Allelopathy in an integrated rye-cotton-beef cattle system
Small grain cover crops offer opportunities for grazing but effects of cover crops on the following row crop are not well understood. Beginning in 1998, rye (Secale cereale L.) planted alternately each September in a 2-paddock rotation, was grazed intermittently by steers from January until early April, was then chemically terminated, and cotton (Gossypium hirsutum L.) was no-till planted in May, in research conducted at New Deal, TX. Following cotton harvest, wheat (Triticum aestivum L.) was no-till planted, grazed out by Angus steers (Bos taurus) by June, and land was fallowed until rye was planted in September. By Feb 2006, wheat failed to establish and was omitted from the rotation. The experiment was a randomized complete block design with 3 replicates. Soil was primarily Pullman clay loam (fine, mixed, superactive, thermic Torrertic Paleustolls). Permanently located caged areas excluded grazing and were harvested as hay. In spring 2005, a second cage, adjacent to the original cage (2005) or randomly located in paddocks (2006 and 2007), investigated effects of 1) zero-grazed; 2) exclusion from grazing in 2005, 2007 or 2008 only; or 3) was always-grazed prior to cotton. In all 3 yrs, rye plant heights were taller (P < 0.01) in previously grazed than zero-grazed rye. Height of cotton plants in July were taller (P < 0.01) when planted into grazed than non-grazed rye in all years. Cotton lint yield in 2005 was greater (P < 0.01) when planted into grazed than ungrazed rye but differences were not significant in 2007 or 2008. Allelopathy was suspected. In a 3-yr small plot study, no-till planting cotton into rye or wheat cover crops reduced (P < 0.05) growth, lint and seed yield of cotton compared with no cover crop. Greenhouse trials verified cotton plant suppression by increasing rates of small grain residues and by direct application of 2-benzoxazolinone (BOA), a known allelopathic chemical. Allelopathic compounds, 2, 4-dihydroxy-7-methoxy-1, 4-benzoxazin-3-one (DIMBOA), 2-benzoxazolinone (BOA), 2, 4-dihydroxy-1,4-benxozaxin-3-one (DIBOA), from rye and wheat were detected in soil and plant material in greenhouse and field experiments. Grazing the cover crop by cattle may help alleviate these negative effects. Cover crops have environmental benefits but negative effects need further investigation.