• Herbicides designed to protect corn plants sometimes harm them instead.Corn image by DSL from Fotolia.com
  
  Although herbicides are intended to kill unwanted plants in order to enable food crops to thrive, they sometimes have the unintended consequence of also injuring the plants they were designed to protect. In addition to injuring the plants themselves, herbicides can also harm food crops by selecting for resistant strains of parasitic plants, killing nonresistant varieties and enabling survivors to reproduce.
  
  

Amino Acid Synthesis Inhibitors

• A class of herbicides known as amino acid synthesis inhibitors act to prevent the growth of unwanted plants by interfering with their production of amino acids, which are necessary for the growth of healthy plants. But these chemicals can also interfere with the healthy growth of beneficial plants such as corn, especially if they are sprayed at the wrong point in the plant's life cycle. Corn herbicide injury symptoms from amino acid synthesis inhibitors include stunted plant growth, yellowing leaves, shortened roots and malformed leaves.
  
  

Growth Regulator Herbicides

• Growth regulator herbicides help to control unwanted plant growth by interfering with plant hormonal systems that stimulate growth. If they are applied in excessive amounts or are applied when the crops they are designed to protect are at a vulnerable stage in their own growth cycles, these herbicides can cause injury symptoms, which usually affect leaf and stem formations. Leaves can roll and crinkle, and develop irregular edges. In addition, stems may droop and curl, interfering with their ability to support the weight of the plant.
  
  

Photosynthesis Inhibitors

• Photosynthesis inhibitors kill unwanted plants by interfering with their ability to transform sunlight into plant compounds such as sugars. They are usually applied to the soil before the plants begin to grow, and therefore present the risk of working their way into every part of the plant during its developmental cycle. Injury symptoms from photosynthesis inhibitors affects leaf pigmentation, causing mottled leaves and yellowing leaves. Symptoms tend to appear on older leaves before newer ones, and they spread more rapidly on sunny days than on cloudy days.