The soil seedbank allows for long-term persistence of weed species in agricultural fields. Some weed species can persist in the soil seedbank for extended periods. Restricting inputs into the weed seedbank has a large impact on future population density and influences management practices of these weeds in soybean production systems. Harvest weed seed control (HWSC) tactics incorporate mechanical and cultural management strategies to target weed seeds present at harvest. A 3-year trial was initiated to determine if continual use of the HWSC method, narrow windrow burning, selects for earlier seed set and shattering in Louisiana soybean. No shifts in weed populations or shattering time were observed. However, there was a significant reduction in weed density and the weed seed present in the soil seedbank when HWSC and robust herbicide programs were used in combination. Therefore, utilizing multiple effective weed management strategies is imperative in reducing the soil seedbank.
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 ± 0.37% chaff, 36.05 ± 1.2% straw, and 56.7 ± 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.
The United States (US) is ranked second in the world in exporting soybean with Louisiana ranked 17th for exporting agricultural products, including soybean. Importing countries maintain high standards for the level of foreign material (FM) accepted in soybean grain shipments. While it has been identified that changes are needed, there is limited research on what comprises the FM, specifically weed seeds, in these samples. The objective of the study was to determine what proportion of the FM consists of weed seeds and to quantify those seeds in Louisiana grain elevators and in Mississippi River Valley (MRV) barges bound for Louisiana. Fifty soybean samples were taken from barges traveling down the Mississippi River Valley (MRV) as well as 56 total samples from Louisiana grain elevators. Each sample was weighed, and each fraction of the FM was separated, weighed, and counted to determine the exact proportion of each fraction. Twelve different weed seeds were found within the FM samples. The amount of weed seeds found was not consistent based on where the sample was retrieved, grain elevator or MRV barge, however the total FM was <1% for all MRV barge samples. A better understanding of the FM in soybean grain samples will allow the midsouthern US to know more accurately which weed seeds are found within grain bound for exportation.
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