Quadrat misuse: Confessions of a cover crop researcher's biomass sampling journey

Authors: Brennan, Eric

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/12/2023
Publication Date: 8/24/2023
Citation: Brennan, E.B. 2023. Quadrat misuse: Confessions of a cover crop researcher's biomass sampling journey. Agronomy Journal. 115(5):2275-2285. https://doi.org/10.1002/agj2.21411.
DOI: https://doi.org/10.1002/agj2.21411

Interpretive Summary: When researchers sample cover crop biomass, they usually use rectangular frames made of plastic, metal, or wood to mark a small area where the cover crop shoot biomass will be sampled within a larger experimental plot. For example, quadrats are generally less than 1 m wide by 1 m long in size. Quadrat sampling saves time and provide small biomass samples that can be converted to meaningful units such as the amount of oven-dry biomass in pounds per acre or kilograms per hectare. But in fields where the cover crop has been planted in narrow rows using a grain drill, quadrats can be an inaccurate way to estimate biomass unless the width of the quadrat, that runs perpendicular to the rows, is a multiple of the row spacing. For example, in a cover crop planted in rows that are 6 inches (15.24 centimeters) apart, the correct width of a quadrat to sample three rows would be 18 inches (45.72 inches. If the quadrat width is not a multiple of the row spacing, the quadrat will either overestimate or underestimate the correct amount of biomass depending on the row spacing and number of rows that the quadrat covered. A survey of cover crop papers published during the past 5 years in two agricultural journals found that the incorrect quadrat size was used in most papers which indicates that most cover crop biomass data is inaccurate. Steps are described to correct these errors in published research, and improved sample procedures in future research. Accurate data on cover crop biomass is important to understand many different benefits of cover cropping such as the amount of biomass being added to the soil and the amount of nitrogen that a cover crop has scavenged and prevented from leaching into the groundwater. Furthermore, accurate cover crop biomass data may also help farmers comply with new regulations that contain cover crop biomass requirements.

Technical Abstract: Rectangular quadrats are commonly used in cover crop research to sample shoot biomass in fields where the cover crops are planted in narrow rows. However, these quadrats do not accurately estimate cover crop biomass unless the width dimension of the quadrat, that runs perpendicular to the rows, is a multiple of the cover crop row spacing. Examples with various quadrat sizes, row spacings, and quadrat placements are used to illustrate how researchers can inadvertently underestimate biomass by up to 49% or overestimate it by up to 27%. A survey of 92 articles of cover crop biomass data published in the Agronomy Journal and Weed Technology from 2017 to 2021 indicates that most studies used an incorrect quadrat size and thus inaccurately estimated cover crop biomass. This pervasive error raises important questions about most published cover crop biomass data and associated metrics (nitrogen uptake, weed biomass, etc.). Approaches to correct the error in published research are described along with suggestions to ensure more accurate sampling practices for future cover crop research. Accurate data on cover crop biomass is important to understand the many benefits of cover cropping and can also help farmers comply with emerging regulations that contain cover crop biomass requirements.