By Daniel Hudson, UVM Extension Agronomist
Dozens of scientific papers have been written about what plants can and cannot sense. A little investigation reveals that plants are a lot more ‘aware’ than we give them credit for, and we have probably only scratched the surface. Researchers have demonstrated, to varying degrees, that plants can see, smell, and feel (physically, not emotionally), and respond to these senses. It has been demonstrated, for example, that a corn plant that is being chewed on by a caterpillar can emits compounds that attract predatory or parasitic
insects that find soft little caterpillars to be very useful. That is why some farmers (particularly organic farmers) may choose to manage the landscape around their fields to attract insects that prey upon crop pests.
Perhaps more important to conventional corn producers is research that has recently described how corn plants can actually ‘see’ and respond to weed competition before the weeds have significantly competed for nutrients, water, or light. That means that crop yield can be reduced by
weeds even if the corn plant has all of the physical resources that corn plants’ should’ need! If I were to try to summarize the classic approach to teaching weed control for row crops in one sentence, it would be something like, ‘Control the weeds before they get to be 4 inches high or they will extract too many nutrients from the soil, use too much water, or start shading your crop.” While that concept is true, we now know that it is far from complete. Weeds reflect light differently than bare soil and this difference is detected by the corn plant.
Recent research has shown that even young corn plants ‘see’ the difference in the quality of reflected light resulting from the weeds around them and alter their growth pattern in a way that increases the likelihood that the corn plant will make it to maturity in a very weedy environment.
Unfortunately, the fact that corn plants can see weeds and respond is not the good news it may seem to be. Why? The top priority of all plants seems to be to make it to maturity and produce seed – at least some seed. The priority of survival trumps (and partially negates) the theoretical genetic ability of the plant to produce optimal amounts of seed or biomass. If you were building your home in an area with a high crime rate, you would invest more in security systems and physical barriers than you would if you were to build your “dream home” in an area with no crime. This would mean that you would have fewer resources to spend on your personal swimming pool, motocross track, and shuffleboard court. It is no different with corn plants, and there are probably many similarities among other plant species. If corn plants detect weeds early in life, they make preemptive adjustments to make it through hard times, and the architecture and yield of the corn plants will be impacted. Once the plants have made the physiological commitment to being a tall-spindly plant (to stay ahead of the weeds), there is no known way to fix the problem and a certain fraction of the yield will be lost. Most crop producers and agronomists have seen this phenomenon in the field but may not have known exactly why it was taking place.
- So, what is the practical outworking of this information? Don’t assume that the only bad thing that weeds can do to your corn crop is steal nutrients from it. Evidence of local competition signals to the plant, ‘you better get going if you are going to stay ahead of the weeds.’ The corn plants know nothing of your plans to spray next week, so they take the hint and make preparations to compete with weeds for the rest of the season.
- How many weeds do I need to have before the corn plant ‘decides’ to alter its growth pattern? While we do not know exactly how many weeds we need to pass the point of no return. Many of us have probably looked at a field and said, “Yikes! Look at all those weeds…but they are less than 4” tall, so they probably won’t take much yield away….” If the visual effect of the field makes you say “yikes” it is very likely that the corn plant already got that memo. Don’t adopt the idea that every weed seed needs to germinate before you take steps to control any of the weeds.
- Is this process influenced by corn variety, environmental factors, or soil conditions? Very likely, but these particulars and relationships are not yet known.
Whether this information will change how you manage weeds in your corn acres this year or not, understanding the biology of the crops and pests we are dealing with are critical to the long-term success of your integrated pest management program.