Distribution of common cocklebur and palmer amaranth seed exiting the combine for harvest weed seed control in soybean

Harvest weed seed control (HWSC) tactics are being investigated for herbicide resistance management by reducing the number of weed seeds entering the soil seedbank. Weed seed retention and the location where weed seeds exit the combine (chaff, straw, or grain) are factors influencing potential HWSC success. An experiment was conducted in 2014 and 2015 in Keiser, AR, to determine where the seeds of common cocklebur ( Xanthium strumarium L.) and Palmer amaranth ( Amaranthus palmeri S. Wats.) exited the combine during soybean [Glycine max (L.) Merr.] harvest. Plots within soybean production fields containing infestations of common cocklebur and Palmer amaranth were harvested with a combine and the grain, chaff (top sieve), and straw (rotor) fractions were collected. The number of seeds in each fraction was determined for both species. The grain fraction contained 6 and 13% of the total Palmer amaranth and common cocklebur seed entering the combine, with 75 and 85% of the common cocklebur and Palmer amaranth seed, respectively, in the chaff fraction. As the chaff fraction is the target for many HWSC systems (e.g., integrated Harrington Seed Destructor, chaff carts, chaff lining, and chaff tramlining), these proportions indicate the potential success of these systems. The HWSC systems that target all harvest residues (e.g. narrow windrow burning and bale direct) would potentially control high proportions of common cocklebur and Palmer amaranth seed (94 and 87% respectively) during harvest. Preventing this seed from entering the seedbank would improve weed management programs and lessen the likelihood of selecting for herbicide resistance.