currently conducting two follow-up studies to determine the suppressive ability of competitive cultivars identified in this project against natural weed populations and to map QTL for weed suppressive ability in a population derived from a cross between a weed suppressive and non-weed suppressive parent based on the results of this study.