Allelopathic effects of small grain cover crops on cotton plant growth and yields

• 2007 annual report
• 2009 annual report

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.

Introduction

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 The main irrigation resource is the Ogallala aquifer but water withdrawn for agricultural use has greatly exceeded potential recharge for many years.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.

Specific Objectives:

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.