Shade and soil moisture as factors in competition between selected crops and field bindweed, Convolvulus arvensis

Crops vary widely in their ability to compete with bindweed. This is indicated by the survival of bindweed over the periods of controlled cropping reported in this study (Table 1). Under certain conditions, a crop and bindweed may grow together more or less normally without any apparent competition. During the seasons of ample moisture on fertile soil, spring-sown oats and bindweed develop normally as companion plants with neither apparently reducing the available supply of essential growth elements to the detriment of the other. The same conditions often exist where corn is growing on bindweed-infested land. When the essential growth factors, such as soil moisture or soil nutrients, are not readily available in quantities sufficient for the development of both the crop and weed, competition develops and competitive forces are reflected in the reactions of the plants. This competition ultimately determines the dominance of the associated species or individuals. Field observations indicate clearly that where competition develops for soil moisture, bindweed competes successfully with practically all crop plants. This study indicates that in southwestern Minnesota, competition for available soil moisture is not normally the prime competitive factor in controlling field bindweed growth with crop plants. The soil moisture data obtained in 1939 and other seasons indicate that bindweed, growing normally and undisturbed, reduces soil moisture almost as rapidly as where a crop of rye is growing in competition with it. Therefore the growth of bindweed would not be expected to be greatly impaired if the rye was competing for this factor alone. When soil moisture was maintained at a high level by a preceding period of intensive cultivation, bindweed growing in competition with sorghum or soybeans produced abnormal growth and was forced into dormancy, indicating quite clearly that soil moisture was not the critical factor. Further, when the rye, soybeans, or sorghum was harvested, bindweed resumed active growth without additional soil moisture. This study indicates that, where soil moisture can be conserved or is ample and where essential soil nutrients are plentiful, light is the prime factor around which competitive forces develop. Furthermore, selected crops used in this study can be manipulated by cultural practices so that competition for light becomes a limiting factor in bindweed development and an important practical control measure. The choice of crops and the cultural practices used in their production are most important factors in predetermining the outcome of this controlled competition. Only when the bindweed is shaded sufficiently by the canopy of the crop are these forces operative. Fall-sown rye or wheat, when preceded by a period of intensive cultivation, intercepts such a high percentage of available sunlight that the subsequent growth and development of the bindweed are hindered. Alfalfa has the inherent ability to compete more successfully with bindweed for soil moisture and soil nutrients than any other crop included in this study. It is also an excellent competitor for sunlight. Maintenance of stands is perhaps the limiting factor in the use of alfalfa in bindweed control as the balance of competitive forces eventually eliminates weaker members of the crop population as well as the weed. The use of summer-planted competitive crops allows the farmer to take advantage of a time when bindweed can be cultivated most effectively i.e., during the period of greatest growth vigor. Millet, sorghum soybeans, and Sudan grass demonstrated their ability to germinate and grow rapidly when planted in midsummer and to maintain a deep, dense shade canopy above their attenuated bindweed companions. Hemp and sunflowers lack the uniformity and rapidity of growth to meet these requirements as is shown by the extent of bindweed survival. Soybeans and alfalfa show a lower daily fluctuation in shade value than do any of the crops of the grass family that were studied. Plants whose leaves roll or fold in response to transpiration deficits show greater daily variations in shade value than do those lacking this response phenomenon.