Research has shown that some cover crops are allelopathic and can inhibit weed germination and growth. Additionally, negative allelopathic effects have been documented in cash crops planted into cover crop residue. However, little literature exists comparing relative the allelopathic potential of cover crops producers utilize in conservation-agriculture systems. This study assessed the effects of twelve cover crop extracts on radish ( Raphanus sativus L.) and cotton ( Gossypium hirsutum L.) radicle elongation, in three trials, using an extract-agar bioassay. In Trial 1 the cover crops were black oat ( Avena strigosa Schreb) cv. SoilSaver, crimson clover ( Trifolium incarnatum L.) cv. AU Robin, white lupin ( Lupinus albus L.) cvs. AU Homer and AU Alpha, rye ( Secale cereale L.) cv. Elbon, wheat ( Triticum aestivum L.) cv. Vigoro Grazer, and triticale (* Triticosecale Wittmack) cv. Trical 2700. In Trial 2 the cover crops were forage rape ( Brassica napus L. var. napus) cv. Licapo, sunn hemp ( Crotalaria juncea L.), Austrian winter field pea ( Pisum sativum spp. arvense L. Poir), black medic ( Medicago lupilina L.), hairy vetch ( Vicia villosa Roth), black oat cv. SoilSaver, and crimson clover cv. AU Robin. Cotton was evaluated using the same bioassay and all of the cover crops mentioned above in a single trial (Trial 3). All cover crop extracts inhibited radicle elongation compared to water. Allelopathic potential was highly variable among cultivars within a cover crop species, and within a cultivar. Allelopathic differences among cover crops give an additional weed control tool in conservation systems. However, winter cover selection may impact on cash crop performance if producers plant their crop into green residue.
