Management of Palmer amaranth in Illinois
Palmer amaranth is a weed species that must be thoughtfully and carefully managed; simply attempting to control Palmer amaranth often leads to ineffective herbicide applications, substantial crop yield loss, and increasing weed infestations. Ignored or otherwise not effectively managed, Palmer amaranth can reduce corn and soybean yield to near zero. The threat of Palmer amaranth during the 2014 growing season is very real across a large portion of Illinois.
In January 2014, the weed science program at the University of Illinois developed recommendations for management of Palmer amaranth in agronomic crops. These recommendations were developed in accordance with the somewhat unique growth characteristics of this weed species. The goals of the recommendations are twofold: 1) to reduce the potential for Palmer amaranth to negatively impact crop yield, and 2) to reduce Palmer amaranth seed production that ultimately augments the soil seed bank and perpetuates the species.
Before delineating the specific management recommendations, we present three general principles of Palmer amaranth management:
1. Prevention is preferable to eradication. Prevention refers to utilizing tactics that prevent weed seed introduction and weed seed production. Palmer amaranth is not native to Illinois, so any population discovered in the state originated from seed that somehow was moved into the state. The myriad of ways in which Palmer amaranth seeds can be transported, however, makes preventing seed introduction extremely challenging. Once Palmer amaranth populations become established, utilizing any and all tactics to prevent seed production becomes of paramount importance.
2. It is not uncommon for annual herbicide costs to at least double once Palmer amaranth becomes established. There are simply no soil- or foliar-applied herbicides that will provide sufficient control of Palmer amaranth throughout the entire growing season. At least three to five herbicide applications per growing season are common in areas where Palmer amaranth is well established.
3.Control of Palmer amaranth should not be less than 100 percent; in other words, the threshold for this invasive and extremely competitive species is zero. Female Palmer amaranth plants produce tremendous amounts of seed and in less than five years a few surviving plants can produce enough seed to completely shift the weed spectrum in any particular field.
Species Biology: Germination and emergence of Palmer amaranth
Palmer amaranth seed germination and seedling emergence are best described as continuous. Similar to waterhemp, multiple Palmer amaranth emergence events are possible throughout much of the growing season. However, previous research has demonstrated that Palmer amaranth seed has a higher germination rate than most other Amaranthus species (including waterhemp), and demonstrates a germination percentage higher than waterhemp at both low and high temperatures. These germination and emergence characteristics help explain why Palmer amaranth can seemingly “displace” waterhemp from a field within only a few years after Palmer’s introduction. Palmer amaranth that emerges before waterhemp in the spring and later in the growing season after waterhemp emergence has stopped, gives the species a competitive advantage over waterhemp and most other weed species.
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