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Did you know that weeds can reduce crop yield EVEN IF the weeds don't actually use resources the crop needs? Our paper describing this phenomenon in Beta vulgaris is 'officially' published today in Weed Science. cambridge.org/core/journals/…
If you know anything about weeds, it is that they use water, nutrients, and light that the crop needs. And if the weeds use these necessary resources, then the crop can't have them. And this is true - but it is just *one way* that weeds reduce crop yield.
But plants interact in other ways too. Plants can 'sense' other plants around them. And if they sense other plants around, many plants will start to grow differently.
Think of it like this - when the weather forecast suggests a major storm coming in, it isn't unusual for people to go stock up on bread & milk. And soon EVERYONE is going to get bread and milk, because they don't want to be the one left in the storm without bread and milk.
So there's a rush a few days before the storm, with everyone trying to get their supplies, even though there's not actually a storm. It is only the threat of a storm.
This is kind of what plants do when they sense lots of other plants around. They 'prepare' themselves for the oncoming competition, because they sense they're in an area where lots of plants are growing, which means resources are going to become scarce.
One of the most scarce resources for plants growing close together is light. It seems like there's a lot of light out there, but only the tallest plants get the best quality light. So when plants sense other plants, most will try to grow tall. Taller than they normally would.
But that 'decision' by the plant to grow tall has a cost - if you put all your resources into growing tall, then you put less into making new leaves, or roots, or other things.

This trade-off is often called 'shade avoidance syndrome'.
This shade avoidance effect has been known about for quite a long time, but has only recently been studied in the context of crop/weed interactions. Clarence Swanton and others (hi @page_er!) have done substantial work on shade avoidance in corn and soybean.
We wanted to know how this affected plants that aren't grown for seed - root crops and leaf crops. So we used Beta vulgaris, which includes root crops table beetand sugar beet, and leafy vegetable Swiss chard.
We grew the Beta crops to ensure the Beta vulgaris plants had all the water and nutrients and light it needed. (We copied the design of this from some of Swanton's previous work.) The weed roots (bluegrass sod) were separated, so there was no root interaction.
And we kept the grass clipped short, so there was no direct shading of the crop. The beets had everything they needed.
But even so, the beets surrounded by weeds grew less - one of the most obvious and repeatable things we observed was that beets surrounded by weeds had fewer leaves than beets surrounded by soil.
At the end of the season, the weeds reduced leaf biomass, leaf area, and root biomass. A LOT. Way more than we expected.
So what does this all mean? Well, more than anything, I think it means we still don't have a very good understanding of *how* weeds reduce crop yields. If shade avoidance is a major contributor to crop yield loss, then this could lead to a paradigm shift in weed management.
It would mean that in the short term, keeping crops weed free at emergence is critically important. It also means using cover crops at crop establishment could have a major drawback.
But it also means that if we could figure out how to 'turn off' the early shade avoidance response in plants, then we might be able to worry *less* about weeds. Maybe even let them grow.
<obligatory plug for more public research funding>
This work has been funded by growers, Ag Experiment Station, and Federal grants (@USDA_NIFA), and without support from Federal sources for basic weed science research, this work wouldn't be possible.
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