Alleopathy in An Integrated Rye-Cotton-Beef Cattle System.

Cotton (Gossypium hirsutum L.) and beef cattle represent two major monocultures in west Texas but little integration of these industries occurs.  In 1997, long-term research began comparing a cotton monoculture with an integrated system that included cotton in a 2-paddock rotation with rye (Secale cereal L.) and wheat (Triticum aestivum L.) grazed by beef steers.  Cotton was no-till planted into grazed rye each year in alternating paddocks. Wheat, planted after cotton was grazed out in spring and then fallowed until rye was fall planted. Permanent exclosures (4.8 X 4.8 m) included in each paddock allowed measurement of grazing effects vs no grazing on soils and plants. Each system was replicated three times in a complete randomized block design. By year 6, rye and cotton growth within exclosures was suppressed compared with rye and cotton yields where grazing occurred and allelopathy was suspected.  Initially, 2-benzoxazolinone (BOA), a known alleopathic compound from rye was identified in soils from the experimental area. A greenhouse study investigated inclusion of 6 levels (0, 800, 1,600, 3,200, 6,400, and 12,800 kg ha^-1) of ground, dried rye and 0, 500, and 1,000 µm of BOA mixed with soil. Cotton was planted into plots and grown for 46 d.  Growth of cotton was depressed with increasing rates of BOA and at the highest rates of rye inclusion.  Soil samples collected in February 2008 from the field showed that more 2,4-dihydroxy-1,4-benxozaxin-3-one (DIBOA), but not BOA (a degradation product of DIBOA) , was present in soils where rye was grown than in the continuous cotton system (7054 vs. 580). Soils where rye was ungrazed during 10 yr had more DIBOA (11,915) than soils in grazed areas (4,646).  Allelopathy from cover crop rye can suppress growth and yield of cotton and rye and grazing reduces this effect.