Interest in cover crops in no-till farming is increasing [1
]. Conservational agriculture systems that utilize high residue cover crops offer many benefits, including enhanced water infiltration, lower soil water evaporation, increased soil organic matter, and increased soil biodiversity. Additionally, high biomass yielding cover crops can suppress early-season weeds [2
]. Moreover, cover crops can suppress weeds through the production of allelopathic secondary metabolites. Allelopathy has been defined as any direct or indirect harmful or beneficial effect by one plant on another through production of chemical compounds that escape into the environment [3
These compounds have been used as natural herbicides [5
]. Unfortunately, these chemicals can also impact the growth and development of the following row crop [6
]. Subsequently, it is important to determine the allelopathic compatibility of cover crops with row crops before incorporating them into agricultural systems, as phytotoxins released by cover crops could affect the–establishment of row crops [8
In their study, Allen et al. [7
] showed that the use of rye and wheat in the rotation reduced the growth of cotton, and that rye allelopathy was the cause of this suppression. Regardless of specific chemical involvement, the suppression of cotton following small grain cover crops was repeatable [7
]. Wheat and rye have been recognized for their suppression of germination and radicle elongation of several species via the release of benzoxazinoid allelochemicals [9
]. Bauer and Reeves [10
] reported that black oat (Avena strigosa Schreb
), rye and crimson clover (Trifolium incarnatum L.
) residues inhibited tap root elongation in greenhouse grown radish (Raphanus sativus L.
) and cotton (Gossypium hirsutum L.
) plants. Price et al. [6
] indicated that cotton radicle elongation was inhibited by extracts of forage rape (≤ 19%), wheat (≤ 23%), rye (≤ 26%), triticale (≤ 28%), black oat (≤ 34%), crimson clover (≤ 30%), white lupin (≤ 40%), sunn hemp (≤ 35%), hairy vetch (≤ 45%), and black medic (≤ 33%). Cover crop species differ in their allelopathic potential [11
]. Cereal rye and soft red winter wheat (Triticum aestivum L.
) are the two most commonly recommended winter cover crops for row crop production in the southeastern U.S. [6
]. Both of these cover crops contain allelopathic compounds that inhibit weed growth [6
Based on germination and radicle elongation of the response species tested in a study by Geddes et al. [9
], hairy vetch did not chemically inhibit germination of any response species, and was in fact found to stimulate the germination of lamb’s quarters under the study conditions. Therefore, hairy vetch might not be a good candidate species for weed-suppressive allelopathic mulch [9
]. Baldwin and Creamer [15
] reported that winter pea (Pisum sativum
) grows fast and vigorously, and while growing as a cover crop, can suppress weeds due to its allelopathic impact.
A winter cover crop’s allelopathic potential is an important attribute which is typically overlooked (Price et al.) [6
]. The information that ranks the relative allelopathic potential of these cover crops, or of other traditional and non-traditional cover crops available to producers, is limited [6
]. The complexity of allelopathic interactions complicates conclusive field research [16
]. In row crops production, it is extraordinarily difficult to distinguish mechanisms of interference between chemically allelopathic and physical mulch-residue effects in the no-till system [12
]. Therefore, laboratory screening allows for the selection of promising species or genotypes for field evaluation [6
To minimize the risk of integrating cover crops into the cotton production system, cotton producers need more information on how allelopathic properties change with different cover crop species and cover crop termination dates. Therefore, the current study was designed to fill the gap in the allelopathic potential ranking of available cover crop species. The objectives of this study were to assess the relative effects of (1) different cover crop species and (2) different termination timings of a blend cover crop species’ extracts on cotton germination and seedling development in controlled environments.
Cotton seeds placed within cover crops extracts (i.e., ‘annual rye’, ‘winter wheat’, ‘oats’, ‘hairy vetch’, and ‘winter pea’) were characterized by delayed and inhibited germination relative to deionized water (check) (Figure 2
). The allelopathic effect on germination varied and depended on extracted species (Table 1
). This suggests that the allelopathic potential of the cover crops is variable. This may be very important to a producer in selecting a cover crop cultivar and understanding the negative impacts allelopathy might have on cotton seedlings.
In the current study, the decline in cotton germination rate, root and shoot lengths, dry weight, and overall seedling length with increased cover crops residue rate were significant (Table 1
). Sahoo et al. [19
] indicated in a study that the aqueous leaf extracts of Leucaena leucocephala
and Tectona grandis
reduced the fresh and dry weights of maize (Zea mays
L.). Bruce et al. [20
] reported negative impacts of wheat residue on emergence and growth of canola (Brassica napus L.).
In a study by Allen et al. [7
], the observed suppression effect of cover crops on cotton lint and seed yield was consistent with results of the long-term system study when both rye and wheat were present in the rotation, providing further evidence of a relationship between suppressed cotton growth, development, and yield with allelopathic compounds.
In our study, the ‘winter pea’ extracts drastically reduced the length of cotton seedlings compared to other cover crop types (Table 1
). Bauer and Reeves [10
] reported that the tap root elongation in greenhouse grown radish (Raphanus sativus L.
) and cotton (Gossypium hirsutum L.
) plants were inhibited by the residues of black oat (Avena trigose Schreb
), rye and crimson clover (Trifolium incarnatum L.
]. Hulugalle et al. [22
] showed that in general, reductions in germination, emergence, dry matter production and root length density of cotton seedlings were affected by winter legumes, summer legumes, cereal extracts and a control group, in that order. They indicated that compared with cereals, leguminous rotation crops, particularly winter legumes, reduced emergence, growth and lint yield, and resulted in poor fiber quality in cotton [22
]. Hill et al. [23
] concluded that effects of aqueous extracts of ‘hairy vetch’ and ‘cowpea’ vary with species and extract concentration. The allelopathic effects are generally attributable to the production and exudation of particular toxic chemicals, which differ in concentrations among cultivars of the same species and are perceived with varying levels of susceptibility among cultivars of other species [24
Price et al. [6
] reported that cover crop extracts decreased cotton radical elongation ≤49% depending on species. The cotton radicle elongation was inhibited by forage ‘rape’ (≤19%), ‘wheat’ (≤23%), ‘rye’ (≤26%), ‘triticale’ (≤28%), ‘black oat’ (≤34%), ‘crimson clover’ (≤30%), ‘white lupin’ (≤40%), ‘sunn hemp’ (≤35%), ‘hairy vetch’ (≤45%), and ‘black medic’ (≤33%). In greenhouse studies, in which the leachates of ground dried residues of the three cover crops were evaluated based on germination, plant height, and dry weight of goosegrass, smooth amaranth (A. hybridus L.
), bell pepper, and tomato, Adler and Chase [26
] found that goosegrass (Eleusine indica L.
) germination was inhibited in a similar manner by residues of the three cover crops by up to 80% or less compared to the control [26
]. Selection of crop species based on their allelopathic potential is perhaps one of the best strategies for reducing their negative impact on row crops and taking advantage of allelopathy for suppressing weeds.
In the current study, all extracts of a blend cover crop at different termination timings inhibited root elongation and germination rate, with the most negative impacts occurring under termination at planting (Table 2
and Figure 5
). The current results show that cover crop termination timing can minimize the potential allelopathic impacts on an emerging cotton crop, and early termination timing will minimize the allelopathic impacts on emerging weeds. Furthermore, applying split termination of cover crops within the row ‘strip /6-wk’ has shown the potential to minimize the negative allelopathic impacts of the cover crop on the row crop germination by 22% compared with ‘at planting’, while achieving cultural control benefits for weed management. Finally, the longer the interval between cover crop termination and row crop planting, the less likely allelopathic compounds will affect crop emergence and growth. Differences in the sensitivity of crop cultivars to allelopathic impacts of cover crops may also occur [26