Roundup (active ingredient glyphosate) is the most heavily used weed killer (herbicide) in the world. Glyphosate is an organophosphorus compound that inhibits the function of an enzyme that is critical for the biosynthesis of an essential amino acid needed for plant growth. It is commonly used to kill annual broadleaf weeds and grasses that may compete with crops. Glyphosate was developed by Monsanto. Monsanto also pioneered the use of crops that have been engineered to be resistant to glyphosate, which has led to glyphosate becoming the most widely used herbicide in the world. After about 20 years of widescale use, glyphosate resistant weeds have emerged that render it less effective. The appearance of glyphosate resistant weeds prompted Monsanto and other agrochemical companies to subsequently engineer crops that are resistant to both glyphosate and other herbicides to try and reduce the impact of glyphosate resistant weeds. The proliferation of multi-herbicide resistant crops is not without controversy.
The movie above shows the effect of glyphosate on common lawn grass seedlings over a 2-day period after the grass was sprayed with a formulation commonly used in home gardens.