In west Texas, no-till planting cotton into rye or wheat cover crops reduced growth and lint yield of cotton compared with no cover crop. Production of allelopathic compounds, a known phenomenon in small grains, was suspected. Known allelopathic compounds from rye were detected in soil [2-benzoxazolinone (BOA)] and plant material [2,4-dihydroxy-1,4-benxozaxin-3-one (DIBOA)] in greenhouse and field experiments. Field and greenhouse trials verified cotton plant suppression by these small grain residues and by direct application of allelopathic chemicals. Grazing the cover crop by cattle may help alleviate these negative effects. Cover crops have environmental benefits but negative effects need further investigation.
Cotton (Gossypium hirsutum L.) is an important crop in the Texas High Plains, grown largely with irrigation. The main irrigation resource is the Ogallala aquifer but water withdrawn for agricultural use has greatly exceeded potential recharge for many years. Intensive monoculture cropping systems have contributed to this decline. A SARE-funded 10-yr integrated crop-livestock system showed a 25% decrease in water use and decreased fertilizer inputs but had variable effects on cotton yield and profitability compared with a monoculture cotton system (Allen et al., 2005; Allen et al., 2007). Furthermore, Hou et al. (2005) found that growth of both rye and cotton were lower inside of caged, non-grazed areas compared with where the cover crop had been grazed. Soil fertility, soil moisture, and presence of diseases failed to explain this difference.
Cotton in the integrated system was grown in alternate rotation with small grains. Prior to planting cotton, ‘Maton’ rye (Secale cereale L.) was used as a cover crop and was harvested by grazing steers. Wheat (Triticum aestivum L.) provided grazing in the alternate paddock of the rotation. Cover crops can help reduce soil erosion (Meisinger et al., 1991), maintain or increase soil fertility (Doran and Smith, 1991; Nova, 1995), suppress weed growth (Barnes and Putnam, 1983), increase nitrogen scavenging (Jackson et al., 1993) and decrease off-farm energy use (Ess et al., 1984).
Small grain cover crops are also known to produce allelopathic chemicals. Allelopathy is defined by Molisch (1937) as production of specific biochemicals of plant origin that promote or inhibit the growth of other plants. According to Fuerst and Putnam (1983), there are four criteria to indicate the existence of allelochemicals. They are “(1) identify the specific interfered symptom, (2) isolate, identify, and synthesize the released chemicals, bioassay the toxins; (3) simulate the interference of toxin as in the natural condition; and (4) identify the quantity of toxin released and the uptake in the plant.”
Reberg-Horton et al. (2005) reported that phytotoxicity found in aqueous extracts from rye tissue was correlated with concentrations of 2,4-dihydroxy-1, 4-(2H)benzoxazine-3-one (DIBOA). Concentrations of DIBOA in rye tissue differed due to harvest date and rye cultivar. Concentrations in all cultivars tested were generally lower when rye was harvested at later stages of maturity.
Marcias et al., (2005) reported that DIBOA in soil was transformed primarily into 2-benzoxazolinone (BOA). Understrup et al. (2005) showed that allelochemicals were present in soils and suggested that exploitability for crop protection by BOA was dependant on the existing concentration of BOA in the soil and the timing of incorporation of hydroxamic acid synthesizing crops into that soil. In their work, the half-life of BOA in soil increased as level of BOA was increased and was 31 days when added at 30,000 nmol BOA g soil-1. These authors further reported that as the concentration of BOA in soil increased, the formation of 2-amino-(3H)-phenoxazin-3-one (APO) was favored and that this compound would have a stronger sorption in soil, making it less likely to leach and less bioavailable. APO is one of the main degradation products of BOA.
In the long-term integrated crop/livestock experiment described above, it was not known whether concentrations of allelochemicals had accumulated over time within the rotation of rye and wheat. In rye, DIBOA has been reported to be the allelochemical of highest concentration (Barnes et al., 1987) while in wheat, 2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) is the most abundant allelochemical ( Bohidar et al., 1986).
The existence of allelopathy can make rye a natural herbicide that could contribute to both sustainable agriculture and profitability through suppression of unwanted plants. The problem with rye is that it not only suppresses weeds but may also suppress cotton growth within crop rotations ( Hou et al., 2005).
According to Hou’s work (Hou et al., 2005; F. Hou, unpublished data, Texas Tech Univ. Lubbock), rye population, basal cover, tiller numbers, tiller weights, total biomass and plant height were all greater in the previously grazed rye than in the 0-grazed rye. Soil water was higher in grazed than in ungrazed rye in April and cotton plant height, density, bolls per m-1 row and cotton lint yield were also higher where rye had been previously grazed then within the non-grazed rye areas. The result showed grazing had improved the growth and productivity of cotton and rye and the effect was consistent within the rotation between paddocks.
It’s interesting that allelopathy can be influenced by plant stress as the stressed plant tends to release more allelochemicals (Reigosa et al., 2002) but Hou’s research showed that grazing, one of the stress factors, appeared to reduce allelopathic effects on cotton. Results from Reberg-Horton et al. (2005) would appear to fail to explain why grazing appears to diminish the negative effect as grazing would likely maintain plants in an earlier stage of maturity than zero-grazed plants. Their results suggested that allelopathic effects diminished with aging of the plant.
Livestock grazing can have several effects including cattle trampling and compaction of soil, soil or plant pathogens, nutrient cycling through feces and urine of the cattle, cattle saliva introducing compounds with biological activity to plant being grazed, and other potential soil-plant-animal-climate interactions. However, analysis of soil and plant samples did not suggest that any of the above possibilities adequately explained the observed effects.
The use of small grain cover crops is important in reducing soil erosion, weed control, retention of soil nutrients and organic matter, and other benefits derived. A possible suppression of the cotton by such a cover crop in both stand establishment and in growth and yield of cotton would translate into large economic consequences. It is possible that cotton (and perhaps other target crops) yield potential in such systems has been lower than might be otherwise expected and that this has gone unnoticed but needs further investigation. Thus, the purpose of our research was to investigate allelopathy as the possible cause of this observed suppression and to compare wheat with rye as cover crops and for their potential to suppress growth and lint yield of a following cotton crop.
The overall objective is to identify the cause of small grain cover crop suppression on growth of rye and cotton and to alleviate this suppression through grazing management and/or selection of small grain species and varieties that minimize this effect.
1. To investigate whether BOA is present in soils when rye and wheat have been grown in alternate rotation for 9 yr and whether past grazing affects concentrations.
2. To determine whether DIBOA is present in Maton rye and to investigate effects of grazing vs. hay on concentrations in aerial plant parts.
3. To investigate differences in concentrations of DIBOA in Lockett wheat and four varieties of rye and the effects of these forages as cover crops on subsequent establishment, growth, and yield of cotton.
4. To determine the biological activity of rye residues on germination and initial growth of cotton.
5. To determine effects of grazing vs. no grazing on growth of rye and the following crop of no-till planted cotton.
The initial study was conducted within the SARE-funded long-term integrated crop-livestock system which began in 1997 to investigate differences between a cotton monoculture and the integrated system (Allen et al., 2005). This research is located at the Texas Tech University research farm (101°47′W; 33°45′N; 993 m elevation) in the Southern High Plains. Soils were primarily Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustolls).
Briefly, System 1 was a cotton monoculture cultivated continuously by practices recommended by state extension specialists (Allen et al., 2005). System 2 was a 3-paddock system where about 54% of the total area was established in a perennial warm-season grass. The remaining area was equally divided into two paddocks where cotton was grown in alternate rotation with wheat and rye. ‘Maton’ rye was planted in early September each year and was subsequently grazed out by Angus stocker steers. Cotton was no-till planted into the rye residue. Following cotton harvest in autumn, ‘Lockett’ wheat was no-till planted into the cotton stubble. In spring, wheat was grazed following rye in the alternate paddock. After wheat graze-out, the land was fallowed until rye was again planted in early September. Both systems were replicated 3 times in a complete randomized block design. Both systems were irrigated by subsurface drip irrigation. See Allen et al. (2005) for full description of these systems.
Permanent sites within each rye and wheat paddock replication have excluded grazing by steers from the beginning of the experiment. Additionally, a second cage within each block was placed randomly in the field to prevent grazing in yr 9 and 10 only. Thus, for objectives 1 and 4 below, there were 4 treatments as follows: 1) ungrazed rye (zero grazed); 2) ungrazed rye in 2007 and 2008 only [grazed in years 1 to 8 (9) and ungrazed in yr 9 (10)]; 3) the always grazed area which was the remainder of the rye paddock, and 4) cotton planted where no cover crop was used (the continuous cotton system).
In January 2007(8) and April 2007(8), soil samples (0 to 8 cm) were collected from each replication of the grazed and non-grazed (zero-grazed and ungrazed in 2007(8) only) rye, and the continuous cotton. Soil was gently crushed and stored at -4ºC without further drying (field moisture conditions). Soils were extracted for presence of BOA following procedures of Understrup et al. (2005) and analyzed by high performance liquid chromatography (HPLC).
Whole-plant samples of rye harvested at ground level were collected in April of 2007 and 2008 at the time grazing was terminated. Samples were collected from the three treatment areas as 1) grazed; 2) zero-grazed; and 3) ungrazed only in 2007 and 2008. Samples were placed in paper bags and dried at 60°C in a forced air oven until a constant weight was achieved. Tissue were ground in a Wiley mill with a 2-mm mesh screen and stored at room temperature to exclude light. Presence of DIBOA was determined following the procedures of Reberg-Horton et al. (2005).
Two field experiments were conducted to meet Objective 3.
Experiment 1. Within the 3 replicate rye paddocks in the integrated system described above, small plots (12 m X 12 m) of four rye varieties were planted in September of 2007 and 2008 to include (1) ‘Maton;’ (2) ‘Elbon;’ (3) ‘Wrensabruzzi;’ and (4) ‘Oklon.’ Biomass was determined in February and May 2007 and 2008 immediately prior to the beginning of grazing and at the termination of grazing each year by clipping two, 0.1-m2 quadrates from within each treatment replication. Forage was dried to a constant weight at 60ºC and weighed to determine biomass. Subsamples were retained for determination of DIBOA as described above. These plots were resampled for DIBOA analysis at the end of grazing in April each yr following graze-out of rye.
Effects of rye varieties on germination (plants per m row), growth (plant height), and yield of cotton was determined by machine harvest of the entire plot. Cotton was weighed and ginned to determine yield of lint and seed.
Maton rye and Lockett wheat were planted into plots measuring 21 X 24 m in a randomized block design with 4 replicates per treatment. A control was included in which no cover crop was planted. Cover crops were allowed to grow until mid- to late-April, were sampled for total dry biomass as described above, harvested and removed as hay. Regrowth was terminated with glyphosate. Cotton was no-till planted into one-half of each treatment plot in an annual rotation between plot halves. Effects of cover crops or no cover crop on growth and yield of cotton was measured as described under Experiment 1 above. In year 2, cover crops were replanted into the same treatment plots on the half of each plot where cotton was not planted the previous spring. Cover crops were grown, terminated, cotton was no-till planted, and samples were collected as described above. Soil samples were collected and assayed as described above to determine presence of allelopathic chemicals. Samples were collected in May and November, 2007; February, June, and December of 2008; and May 2009. This experiment is ongoing.
Two greenhouse studies were conducted to determine the biological activity of rye (rye and wheat in experiment 2) on germination and initial growth of cotton.
Experiment 1. First greenhouse experiment.
In 2007, rye harvested under Objective 2 from the zero-grazed plots was used for this Objective. Ground rye was added to the top 3 cm of soil in pots in a greenhouse experiment to determine the effect of rye on germination of cotton. Rye was added at a rate equivalent to 0, 800, 1,600, 3,200, 6,400, 12,800 kg ha-1 to approximate levels of plant residue that would reasonably be left as biomass prior to no-till establishment of a crop. Allelochemical BOA [2-benzoxazolinone] purchased from Fisher’s was added to pots and mixed with the top 3 cm of soil at a rate of 0, 500 and 1,000 µmol/g.
A Pullman soil was collected from an area where no small grains had been grown for at least the past 5 years. Soils were thoroughly mixed, and placed into pots after mixing with each of the rye treatments. Cotton seed (5 per pot) were planted. Numbers of seed germinating and time to germination were recorded. Plants were grown for 4 wks and plant heights were recorded. Plants were harvested at the end of 4 wks, dried at 60°C to a constant weight, and weighed.
Each treatment was replicated 4 times in a completely randomized experiment. Pots were rerandomized weekly to minimize environmental variation within the greenhouse. Soils were analyzed at the end of the experiment for the presence of BOA.
A second complete set of treatment pots were set up in this experiment as described above and treated similarly to the pots described above with the exception that no cotton was planted in these pots. These pots were used to sample soils for BOA at 7-day intervals during the experiment. At the end of the experiment, these pots were placed inside in storage at room temperature for reuse in Experiment 2 described below.
Experiment 2. Second greenhouse experiment.
In 2008, rye and wheat harvested under Objective 3 from the wheat/rye plots were used for this objective. A Pullman soil was collected from an area where no small grains had been grown for at least the past 5 years. Soils were thoroughly mixed and placed into pots as described for Experiment 1. Ground rye and wheat were added separately to soil in pots in a greenhouse experiment as described above to determine the effect of wheat and rye on germination and growth of cotton. Ground plant tissue was mixed with about the top 3 cm of soil at a rate equivalent to 0, 6,400 and 12,800 kg ha -1 to approximate levels of plant residue that would be reasonably left as biomass prior to no-till establishment of a crop. Allelochemical BOA [ 2-benzoxazolinone] saved from 2007 was added to pots and mixed thoroughly with soil at a rate of 0, 500 and 1,000 µmol/g. Cotton seed (5 per pot) were planted into these pots and all pots were watered. Numbers of seed germinating and time to germination were recorded. Plants were grown for 6 wks and plant heights were recorded. Plants were harvested at the end of 6 wks, dried at 60°C, and weighed.
The second complete set of treatment pots saved from 2007’s greenhouse experiment were placed in the greenhouse after being maintained without watering at room temperature for 9 months. Cotton seed (5 per pot) were planted into these pots and all pots were watered. Numbers of seed germinating and time to germination were recorded. Plants were grown for 6 wks and plant heights were recorded. Plants were harvested at the end of 6 wks, dried at 60°C, and weighed.
Objective 5. Rye.
For this objective the protocol described by Dr. Fujiang Hou was used (unpublished data) for determine effects of grazing vs. no-grazing of rye on growth of rye and on the following cotton crop. In 2007 and 2008, when grazing began in January, and when grazing ended in April, total biomass was determined by clipping forage within a 0.1-m2 quadrat at ground level (0.24 m2 quadrat was used in caged area). Four samples were collected from the grazed area and two samples were taken from caged areas. Samples were dried at 60 °C to a constant weight to determine dry biomass.
From grazing initiation in January to termination in April, rye plant extended leaf height was measured at 28-d intervals. Thirty random sampling points were measured in grazed area and 20 random sampling points were measured in caged area.
In autumn of 2008, rye in the alternate paddock was sampled in as described above. Additionally, at the time cotton was planted but prior to initial irrigation events, soil was sampled to determine gravimetric soil moisture.
Cotton plant height (cm) and plant density (plants m-1 within a row) were determined within each of the three treatment areas described above in both 2007 and 2008. Plant heights were determined by measuring 15 plants randomly within each previously caged area and 30 plants within the grazed area. Plant density was determined by counting plants within 1 m of row at 3 random sampling points within caged areas and at 20 random sampling points within grazed areas in July and October, 2007 and 2008. After harvest, cotton was weighed and ginned to determine yield of lint and seed.
The known allelopathic compound from rye, 2-benzoxazolinone (BOA) was detected from the research site when the study began. Later during 2007, after above normal amounts of rainfall, only trace amounts of this allelochemical was detected in soil. In 2008, analysis of soil samples collected from the research site showed that more 2,4-dihydroxy-1,4-benxozaxin-3-one (DIBOA), the major allelochemical in rye, was present in the soil where rye was grown verses the continuous cotton site (7054 vs. 580 P < 0.003); numbers represent peaks on the HPLC indicating differences in concentration). More DIBOA was found in the soil where rye was ungrazed for 10 yr than soils in grazed areas (11,915 vs. 4,646; P < 0.002) suggesting that a known allelopathic chemical produced by rye was present where rye was grown and that grazing reduced the amount of this chemical in soil.
Data for this objective are incomplete at this time. Samples are in the process of being analyzed.
In February, immediately prior to grazing, there was less (P < 0.01) Maton biomass than the mean of the other three rye varieties (1,260 vs. 1,830 kg/ha). Wrensabruzzi had more biomass than the mean of Oklon and Elbon ryes (2,112 vs. 1,680 kg/ha; P < 0.06) prior to grazing. By the end of the grazing season in May, there were no differences in rye biomass among these four varieties. Likewise, yield of cotton no-till planted into these four rye varieties did not differ.
Both rye and wheat cover crops increased emergence rate of cotton and following a hail event, plant survival and initial growth of cotton were greater compared with no cover crop. However, when cotton was harvested in November, cotton yield was higher (P < 0.0001) where no cover crop was planted compared with where wheat or rye was planted. Cotton yield was also higher (P < 0.05) where rye was planted compared with where wheat was planted suggesting that wheat may have greater allelopathic effects than rye. This experiment is being continued for a 3rd year (2009).
Weight of cotton plants at the end of the first greenhouse experiment was reduced linearly (P < 0.01) by addition of either rye residue or by direct application of BOA. BOA was detected in soil from all treatments except for the controls.
Cotton plant heights and weights were reduced linearly (P < 0.05) by both wheat and rye residue. Likewise, plant height and plant weight were reduced linearly (P < 0.001) in cotton grown in soils treated with rye residue in Experiment 1 and then returned to the greenhouse after 9 months of maintaining these soils without water at room temperature.
Cotton no-till planted into rye stubble that had been grazed was taller (P < 0.05) in July than cotton planted into rye stubble that was not grazed but was harvested as hay prior to planting cotton. In October, these relationships were numerically similar but differences were no longer significant. In 2008, there were more cotton plants per m of row where cotton was planted into previously grazed rye compared with rye that was harvested as hay and not grazed. This difference was not observed in 2007, however. No effects of treatments were observed on cotton lint yield in either year.
Preliminary studies by Hou et al. (2005) in a long-term field study showed clearly that cotton no-till planted into a previously ungrazed rye cover crop had lower plant populations, fewer boles per plant, and lower lint yield than when cotton was planted into rye that was previously grazed during the spring by stocker steers. Attempts to identify contributing factors including differences in soil fertility, diseases, insects, and soil moisture failed to explain this effect. Furthermore, rye growth per se was suppressed where no grazing occurred compared with where grazing had previously occurred. Allelopathy, a known phenomenon in small grains, was suspected as a possible cause.
Our work has identified known allelopathic compounds in both soils and plants in the same fields with the same cropping rotations investigated by Dr. Hou. Thus, the presence of these compounds has been established. Rye biomass from these fields was taken into the greenhouse and added to soils. The purified allelopathic chemical purchased from Fisher Scientific (Houston, TX) was also added as a treatment. Both the rye and the chemical reduced growth of the cotton plants. Again, the allelopathic chemical was recovered from all soils at the end of the experiment except for the controls.
A small-plot field study was then conducted to determine if both wheat and rye had a suppressing effect on cotton growth. Both were found to reduce yield of cotton lint compared with the control. Wheat and rye biomass from this experiment was then used in the second greenhouse experiment and again, cotton growth was suppressed linearly by both small grains.
Thus, it appears that both small grains do produce allelopathic chemicals that have negative effects on the following cotton crop. In the original integrated system, both wheat and rye were grown in alternate rotation with cotton. It now appears that the suppression observed by Hou et al., (2005) was likely due to both small grains in this rotation. The greenhouse experiment with soil treated with rye residue and held for 9 months prior to planting cotton verified the effectiveness of the suppressing effect was sufficiently long to have occurred in the field rotation.
Educational & Outreach Activities
Li, Yue, J. Chen, F. Hou, P. Brown, and V. Allen. 2008. Alleopathy in an integrated rye-cotton-beef cattle system. Proc. Oct. 5-9, 2008 Joint Meeting, ASA, CSSA, SSSA, and GSA, Houston, TX. http://acs.confex.com/crops/2008am/techprogram/P40407.HTM.
Li, Yue. Ph.D. dissertation in progress.
This research is directly focused on increasing the yield of the target crop (cotton) in a rotation with small grains that provide soil protection and grazing opportunities for livestock. The research is providing a biochemical explanation for the observed depression of the target crop in this rotation compared with a cotton monoculture. The depression occurs in both yield and plant populations of cotton. Much data exists in the literature demonstrating the beneficial effects of cover crops and crop rotations compared with conventional tillage and monoculture cropping practices. However, this long term integrated system research has failed to show this benefit. Our research suggests that allelopathy is a major factor in explaining this effect. The semi-arid climate is suspected as a contributing factor to magnifying the negative effects of allelopathy on the following target crop. The unusually high rainfall received in 2007 and the trace amount of BOA and non-detectable amounts of DIBOA in the collected soil samples compared with the much larger concentration of DIBOA in the drier spring of 2008 suggests that precipitation may influence this effect and that this may be a particular problem in similar semi-arid regions. The use of cover crops is of particular importance in such regions to prevent high soil losses from wind erosion.
The larger observed effect of wheat than rye in suppressing cotton growth suggests opportunities to select species and perhaps varieties of small grains to minimize the negative effects of allelopathy on the following crop. Furthermore, grazing by livestock offers opportunities to utilize the cover crops while reducing the allelopathic effect giving producers a management strategy to reduce this suppression while protecting the soil from wind erosion. Although not consistently observed, grazing has increased yield of the following cotton crop by as much as one half a bale per acre compared with cotton grown following non-grazed rye. This was due in part to the greater number of plants that germinated and established where grazing occurred. A common strategy used by area producers to insure desired plant populations in no-till establishment of cotton into a cover crop has been to increase cotton seeding rates. Given the price of cottonseed, not needing to increase seeding rates is a significant economic advantage to producers. Our research indicates that the allelopathic depression may vary among different small grain cover crops. Although we could select different small grains or the species which release less allelochemicals but we may also abate their useful influence as a natural herbicide. As previous data has suggested, introducing cattle grazing into the system may help alleviate the depression. Further study on selecting genetic tolerance in cotton species would also be a promising way to solve the problem.
No economic analysis has been conducted with this research thus far.
Although no formal programs are yet targeting farmer adoption, producers are aware of the challenges of no-till planting cotton in this environment and of the research that we are conducting.
It is too early in our investigations to make specific recommendations to producers but we are informing producers of both the benefits and the potential detriment to using small grain cover crops in this region. As this research is completed, we anticipate being able to make specific recommendations to producers. More research will be required to fully develop strategies for retaining the positive benefits of cover crops while avoiding the negative effects of allelopathy on the target crop.
Areas needing additional study
Areas of further research now needed include:
1. Identification of other unknown allelochemicals indicated by HPLC analysis of our samples.
2. Identification of genetic differences within target crops that have resistance to allelochemicals.
3. Identify the mode of action of allelochemicals on cotton.
4. Further investigation of the grazing impact on reducing allelochemical effects on the target crop are needed.