Whether you are a specialty crop farmer, grain farmer, health-minded conventional farmer, or fully committed organic farmer, weed infestations are a negative consequence of human intervention during crop production. While weather conditions might affect the growth of the problem, it is our farming systems that most affects our types of weed species and the level of nuisance they become. Competition for space, nutrients, water, and light, or hindering planting or harvesting are where plants become unwanted weeds. Keeping the advantage to the crops and not the weeds is part of making good crop management choices. In preventative weed management, like so many other aspects of farming, diversity is key. And crop rotation is how you reach this higher level of diversity. The principle is to produce an ever-changing environment that keeps weeds from getting established long enough to spread. This ever changing environment needs a crop rotation that includes different crop types with different lengths during the growing seasons. A diverse crop rotation should include annuals, biennials, and perennials. Crop types can include everything from open row grain crops like corn, to leaf-canopy crops like turnips, to competitive drilled cereals like oats or winter rye, and longer term legumes like clovers, which all together help to diversify a crop rotation. Adding beneficial plant types during different seasons, such as summer, fall, or spring planted cover crops to cover the bare soil between food crops, adds diversity to weed management. Not giving the weed seeds a chance to establish is just one benefit of many that cover crops give to a rotation. Having land that can be planted to a nutrient building ley, like a pasture or soil building mix for more than one year, flushes out those harder weed problem areas using a longer period of time to deplete the seed bank. In order to be successful at minimizing weeds and building nutrients, you need to plan out a long term crop rotation that fits your type of farming system. Be flexible to changing the plan to fix parameters like sustaining an income, following supply and demand, minimizing inputs, preserving or improving soil health, and preventing environmental issues.
(Source – http://www.ilstewards.org/words-from-woody-the-effects-of-crop-rotation-on-weeds/)