From the previous section, it is clear that weeds cause considerable economic damage to agriculture and the overall U.S. economy. One of the points made was that weeds lower crop yields. If forage crop yields are lowered, there may be a corresponding decrease in animal production in a given operation. What do weeds do that causes these problems?

1. Weeds compete with forage crops for light, water, and soil nutrients. This means lower amounts of light, water, and nutrients are available to support crop growth. It has been estimated that in some cases for each pound of weed dry matter produced, crop dry matter production is reduced by one pound.

2. Weeds in hay or pasture (e.g. wild garlic, leafy spurge, cocklebur) may reduce the palatability of the forage and lower animal consumption, thus lowering animal production.

3. Weeds with toxic substances (e.g. certain alkaloids) may lower animal rate of gain, cause diseases, cause abortions, cause birth defects, or kill animals.