Quantifying Nutrient and Economic Consequences of Residue Loss from Harvest Weed Seed Control

Harvest weed seed control (HWSC) methods destroy, remove, or concentrate weed seeds collected during harvest. Depending on the method of HWSC, chaff and straw fractions may also be destroyed, removed, or concentrated. Observations at soybean (Glycine max (L.) Merr.) harvest in this study estimated the distribution of aboveground biomass between seed, straw, and chaff fractions and the nutrient composition of straw and chaff. Measurements were combined to predict nutrient consequences of HWSC, which have not been documented. The average harvest index of soybean was 0.57:1. Soybean biomass that enters the combine partitions into 7.25 $\pm$ 0.37% chaff, 36.05 $\pm$ 1.2% straw, and 56.7 $\pm$ 1.2% seed. Chaff and straw residues equal 13.4% and 68.5% of the seed weight, respectively. In a soybean crop yielding 3368 kg ha⁻¹ (50 bu a⁻¹), chaff yields 9.4, 0.8, 5.0, and 0.6 kg ha⁻¹ and straw 31.6, 2.1, 1.1, and 2.0 kg ha⁻¹ of N, P, K, and S, respectively. Using 5-year average fertilizer prices ending in 2021, the cost to replace chaff, straw, and the combination of both residues is USD 1.58, USD 5.88, and USD 7.46, respectively. These results give insight into the nutrient consequences and replacement costs of HWSC.