Increasing cases of multiple herbicide-resistant weeds is a serious concern for soybean growers. The annual impact of herbicide-resistant weeds on growers was once estimated by USB at over $2 billion and has continued to increase as the problem spreads, especially for Palmer amaranth, waterhemp, and horseweed (i.e. marestail). Waterhemp is one of the most troublesome weeds in Iowa soybean production. Waterhemp resistant to six different group of herbicides (HG 2, 4, 5, 9, 14, and 27) has been reported. The multiple resistant (glyphosate, ALS, and PPO-resistant) waterhemp epidemic in soybean production has rendered valuable herbicides ineffective, increased weed control costs, prompted a reversion to tillage, and increased use of herbicides with more adverse environmental impacts. This warrants the implementation of multi-tactic approaches to manage resistant weed seed banks in soybean.
US soybean growers, researchers, and industry representatives can draw from the success of the Australian experience with herbicide-resistant weeds. Australia’s joint effort among the public sector, commodity groups, and industry resulted in ~75% adoption of integrated weed management (IWM) tactics. Australian growers incorporated a suite of non-chemical practices, known as harvest weed seed control (HWSC), into their IWM programs, which allowed them to target multiple herbicide-resistant weed populations at harvest. HWSC strategies involve collection and/or destruction of weed seeds during crop harvest, thus minimizing weed seedbank additions. Weed seeds are intercepted by a combine and separated from the bulk crop residue and grain for subsequent management. The most effective forms of HWSC include impact mills and chaff lining or chaff tramlining. The success of HWSC relies on the propensity of annual weed species to retain seeds until soybean harvest. For example, weed seed retention was greater than 90% for Palmer amaranth, waterhemp, and common lambsquarters up to three weeks after soybean maturity.
Chaff lining technology of HWSC holds a great promise in Iowa soybean production since it relatively inexpensive (investment less than $5000) with minimum modifications to the combine. A majority of the weed seeds are in the chaff fraction of the chaff + straw that get spread out from the rear of the combine. Chaff lining is a HWSC tactic that confines the chaff material between stubble rows during harvest and relies on a mulch effect to prevent or reduce weed seed germination and emergence. These chaff rows are typically established by retrofitting a combine with a baffle to separate the chaff from the crop residue or straw and using a chute at the rear of the combine that collects and places chaff into narrow rows (18-24 inches wide by 6-8 inches deep). Crop residues (the chaff fraction that includes weed seeds) are deposited directly behind the combine in those narrow rows. Based on the first research conducted in 2020-2021 in Iowa (funded by ISA), chaff lining concentrated weed seeds to less than 5% of the field, rather than spreading them across the whole field as with a conventional harvest. The concentration of the chaff material places weed seeds in an environment unsuitable for germination and emergence, if left undisturbed. Chaff lines can then be targeted with additional weed control tactics such as shielded sprayers/herbicides. The process of chaff lining, by its nature, can ultimately help decrease weed seed contamination of soybean grain.
This HWSC technology of chaff lining has not been fully tested yet in US soybean. We were the first one to use a commercial chaff liner attached to the rear of a combine to test the efficacy of this technology in 2020-2021. The focus of year 2 of this project will be on the practical implementation of chaff lining in Iowa soybean production systems and design BMP’s based upon the results of herbicide interactions with chaff lining and weed seed decay/emergence.