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. On a smaller scale, as a farmer combines his crops and moves from field to field, he may contaminate the entire farm with numerous weed seeds. Many weed seeds pass through the digestive tracts of animals and remain viable. When manure is used to fertilize cropland, weed seeds are systematically introduced across fields. Of the avenues of seed introduction discussed, the production of seed by mature plants in the field introduces the largest quantity of seed into the soil.