Roundup, a Success Story with a Twist

Paul Botting

 

Most of us have used Roundup (glyphosate) in our gardens.  In fact it is hard to imagine gardening without it.  Its effectiveness as a broad-spectrum herbicide and its safety are unparalleled with no apparent effect on animals. Quick-acting and environmentally benign, it seems to be the perfect answer to weeds.

 

But its use in agriculture far outstrips its backyard use.  The convergence of no-till farming and genetically modified (GM)  crops has led to agriculture unlike any in the past.  By planting GM crops that are glyphosate-resistant (GR), farmers are able to kill the weeds with Roundup without harming the crop.  The fields are clear of weeds.  This eliminates the need for costly machine and hand-cultivation of the field and  reduces some of the pests that feed on both the weeds and the crop.  Less pesticide is required.  Because the fields are not cultivated, there is only one sixth of the soil erosion in no-till fields.  Soil compaction and fuel use is greatly reduced.

 

The history of this revolution covers a 22 year period.  Monsanto introduced Roundup in 1974.  By 1986 the mechanism by which glyphosate worked was understood well enough for scientists to begin developing, through  genetic modification,  glyphosate-resistant soybeans. In 1996, Roundup Ready soybeans reached the market.  Currently, there are GR soybeans, canola, cotton, corn, sugar beets and alfalfa, and GR soybeans make up 90% of soybeans planted in the US.  Considering how conservative farmers are, this is truly revolutionary. 

 

But there is a downside to this story.  The development of resistance to Roundup was originally thought impossible because of nature of Roundup's mechanism.  It kills by inhibiting an essential plant enzyme known as EPSPS.  Without this enzyme the plant quickly dies.  This is also the source of its safety to animals.  EPSPS is not present in animals and therefore this is a vulnerability unique to plants.  In plants,  glyphosate is absorbed quickly and moves to growth points where it kills the plants.

 

In GR weeds, Roundup is restricted to leaf tips, rather than the whole plant.  Its movement to growth points is prevented so it cannot attack the growth points. This appearance of GR weeds is a source of real worry to plant scientists and farmers.  Worldwide, there are 12 weed species that have developed this resistance.  The economic impact of this development could be immense.

 

What can be done about it? There are several attacks being investigated.  The simplest involves using traditional proven methods of weed management, such as crop rotation. Another approach is to develop GM crop varieties with higher resistance to glyphosate, thus allowing the use of more concentrated  solutions of  glyphosate.  Developing GM crops with resistance to other herbicides would allow alternating crops and herbicides to reduce the probability of resistant weeds surviving. Dicamba is one such herbicide with a long history of relatively safe agricultural use.  Experimental Dicamba-resistant versions of Arabidopsis thaliana, tomato, and tobacco have been created and used successfully in studies.  And finally, developing GM crops with resistance to multiple pesticides, and for that matter other environmental factors such as heat, drought, or insect pests could lead to much higher yields at lower costs.

 

What does this all mean to us as backyard gardeners?  First, we may not be able to depend on Roundup very much longer.  We may encounter GR weeds in our own gardens.  If GR weeds develop spontaneously in our gardens, agricultural interests will naturally want to restrict our use of Roundup.  We will have to be careful to control invasive species more effectively.  The more out of bounds such a species becomes in our gardens, the more likely it will develop glyphosate-resistance.  We may have to alternate between several herbicides to control weeds.  Hand-weeding of weeds remaining after spraying with Roundup becomes even more essential.

 

For a more in-depth article on Roundup see the 25 May 2007 issue of Science (Vol 316), p1114.  For a paper on developing Dicamba-resistant crops see p1185 of the same issue of Science.