Journal of Chemical Ecology, Vol. 31, No. 1, January 2005 (  C 2005) DOI: 10.1007/s10886-005-0983-3 CHANGES OVER TIME IN THE ALLELOCHEMICAL CONTENT OF TEN CULTIVARS OF RYE (Secale cereale L.) S. CHRIS REBERG-HORTON, 1 JAMES D. BURTON, 1,∗ DAVID A. DANEHOWER, 2 GUOYING MA, 1 DAVID W. MONKS, 1 J. PAUL MURPHY, 2 NOAH N. RANELLS, 2 JOHN D. WILLIAMSON, 1 and NANCY G. CREAMER 1 1 Horticulture Department, North Carolina State University, Raleigh North Carolina 27695-7609, USA 2 Crop Science Department, North Carolina State University, Raleigh North Carolina 27695-7620, USA (Received January 15, 2004; accepted August 15, 2004) Abstract—Published studies focused on characterizing the allelopathy-based weed suppression by rye cover crop mulch have provided varying and incon- sistent estimates of weed suppression. Studies were initiated to examine several factors that could influence the weed suppressiveness of rye: kill date, cultivar, and soil fertility. Ten cultivars of rye were planted with four rates of nitrogen fertilization, and tissue from each of these treatment combinations was harvested three times during the growing season. Concentrations of a known rye allelo- chemical DIBOA (2,4-dihydroxy-1,4-(2H)benzoxazine-3-one) were quantified from the harvested rye tissue using high performance liquid chromatography (HPLC). Phytotoxicity observed from aqueous extracts of the harvested rye tissue correlated with the levels of DIBOA recovered in harvested tissue. The amount of DIBOA in rye tissue varied depending on harvest date and rye cul- tivar, but was generally lower with all cultivars when rye was harvested later in the season. However, the late maturing variety ‘Wheeler’ retained greater concentrations of DIBOA in comparison to other rye cultivars when harvested later in the season. The decline in DIBOA concentrations as rye matures, and the fact that many rye cultivars mature at different rates may help explain why estimates of weed suppression from allelopathic agents in rye have varied so widely in the literature. Key Words—Allelopathy, cover crop, residue, redroot pigweed (Amaranthus retroflexus L.), goosegrass (Eleusine indica L. Gaertn.), 2,4-dihydroxy-1,4- (2H)benzoxazine-3-one, DIBOA, maturity, phenology, rye (Secale cereale L.). ∗ To whom correspondence should be addressed. E-mail: jim burton@ncsu.edu 179 0098-0331/05/0100-0179/0 C  2005 Springer Science+Business Media, Inc.