Weeds growing along field margins mature and weed seed is easily carried into fields. Controlling weeds along field margins is an inexpensive way of reducing the introduction of new weed species. Surface irrigation water has also been shown to carry many kinds of weed seed into cropland. A study conducted in western Nebraska showed that surface irrigation water could contain up to 77 different kinds of weed seed and deposit 9 seeds per square yard, or approximately 38,000 seeds per acre during an irrigation season. The removal of weed seed from irrigation water with seed screens and the control of weeds on ditch-banks provides an effective way to reduce weed seed in irrigation water.
Man also introduces weed seed into cropland.
As crop seed is planted, weed seed is often a contaminant. Each pound of alfalfa seed contained an average of 400 weed seeds per pound. If the alfalfa seeds are planted at a rate of 10 pounds per acre, approximately 4000 weed seeds will have been planted per acre. Combines also play an important role in spreading weed seed. Jointed goatgrass was spread throughout most of the wheat belt as custom combine crews moved north with the wheat harvest.