Commercial genetically modified crops are commonly optimized to be resistant to glyphosate, which most people know as Roundup. 

Glyphosate inhibits plant growth by blocking an enzyme known as EPSP synthase, which is involved in as much as 35% of a plant’s mass. The Monsanto GMO technique, for example, involved inserting genes derived from bacteria that infect plants into a crop’s genome to boost EPSP-synthase production.  They quite cleverly used nature to make crops pesticide-resistant, so farmers can wipe out weeds from the fields without damaging their crops, which means better yields, more food at cheaper prices and a lower environmental footprint, including global warming emissions.

A new study has shown that it may be possible for transgenes like glyphosate resistance to make other plants more competitive in survival and reproduction, such as a weedy form of rice, even in the absence of the herbicide, and without genetic modification.

Genetically modified crops pass benefits to weeds by Jane Qiu, Nature News

Citation: Wei Wang, Hui Xia, Xiao Yang, Ting Xu, Hong Jiang Si, Xing Xing Cai, Feng Wang, Jun Su, Allison A. Snow, Bao-Rong Lu, 'A novel 5-enolpyruvoylshikimate-3-phosphate (EPSP) synthase transgene for glyphosate resistance stimulates growth and fecundity in weedy rice (Oryza sativa) without herbicide', New Phytologist 1 AUG 2013 DOI: 10.1111/nph.12428