| Agricultural knowledge has been hindered by a combination of tradition and empiricism without theory; ecology may provide a useful theoretical framework. Ecological principles were applied to the experimental study of crop-weed interactions and the influences of cropping system, site, soil fertility, and plant mulches. The findings are in five papers. (I) Weed Control and Polycultural Advantage. In two of three sites, weed biomass was less in corn-legume polycultures than in monocultures; this effect did not increase corn yield but benefited legume yield and contributed in some cases to overyielding (RYT or LER > 1). When certain relationships are in effect among crops and weeds, it is predicted that polycultural overyielding, especially in monetary terms, will be more common where weeding input is low than where it is high. (II) A Comparison of Weed Communities in Some Traditional Cropping Systems in Tabasco. Polyculture and bean monoculture reduced weed biomass compared to corn and squash monocultures. Weed species composition reflected site and cropping period more than specific crops present, therefore it is probably not as easily manipulated as weed biomass. (III) Aquatic Mulches in a Reconstructed Chinampa in Tropical Mexico. Without mulch there was low growth of both crop (corn and cowpea) and weeds, and the yield of crops in weedy plots was not significantly different from in weeded. The addition of mulch increased both crop yield and weed growth, such that weeds reduced crop yield compared to weeded controls. Under low soil fertility, the full benefits of mulch were accrued by crops only when weeds were removed. A more apparent weed-control effect of mulch is expected where soil fertility is higher. (IV) Crop-Weed Interactions: The Effects of Plant Litter and Soil Fertility. At a site with relatively rich soil in s.e. Michigan, straw mulch enhanced corn yields by suppressing weeds and allowing crops to take advantage of nutrients. Nutrient addition was not responsible for the yield enhancement. (V) The Effects of Plant Litter and Soil Fertility on Weed Communities. Straw mulch greatly decreased weed biomass and number of species. Both mulch and corn presence decreased weed biomass, crop-fertilizer-mulch constituting the best combination for weed biomass control. Fertilizer did not affect weed community composition, and the action of mulch did not depend on soil fertility. |
