When annual crops are grown and planted in the spring, there is a considerable amount of time when the soil is not occupied by living plants. This means that for a large portion of the year there are no living plants to take up nutrients, especially nitrate, that can leach out of the soil. This is especially a problem in the Midwest, where many soils have tile drainage, which accentuates the discharge of high-nitrate water into streams and rivers. In addition to not taking up nutrients, the lack of growing plants means that the soils are wetter and more apt to produce runoff and erosion as well as leaching. Thus, rotations that include perennial forages and winter grains help maintain or enhance the quality of both ground and surface waters. And, while intensive use of cover crops helps water quality in a similar way, cover crops should not be viewed as a substitute for a good rotation of economic crops.