Weed-Crop Competition

Project 1: Nitrogen levels and weed competition in soybean

Why study weeds?
Organic farmers have indicated that weed competition is a major concern. Although spraying and tillage are two methods of managing weeds, we wanted to explore reducing weeds’ competitive ability through an ecological framework. Previous research suggests that crops are more competitive against weeds (ie. more able to “tolerate” weeds) in organically managed compared with conventional growing systems. Many factors may contribute to this pattern, including plant resource-partitioning abilities, plant-microbe associations, and other ecological relationships.

The experiment
We conducted field experiments to quantify soil nitrogen pools and weed competition with soybean across varying soil management systems within the Cornell Organic Grain Cropping System Experiment at the Musgrave Research Farm in NY. We investigated the effects of soil management and nitrogen pool diversity on weed-crop competition. By describing poorly understood ecological mechanisms and feedbacks, our research aimed to assist the development of new integrated weed management strategies that reduce the need for herbicides and mechanical cultivation.

We compared four organic management systems for weed suppression, weed community composition, and weed-crop competition in soybean. The four organic management systems were High Fertility (“maximize yield with more inputs”), Low Fertility (“maximize profit with fewer inputs”), Enhanced Weed Management (“improve long-term results with fewer weeds”), and Reduced Tillage (“improve long-term results with improved soil”).

The Reduced Tillage system had the highest weed biomass (~1200 kg/ha). Low Fertility and Enhanced Weed Management systems had lower weed biomass (~200 kg/ha). Across all systems there was an average 9% soybean yield loss due to weeds. We concluded that the Low Fertility system was the most favorable for soybean, due to its competitive yield, good weed suppression, and low input costs.

View our presentation on weed competition in soybean for the Northeast Weed Science Society of America January 2015 meeting

Read Margaret Ball’s thesis on weed-crop competition


Does system diversity influence weed pressure?
Diversity in a cropping system (meaning, more different kinds of plants growing together) can influence soil resources, and systems with more diverse resources may have reduced plant-to-plant competition because of niche partitioning. Niche partitioning refers to how competing species use different resources and thus are better able to coexist. Can agricultural practices that increase diversity decrease weed competition on the crop?

Pots were filled with soil from the annual and perennial systems under the four levels of diversity. Some of the pots had field soil, some had sterilized potting mix that was inoculated with biota from the field soil, and some had sterilized soil that was not inoculated (the control). Then we planted one crop plant seed (sorghum sudangrass) and different densities of weed seeds (common lambsquarters). After eleven weeks in the greenhouse, we harvested the crop and the weeds and weighed them.

The pots with field soil had the nutrient and microbe footprints of the crop diversity treatments, while the inoculated pots maintained constant fertility and mostly differed in their soil microbe communities. These two potting treatments allowed us to study the relative importance of soil microbes in defining weed-crop competition.

We learned that most differences in crop tolerance to weeds were due to diversity. In pots that had soil from a more diverse cropping legacy, weed-crop competition was higher. Across all diversity levels and sources of soil, competition trends were the same in all but one instance. This means that the microbe-inoculated sterilized soil showed the same trends as the field soil, which suggests that microbe communities can replicate weed-crop competition trends.

Check out Uriel Menalled’s poster about this weed-crop competition project.

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