TIMING AND NATURE OF MULCH RETARDATION OF CORN VEGETATIVE DEVELOPMENT

Low soil temperatures that are induced by crop residue have been shown to delay corn (Zea mays L.) development. The exact conditions under which retardation takes place and whether the delay is only due to thermal effects still need to be elucidated. This study was conducted to characterize corn vegetative development, with and without residue cover, and to determine if differences in seed-zone temperature account for the differences in development. The effects of an oat (Avena sativa L.) straw mulch and an inert poplar excelsior (Populus) mulch, applied at second leaf tip emergence and at the time of full extension of the fourth leaf, on seed-zone temperature, time, and thermal time (daily temperature accumulation) between specific vegetative stages, were investigated on a Conover loam (mixed, mesic, Udollic Ochraqualf) under irrigated conditions. Developmental delays occurred as average soil maxima temperatures, under the mulches, were 2.2-degrees-C lower than the bare soil control. When mulches were applied at second leaf tip emergence, soil temperature-based thermal time was significantly higher for the oat mulch than for the popular mulch or the bare soil, until the full extension of the fourth leaf in 1988 and the third leaf in 1989. Modeling equations derived from CERES-MAIZE accurately predicted thermal times to the full extension of the third leaf and sixth leaf for most treatments, but underestimated thermal time for the 1988 oat mulch. When applied at full extension of the fourth leaf, oat straw had minimal effects on development. The differences in thermal time due to the two mulches during the early vegetative stages of corn, suggest that in addition to soil temperature, allelopathy from oat straw may be a source of developmental delay and that its occurrence is weather-dependent.