Since 1996 the large-scale cultivation of commercial transgenic (genetically engineered) crops has grown continuously, with a total of 114 million hectares cultivated in 23 countries in 2007.  Most crops are modified with one or both of two main agronomic input traits?herbicide tolerance and insect resistance.Crop tolerance to otherwise phytotoxic herbicides generally improves weed management, reduces the number and strength of herbicide applications, and allows topical application of herbicide to crop and weeds.  Topical herbicide application can replace directed spraying between crop rows and mechanical weed removal, without damaging crops.  Replacing mechanical treatment not only reduces fuel consumption but also helps conserve soil types vulnerable to erosion and compaction.  Transgenic herbicide-tolerant crops also permit more flexibility in the timing of herbicide application.  The new 'post-emergence' herbicide treatments for herbicide-tolerant crops may actually replace both 'pre-emergence' and 'post-emergence' sprays with conventional herbicides.