Effects of management practices and plant growth regulators on the allelopathic potential of rye (Secale cereale)
One of the benefits of cover crops is weed suppression, but the focused management of cover crops for weed suppression is not often considered. A comparison of winter rye cover crop management variables demonstrated that variety, Spring mowing, and nitrogen after Spring mowing can have an impact on the status of the plant tissue at the time of main crop planting. Cover crop management effected growth stage; biomass allocation between stems, leaves and flowers; and allelochemical levels. These changes could be utilized to improve weed suppression and/or enhance the economic incentive for a grower to utilize cover crops.
The purpose of this project is to study the allelopathic potential of rye (Secale cereale L.) and develop a better understanding of allelochemical biosynthesis within the plant. Cover crops serve a number of purposes including improving soil organic matter, preventing soil erosion, and weed control, among others. Weed control in particular involves both living cover crops and mulches left on the soil surface. This weed control is facilitated by physical suppression, competition, and allelopathy. Allelopathy is the addition of phytotoxic compounds into the soil by an organism, so that the growth of weeds in the close vicinity is reduced. In rye, allelopathy is mainly attributed to the Benzoxazinones (BX), in particular 2,4-dihydroxy-(2H)-1,4-benzoxazin-3(4H)-one (DIBOA) and its breakdown product benzoxazolin-2(3H)-one (BOA) (Barnes et al. 1987). An allelopathic cover crop, such as rye, can significantly contribute to weed control in an agricultural field (Barnes & Putnam 1983).
The weed control provided by rye cover crops varies significantly from year to year (Nagabhushana et al. 1997). These variations in allelochemical production are potentially due to adverse environmental conditions in the field, such as high temperature or low moisture levels. Differential genetics can also determine different plants’ abilities to respond to those unfavorable environmental conditions and might influence their allelopathic potential.
Variations between cultivars are a major source of allelopathic variability; among eight different cultivars a tenfold difference in BX concentrations has been shown (Burgos et al. 1999). Environmental variability also influences BX concentrations and allelopathy, for example plants with low to moderate fertility had higher concentrations than plants with high fertility (Mwaja et al. 1995). In addition, different cultivars vary in their BX concentrations as the plants age, with levels declining concurrently with development of the plants (Reberg-Horton et al. 2005, Rice et al. 2005). Hydroxamic acids are not present in the seed, but rather increase upon germination, peak while the plant is still a young seedling, and subsequently decrease as the plant grows in size (Argandoña et al. 1981).
BX biosynthesis and availability change as rye plants develop (Reberg-Horton et al. 2005, Rice et al. 2005), and BX concentrations are directly related to the phytotoxic ability of the rye plants (Belz & Hurle 2005). However in a field environment, a plant’s allelopathic potential for weed suppression is dependent on both allelochemical content of the plant tissue and the total biomass of the plant (Reberg-Horton et al. 2005). Younger plants have a greater BX concentration and thus have greater phytotoxicity. However, younger plants also have less biomass and therefore provide less physical suppression, and a lower total allelochemical dose (kg/ha). Uncertainty exists whether biomass actually affects the weed suppressive characteristics of rye. Wickliffe (1997) found that increased rye biomass did not, or rarely resulted in increased weed suppression, whereas other groups have found correlations between biomass and weed growth suppression (Creamer et al. 1996). Studying the balance between allelopathy and physical suppression is crucial in determining what constitutes an ideal cover crop management plan that maximizes weed control.
So now the question is how do we achieve this? Can we extend the length of time that a rye plant can synthesize BX, or the length of time the BX are available for weed suppression? At what age are different rye varieties most effective at weed control, and is it possible to influence their allelopathic potential through cultural practices? One promising method we are exploring is the development of new rye cultivars with increased allelopathic potential through breeding programs. Other possibilities employ the use of cultural methods such as specific fertilization regimes, use of plant growth regulators, or mowing to increase weed control.
Improving weed control might be achieved by altering various cultural practices. A number of different questions still need to be answered concerning the role of cover corp management on allelopathic potential and the resulting weed control. What effect would changing the planting times have on allelopathic potential? If younger tissue has the greatest weed suppressive ability, would increasing the sowing rate enhance weed control? Re-seeding different varieties is another option that could affect control, planting one variety in the Fall and re-seeding with another variety in the Spring. Perhaps mowing the rye at specific times during the growing season will increase BX concentrations within shoot regrowth. What are the best mowing times, and what varieties are most influenced by mowing? And when is the best time to harvest for mulch? These questions need to be answered so farmers can put different management plans into practice to enhance weed control in their fields depending on their particular situation.
Another possible cultural practice to increase weed suppression is the use of plant growth regulators. Would spraying a plant growth regulator, such as Ethephon, on the rye affect allelopathic compounds in the plant? Previous work in rye has illustrated the potential for induction of BX biosynthesis using the plant defense hormone, jasmonic acid (Slesak et al. 2001). Another hydroxamic acid, HDMBOA-Glucoside, is induced in maize treated with jasmonic acid (Oikawa et al. 2001), as well as by attack by fungal pathogens and insects (Oikawa et al. 2004). Specific cultural methods such as mowing might mimic wounding, and induce defense signal production, and BX synthesis. An understanding of the role plant growth regulators in BX synthesis might potentially allow for the manipulation of the signaling system to increase the level and duration of BX production and improve rye’s ability to control weeds.
Our ultimate goal with this project is to develop a viable management plan for farmers to provide maximal weed control in their cropping system. We want to examine several different cultural methods such as variable planting dates, mowing, and fertility to see if we can influence BX availability and allelopathic potential of rye.
Field cover crop management treatments will include different Fall planting dates and Spring mowing dates. The impact of N fertilization at mowing will also be considered. The research will also determine if Spring planting of different varieties, particularly late maturing varieties, will affect allelochemical potential of the cover crop mulch. The allelochemical content (BX), growth stage (Feekes) and biomass allocation (dry wt of total tissue per stem and dry wt of leaf and flower tissue per stem).
Materials and Methods
The potential cover crop management options that might alter the allelochemical levels and weed suppressive abilities will be studied in the field. Experimental design for the field trials was a randomized block with three replications. Winter rye varieties used included both facultative (minimal to no vernalization for flowering) and Winter (vernalization required) types. Rye varieties were Wrens Abruzzi (facultative), Wheeler, Spooner, Rymin, and Hancock (Winter types). Wrens Abruzzi was developed in GA for superior performance in the SE piedmont and coastal plains. Wheeler is a tetraploid developed in MI, a cross between a Gator cereal rye from Florida and a perennial oriental rye. Rymin was developed in MN, and Spooner and Hancock were developed and released in Wisconsin. Because of high rainfall and excessive soil moisture, rye was planted December 1, considered a late planting for NC. The seed was no-till drilled into a pasture at 5 cm row spacing, and 50 kg/ha seeding rate. An initial fertilization of 70 kg/ha N was applied. In the Spring, two separate mowings were conducted on 4/6 and 4/16. Within each mowed strip, one half received a 30 kg/ha N application and the other was not fertilized.
The allelochemical content in the un-mowed rye tissue was sampled at four dates in the Spring: 4/04, 4/12, 4/22, and 5/08. The growth stage (Feekes scale) of the different varieties was recorded. For this harvest, a 0.5 m2 quadrant within each replicate was collected by cutting the plants just above the soil surface. The tissue was placed in paper bags and dried at 60 C. A representative sub-sample of the tissue was later ground in a mill using a 1 mm screen. Because of the poor stand this year, the regrowth of the rye plants was sampled differently. Tissue was harvested on 5/12 by collecting 40 individual stems of representative plants from each treatment. The tissue was taken to the lab and separated into stem, leaf, and flower tissue. Each fraction was dried at 60 C and the dry weight recorded.
The allelochemical (BX) content of the tissue was analyzed utilizing a GC method, modified from the protocol of Finney et al. (2005). Dried rye shoot tissue was weighed (0.5 g) and transferred into empty 75 ml SPE (Alltech) reservoir fitted with bottom frits (20 m porosity). After adding 10 ml of 50% ethanol solution the reservoir was mixed with a vortexer for 1 min, the extract was then filtered through the reservoir by attaching it to vacuum manifold (VWR Scientific). An additional 5 ml of deionized (DI) water was filtered through the extracted tissue and the washed tissue was allowed to air dry under vacuum. The filtrate was transferred to a 25 ml volumetric flask, and brought to volume with DI water. A 10 ml aliquot of the filtrate was partitioned three times with equal volumes (5 ml) of EtOAc. The EtOAc layers were combined and dried overnight with sodium sulfate.
A 10 ml aliquot of dried EtOAc and 750 μl of 200 μg/ml octadecanol internal standard dissolved in toluene were reduced to dryness under a N2 stream at 40°C. 400 μl of 1:1 MSTFA–DMF were added to the sample/internal standard residue, the samples were capped under N2, and then heated at 75°C for 30 min. Samples were then transferred to auto-sampler vials for GC analysis.
Gas chromatographic analysis GC analysis was used to resolve and quantify compounds in the plant extract. An Agilent 6890N gas chromatograph equipped with a 20m DB-5 megabore column (0.53 mm diameter, 1.5 μm film thickness, J&W Scientific) was used for analysis. 0.5 L of the derivatized sample was analyzed using splitless injection and an injector temperature of 250°C. Flame ionization detection was utilized with a detector temperature of 325°C. Column flow of the helium (UHP) carrier gas was set to a linear gas velocity of 43.0 cm/s. The initial oven temperature was 100°C, followed by a 5°/min temperature increase to 300°C final temperature. The oven temperature increase began upon injection and the final temperature was maintained for 30 min. Data were collected using the Perkin-Elmer TotalChrom 6.2 system. Quantitation of peaks was done using the internal standard method. Appropriate multi-level calibration curves were run for BOA, DIBOA, and DIBOA-Glu.
Results and Discussion
A study was initiated to determine the impact of cover crop management choices on specific plant factors that play a role in the allelopathic suppression of weeds. Field research was conducted to study at the effect of Spring mowing and N fertilization on five rye varieties (Wrens Abruzzi, Wheeler, Spooner, Rymin, and Hancock).
Because of a wet Fall, the rye was planted late for NC (Dec 1). In addition, the rye was no-till drilled into a pasture (treated with roundup prior to planting) because excessive soil moisture prevented planting in a prepared field. The late planting and cool soil from the pasture cover resulted in a poor stand and slow growth of the rye in the Spring. However, 3 of the 6 reps that were initially planted had sufficient growth to conduct the studies (Figure 1). Given the later planting and slow growth, extrapolating the results of this study should be done conservatively.
Though the growth was not optimal, the experimental results demonstrate the potential of management options to alter the morphological profile of a cover crop. These changes may also offer a grower options for the weed management in the main crop. The image in Figure 1 demonstrates that after a Spring mowing, there can be substantial regrowth of rye. The image also visually demonstrates the importance of N for the regrowth. The left side of the image was regrowth without added N, and the center section shows growth with added N. The addition of N after mowing allowed for greater growth and there was a greater proportion of the biomass that was directed to the leaves, seen visually in Figure 1.
The partitioning of biomass in the regrowth of rye is quantified in Figure 2. The dry weights of the stems, leaves, and flowers were determined for a defined number of stems (g dry wt/stem). Then, the ratios of the +N to -N for each rep were determined, and the results are presented in Figure 2. N addition after mowing had minimal to no impact on the total dry wt/stem, stem wt/stem, or the flower wt/stem. However, the leaf wt per stem increased up to 2 fold for Hancock and Wheeler, 1.5 fold for Spooner and Abruzzi. The +N/-N ratio for Rymin did not increase.
Not surprisingly, these data indicated that N is important for regrowth after mowing. The -N treatments were likely limited for N, thus the regrowth was limited, and the regrowth in the +N treatments demonstrates the biomass allocated to the leaves when N is not limited.
These data also suggest that the biomass allocation within the rye plant may be altered (i.e. higher relative leaf tissue). This alteration may have an impact on either the weed suppressive capacity of the cover crop mulch, or the ability to manage the mulch. A thick rye mulch may provide better physical weed suppression, but may be difficult for the grower to manage, to plant the main crop into. A thick mulch may also keep the soil temperature low, which may slow the growth of the main crop, or make it more susceptible to pathogens or disease. Having the option to manage the cover crop biomass could be advantageous under different circumstances.
Spring mowing and N also had an affect on the maturity of the rye. Mowing delayed the maturity of all the rye varieties, and the addition of N after mowing would slightly delay maturity in some varieties or some mowing dates (Table 1). The Feekes stages for all the Uncut, untreated Winter types (Hancock, Spooner Rymin, and Wheeler) was in the 10.1 (ear emergence) to the 10.5 (ear clear of leaf) range (see figure below), and Abruzzi (facultative) was at a more mature state in the 10.5 to 10.51 (beginning of flowering) range. The earliest mowing (4/6) delayed maturity, but the later mowing (4/16) had the greatest impact on maturity. N fertilization slightly delayed the growth stage of Hancock and Spooner at the first mowing. Delay in maturity may have an important impact on the how the grower will need to kill the cover crop prior the planting the main crop. If a roller-crimper is to be used, the a delay in maturity may not be desirable, as rye does not ‘lay-down’ if it is rolled before a certain stage of maturity is achieved (Chris Reberg-Horton, unpublished results).
The BX levels in the non-mowed treatments were similar in the different rye varieties, but there were significant differences at the different harvest dates (Figure 3). The winter type rye each had similar BX levels, but the biggest difference was in Abruzzi at the April 22 harvest, which had much less BX. By the final harvest, all the varieties were similar. The differences in the BX levels at the different harvest dates suggest that timing can play an important role in the allelochemical levels in tissue. These data demonstrated that there was a greater level of BX in the tissue of the Winter types at the 4/22 harvest that at any other date (Figure 3). Abruzzi had the highest BX content at the 5/8 harvest. The choice of the variety and the timing of the cut has a significant impact on the allelochemical levels, and this may result in differences in weed management.
Spring mowing also had an impact on the BX levels in the tissue, and the effect depended on variety (Figure 4). The greatest difference was seen in the facultative type, Abruzzi, where the first mowing reduced the BX levels in the regrowth by about 50%, and the levels in the second mowing were reduced by about 30%. Abruzzi is the variety most adapted to the Southern US, and possibly the mowing late in the maturity affected its ability to regrow and the capacity for allelochemical biosynthesis (Figure 4). The Winter type rye varieties were relatively less effected by the Spring mowing, possibly because they were at a more juvenile stage of growth. BX levels in Hancock was the least affected by mowing, Wheeler and Rymin intermediate, and Spooner seemed the most affected. In general, N application after mowing did not have an effect on BX levels.
In addition to the field study, several preliminary experiments were carried out in the greenhouse, using the plant growth regulators BTH, Ethephon, and MeJA. These included a dose response experiment, the effect of the plant growth regulators over time, and the combination of mowing and chemical application. A slight decrease in both fresh and dry weights was observed with chemical application, although this decrease is most likely not statistically significant. Remaining work to be done includes GC analysis of the preliminary greenhouse experiments, as well as conducting additional plant growth regulator experiments on other rye varieties.
Impacts and Contributions/Outcomes
Impacts and Contributions/Outcomes
This research project examined the effect cover crop management choices, including variety, Spring mowing and N fertilization, on development and allelochemical levels in Fall planted Winter rye. The underlying concept for the research is that cover crops could be managed specifically to enhance weed suppression by designing systems that result in the highest levels of allelochemicals and biomass in the field at the time when the grower most needs them. Rye is a high biomass producer, but the allelochemical production seems to occur too early in the season for optimum and consistent weed suppression from the cover crop mulch. Thus, this research considered specific management choices that might shift the temporal profile of allelochemical levels that would favor weed suppression.
Results suggest that the choice of variety can have an important effect on the growth and development of the rye in the Spring. Varieties are commonly chosen for their adaptation and performance in the region, but this may not be the best choice for weed management in all situations. Later maturing varieties that are adapted to more Northern climates may be viable options for Southern growers because the peak of allelochemical content may be later in the season. Further research on the role of variety, allelochemical content, and weed suppression are needed.
Spring mowing with N fertilization (if N is limiting) may be useful management options to enhance the weed suppressive abilities of the rye cover crop mulch. It may be advantageous in specific cropping circumstances to have lower total mulch biomass into which the main crop will be planted. This may be important if tillage equipment is not sufficient to plant into a thick rye mulch. There may be an economic incentive to harvest a hay crop in the Spring for livestock, and still plant a main Summer crop. The data presented herein suggest that the cover crop may be managed differently, while maintaining many of the benefits, including the weed suppressive abilities.
This and further research may benefit growers by providing insight into management options that will result in enhanced weed suppression when using rye as a cover crop. Other management methods should be studied to determine their potential to increase the allelochemical content in rye, which will increase the cover crop’s ability to provide weed control.
Professor of Crop Science
North Carolina State University
PO Box 7620
Raleigh, NC 27695-7620
Office Phone: 9195153567
Associate Professor of Horticultural Science
North Carolina State University
232 Kilgore Hall, Box 7609
Raleigh, NC 27695-7609
Office Phone: 9195151211