The presence of Palmer amaranth (Amaranthus palmeri) and waterhemp (Amaranthus tuberculatus [syn rudis]) was limited except for a few localized areas of west Kentucky prior to the year 2000. Between 2005 and 2010 isolated problems with control of these Amaranthus species with glyphosate in grain crops began to develop and were reported in counties in west Kentucky adjacent to major rivers including the Mississippi, Ohio, Cumberland, and Green Rivers. Several county extension agents reported that infestations of these pigweeds often occurred in fields within the floodplains. It was initially thought that excessive flooding caused a rapid spread of both Amaranthus species on bottomlands, but weed seed was also spread on some upland areas with equipment, especially combines and other equipment used at harvest.

 A survey of county extension agents in 2015 along with other observations confirmed Palmer amaranth is now present in 56 counties on an estimated 60,000 acres that extend from west Kentucky eastward to counties within the central parts of Kentucky including three counties northeast of Lexington.  Waterhemp is not as widespread compared with Palmer amaranth but occupies 34 counties on approximately 36,000 acres that include counties that border the lower and upper Ohio River valley and a few isolated counties throughout the state. Introduced populations are considered to be glyphosate-resistant biotypes. Some populations are also known to have ALS-type herbicide resistance.  Recently discovered biotypes with PPO-resistance, along with glyphosate resistance in both Amaranthus species are new threats to grain crop production within Kentucky.  Sources of introductions have included: 1) overflow or rivers and flood plains, 2) importing of harvest equipment and transportation of crops, 3) cotton seed hulls fed to livestock, 3) cover crop seed, and 4) potentially birds and other animals.

  JD Green and JR Martin

WSSA (Feb 2016)