LIGHT TRANSMITTANCE, SOIL-TEMPERATURE, AND SOIL-MOISTURE UNDER RESIDUE OF HAIRY VETCH AND RYE

Cover crop residue on the surface of soils in no-tillage systems can suppress weed emergence and growth. Although allelopathy often is invoked to explain weed suppression by residue, physical alterations of the seed environment could be important as well. This experiment was conducted to determine the light, temperature, and moisture conditions under cover crop residue. Hairy vetch (Vicia villosa Roth) and rye (Secale cereale L.) were desiccated with a contact herbicide and residue rates ranging from one-fourth to four times the natural residue biomass were established in experiments at Beltsville, MD and Ithaca, NY. Photosynthetic photon flux density (PPFD) was determined above and below residue at approximately monthly intervals after initiation of the experiment. Transmittance of PPFD through residue declined according to an exponential decay function of residue biomass. Transmittance was similar through hairy vetch and rye residue initially, but as the experiment progressed, transmittance through hairy vetch residue was greater than that through rye because of faster decomposition of hairy vetch residue. Spectral analysis from 400 to 1100 nm showed a slight increase in transmittance as wavelength increased resulting in a slight lowering of the red (660 nm) to far-red (730 nm) ratio relative to that of unobstructed sunlight. Soil maximum temperature and daily soil temperature amplitude were reduced by cover crop residue. Residue prevented the decline of soil water content during droughty periods. Results indicated that reductions in light transmittance and daily soil temperature amplitude by cover crop residue were sufficient to reduce emergence of weeds but that maintenance of soil moisture could increase weed emergence.