Shading is the major means by which crop plants suppress weeds. Anything about the plant's structure or the way it is managed that hastens ground cover and shading will enhance a crop's ability to compete. Your goal is to get a crop to emerge from the soil first and then to either have a height advantage or grow at a faster rate than the weeds. Cultivars differ in their ability to compete with weeds. Unfortunately, the highest-yielding cultivars are not necessarily the most competitive.
Some crops compete better with weeds than others.
A few compete so well that they are called smother crops. By themselves, smother crops keep weeds in check, in part because these crops are generally solid seeded or planted in closely spaced rows. Examples of smother crops are alfalfa, proso millet, rye, sudangrass, sweetclover, and soybean for hay. Other crops, although they cannot be called smother crops, provide substantial weed control. Among these are most forage and small grain crops and perennial grass sods.