Purdue weed scientists with the assistance of Purdue Extension educators and various agricultural industry reps have identified corn and soybean fields in northwestern Indiana that have Palmer amaranth infestations. At least 50 fields across five counties have been confirmed to have Palmer amaranth, with several fields having overwhelming infestations (Map 1). Palmer amaranth plants were also found along roadsides and in drainage ditches throughout the area of infested fields. This is not the first discovery of Palmer amaranth in Indiana, but is the most significant as previous populations appeared to be confined to select river bottoms in southwestern Indiana. The large number of infested acres and dense infestations in multiple fields across at least four counties indicates that these populations have been present for at least a couple of years. The majority of fields observed with Palmer amaranth infestations have survived multiple applications of glyphosate and PPO inhibiting herbicides.
The impact of Palmer amaranth
The spreading of manure from beef and/or dairy operations with feed rations that contained cotton seed/cotton seed hulls from the southern U.S. that was infested with palmer seed is suspected to be the source of the populations found in northern Indiana. The overwhelming majority of Palmer amaranth populations in the southern U.S. are glyphosate resistant and it is assumed that this is why multiple applications of glyphosate have failed to control the transplanted populations in northwest Indiana.
Palmer amaranth is by far the most competitive of the amaranth species and has had significant economical impact on cotton production in the southern United States. Entire cotton fields have been abandoned solely due to the lack of control of glyphosate-resistant Palmer amaranth. Southern producers have also reverted back to using hand rouging crews to combat competition from Palmer amaranth. Palmer amaranth plants grow rapidly in the summer heat, upwards of 2 inches/day, and can reach heights greater than seven feet. The seed production capabilities of Palmer amaranth are equally impressive with individual female plants capable of producing over 500,000 seeds.
Palmer amaranth identification
The identification of Palmer amaranth is critical to keeping track of where this weed has established itself in Indiana. Unfortunately, it is suspected that the recently identified populations have been misidentified over the last couple of years allowing it to spread without proper control. Without close inspection Palmer amaranth can be confused with other more commonly known amaranth species such as: Redroot or smooth pigweed and waterhemp. The following publications will assist you in correctly identifying amaranth species.