One of the most significant problems facing the field of agriculture is the development of herbicide-resistant weeds. In the fruit and nut crop industry of California, the development of glyphosate-resistance is particularly concerning. Currently, 22 weed species (worldwide) have documented resistance to the herbicide glyphosate (http://www.weedscience.org/In.asp). As of April, 2012, 13 weed species (Palmer amaranth, common waterhemp, common ragweed, giant ragweed, hairy fleabane, horseweed, jungle rice, goosegrass, kochia, Italian ryegrass, rigid ryegrass, annual bluegrass, and Johnsongrass) have glyphosate-resistant biotypes occurring in the US. Four of these species (hairy fleabane, horseweed, junglerice, and rigid ryegrass) occur in California) (http://www.weedscience.org/In.asp). Numerous authors have suggested that herbicide-resistant weeds (including glyphosate-resistant species), tend to possess a common suite of characteristics that have facilitated the development and spread of the resistance trait. These qualities include 1) an annual life cycle, 2) the possibility of multiple generations within a given season/year, 3) genetic diversity aided by the dispersal of pollen and/or seeds 4) high reproductive output (lots of seed), 5) and a rapid turnover of the soil seedbank (http://anrcatalog.ucdavis.edu/pdf/8012.pdf, http://www.weedscience.org/paper/Book_Chapter_I.pdf, http://www.extension.umn.edu/distribution/cropsystems/dc6077.html, http://ag.arizona.edu/crop/pesticides/papers/herbresis.html).
Just a quick summary of these characteristics:
Life cycle and emergence phenology: Most of the species resistant to glyphosate in the US are obligate annuals, or else can behave as annuals in certain environments. Many are capable of germinating and emerging throughout year (i.e. not just in the spring or not just in the fall). For example, scientists have observed multiple seedling flushes for both Palmer amaranth and common waterhemp throughout a traditional agronomic crops growing season. Similarly, both horseweed and hairy fleabane are capable of emerging across the calendar year, depending on soil moisture and temperature conditions.
Pollen and seed dispersal: Palmer amaranth and common waterhemp produce male and female flowers on different plants and are forced out-crossers; both species produce prodigious amounts of wind-borne pollen that can facilitate gene-flow. Hairy fleabane and horseweed produce seed that have a pappus (a tuft of hairs that assists dispersal by wind) and can be transported long distances by air currents. Even without specialized dispersal mechanisms, there is potential for most weed seeds to be moved within and between environments by other mechanisms, including on farm machinery.
Seed production: Many, if not all, of the glyphosate-resistant weed species in the US are capable of producing thousands to hundreds of thousands of seeds per plant, although the extent of seed production is likely influenced by numerous external factors such as time of emergence and the subsequent degree of inter- and intra-specific competition, temperature, and the availability of water and nutrients. Palmer amaranth plants growing in the absence of competition, and with sufficient resources can produce 500,000 to 1,000,000 seeds per female plant.
Seedbank turnover: Despite the occurrence of some seed dormancy mechanisms, all of glyphosate-resistant species in the US are likely to produce short-lived seedbanks. Results from a number of studies suggest that the seedbanks of Palmer amaranth, common waterhemp, giant ragweed, hairy fleabane, horseweed and Johnsongrass are considered to be transient (mostly 1-4 years) and will decrease over time in response to tillage (i.e. depth of burial) and environmental variables (i.e. temperature and moisture effects on inherent seed death, seed pathology and predation, etc…).
Understanding if certain weedy traits facilitate the development of resistance will help agricultural scientists and growers predict whether or not certain weed species are at risk of becoming unmanageable by chemical means. However, this knowledge, alone, is not sufficient to prevent resistance from developing; herbicide resistance is a function of the system that a weed is growing in, as well as the biology of the weed itself. The potential for a weed species to develop a resistance to an herbicide will be dependent upon the frequency with which that herbicide is applied, the use of other weed management practices (i.e. other herbicides and/or mechanical and cultural control measures), the diversity in crops and cropping practices allowed in the system, etc... To really understand evolution and spread of herbicide-resistant/glyphosate-resistant weeds, as well as their future threats to our production systems, we’ll need to fully integrate multiple crop sciences disciplines (i.e. ecology, biology, agronomy, breeding, physiology…). Fortunately, we are heading that way…here at UC Davis and at Universities across the country.