According to the CalFlora website (http://www.calflora.org/), 21 species of amaranths occur (to some extent) in California. While many are non-native, a few, including prostrate pigweed (Amaranthus blitoides) and Palmer amaranth (Amaranthus palmeri), are indigenous. While it may be convenient to lump all of the genus Amaranthus together when considering weed management options, proper identification is important for understanding the potential for crop yield loss (not all amaranths were created equal with respect to competitiveness) and the possibility of herbicide resistance (populations of Palmer amaranth with resistance to glyphosate have been confirmed in the...

Talking Points:

Weed control failures can and do occur.

Weed control failures occur for many reasons, including improper treatment applications and plant size and development at the time of treatment.

Plants that aren't completely controlled can produce viable seed and re-infest fields.

Insufficient control of herbicide-resistant weeds could facilitate the establishment and spread of undesirable traits.

Introduction:

Weed pressure, and the resulting competition for water and nutrients, can significantly impact crop establishment, growth, yield and harvest. Furthermore, there is some concern among growers that non-managed weeds may support...

Glyphosate-resistant (GR) Palmer amaranth is, currently, the most significant weedy pest of cotton grown in the SE US. In order to reduce population size, maximize herbicide efficacy and prevent the development of further resistances, cotton growers must consider using additional mechanical (i.e. tillage) and cultural (i.e. cover crops) weed management strategies. Small plot experiments conducted in Georgia between 2008 and 2010 showed that the use of deep-tillage plus a heavy rye cover crop, when paired with a herbicide program consisting of both residual and POST-applied...

Since it was first identified in 2004, glyphosate-resistant Palmer amaranth has become the most significant weedy pest of cotton in the Southeastern United States. When acceptable control is not realized, and Palmer amaranth is allowed to set seed, population densities can become quite high in infested fields. For example, research conducted by the University of Georgia indicated that Palmer amaranth seed densities exceeded 35,000 seeds per m² in a cotton field where the glyphosate-resistant biotype had been ineffectively managed.

To combat Palmer amaranth, some cotton growers in Georgia have resumed using pre-plant deep-tillage and in-crop cultivation in fields with high weed population densities....