Perennial cover crops in Nicaraguan coffee orchards: Mechanisms and manipulations of plant competition

A widespread renovation of coffee orchards in southern Nicaragua during the early 1980's removed the overstory shade trees from the traditional agroforestry system. In following years weeds and other pests became much more prevalent. This study examined the feasibility of using perennial cover crops to control weeds growing on orchard floors. Three species of cover crops were tested.;Arachis pintoi achieved 100 percent ground cover in three months when plots were manually weeded at monthly intervals up to that date. Arachis most likely suppressed weeds by competing for water and/or nutrients, and continued controlling weeds two years after planting, when the experiment was terminated.;Desmodium ovalifolium required more time to establish than the other two species. A broadcast seeding rather than the in-row seeding method used might have allowed more rapid cover formation. Desmodium controlled weeds up to two years after planting, but the competitive mechanism could not be determined. High plant densities within rows created intraspecific competition in Desmodium during the six month dry season.;Both Arachis and Desmodium caused greater water stress in coffee plants at the end of the dry season. Although soil moisture in the dry season was usually greater at 0-30 cm depth in the plots with these cover crops or uncontrolled weeds, coffee root length densities were lower, resulting in decreased water uptake by the coffee. Cutting the covers during the dry season might alleviate the increased moisture stress. Coffee yields over two years were not different among treatments of Arachis or Desmodium cover crops, unweeded controls, or farmer managed controls.;Commelina diffusa established from transplants rapidly in one farm and not at all on another. Where it established it controlled weeds effectively, apparently by limiting light at the soil surface. We were unable to measure weed suppression beyond nine months after planting or competitive effects on coffee by Commelina because of its failure to establish in one site.;A model was used to simulate cover crop decomposition and nutrient release under a management plan of cutting the cover crops midway through the dry season and placing them as mulch under the coffee canopy. The model did not accurately predict Desmodium decomposition rates, possibly due to the presence of polyphenolics resistant to breakdown, a factor that the model did not incorporate. Arachis decomposition was accurately simulated and a range of nitrogen mineralization rates based on potential Arachis biomass production were predicted.;Before cover crops can be recommended for farmer use, a means of avoiding dry season moisture competition with coffee must be developed. Farmers must also consider the particular cover crop species desirable before they will adopt the practice.