Allelopathic effects of small grain cover crops on cotton plant growth and yields
Production of allelopathic compounds is a well known phenomenon in small grains. Use of small grain cover crops has been recommended as a method to suppress weeds in no-till crop production for many years. In west Texas, a cotton monoculture is being compared with an integrated crop-livestock system that includes rye and wheat in a 2-paddock rotation with cotton. Observed yields in the integrated system have indicated that the alleochemicals released from rye suppress growth and yield of the following cotton crop. Grazing of the rye by beef steers reduced this suppression. Although rye is known for its value as a natural herbicide, its alleopathic characteristic has lowered the yield potential of both the target crop and rye itself. Known alleopathic compounds from rye were detected in both soil [2-benzoxazolinone (BOA)] and plant material [2,4-dihydroxy-1,4-benxozaxin-3-one (DIBOA)] from this site. Data gathered from a greenhouse study also supported the hypothesis that rye has alleopathic influence over cotton growth. Thus far, these studies have indicated that wheat depressed cotton yield even more than rye did, and that this depression can be alleviated at least in part by grazing.
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 and rye extracts on germination and initial root elongation 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 known alleopathic 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, the alleochemicals were hardly detected from the soil sample. In 2008, the analysis of soil samples collected from the research site showed that more 2,4-dihydroxy-1,4-benxozaxin-3-one (DIBOA), the major alleochemical 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 allolopathic chemical produced by rye is present where rye is grown and that grazing reduces the amount of this chemical in soil.
In a greenhouse study, 6 levels (0, 800, 1,600, 3,200, 6,400, 1,2800 kg ha-1) of dried and ground rye and 3 levels (0, 500, 1,000 µm/gram of soil) of 2-benzoxazolinone(BOA) were mixed with soil and planted with cotton. In this 46-d study, the initial seedling emergence rate and growth of cotton were suppressed with increasing rates of BOA and at the highest level of rye inclusion.
In a further experiment, wheat, rye, or no cover crop was used prior to planting cotton. In this study, cover crops increased emergence rate and following a hail event, plant survival and initial growth of cotton were greater compared with no cover crop. However, by the end of the growing season, cotton yield was higher where no cover crop was planted compared with where wheat or rye was planted. Cotton yield was also higher where rye was planted compared with where wheat was planted suggesting that wheat may have greater allelopathic effects than rye.
The work left to be done includes collecting of data and soil/plant samples from the field as was done during 2007. The green house study conducted during 2007 will be repeated in 2008 and this time will include wheat as an additional treatment.
Impacts and Contributions/Outcomes
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. 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 to insure desired plant populations has been to increase seeding rates. Given the price of seed, not needing to increase seeding rates is a significant economic advantage to producers. Our research indicates that the alleopathic depression may vary among different small grain cover crops. Although we could select different small grains or the species which release less alleochemicals but we may also abate their useful influence as a natural herbicide. As previous data has shown, introducing cattle grazing into the system may solve the depression. Further study on selecting genetic tolerance in cotton species would also be a promising way to solve the problem.