Weeds And Water – 5 Things That Matter


To maximize corn yields and profit in low-water environments, pay attention to weed pressure.

Admittedly, that sounds like something from an Agronomy 101 textbook, but weeds rob fields of far more water than might be thought – and that effect becomes even more critical in areas with limited water supplies.

A couple of Texas A&M studies conducted in semi-arid conditions bear out that fact.

The objective of the first of the two studies was to determine the effects of weeds on yield in a low-volume irrigation environment – 4 inches of supplemental water per year plus average in-season precipitation. That study was conducted by Jourdan Bell, Texas A&M AgriLife Extension service agronomist, and graduate student Asilinn Walton,

In West Texas, the daily water requirement for corn can reach 0.35 of an inch per day. However, many producers no longer have the well capacity to meet the crop’s full water demand, so they must optimize every drop that’s pumped out of the ground.

In year two of the initial study, the researchers added a second study with a slightly different angle. They purposely infested plots with weeds – kochia, redroot pigweed, morningglory, velvetleaf and barnyardgrass. Their goal was to determine just how much of that limited water supply was going to the weeds.

Plots included a weedy untreated check and also plots protected by a range of herbicide programs. Various in-field sensors determined water use and evaporation from the soil.

Here are five observations from the two studies, both of which were conducted in the Texas Panhandle.

#1. Weeds gulp water

Detailed data from the second study are still being compiled, but the initial results showed that 10 more inches of water were used in weed-infested plots than in plots where weeds were controlled.

If you’re counting, that 10 inches of water in weed plots is six inches more than what was supplementally applied to the crop that year. According to Bell, a lot of this wasted water came from moisture stored in the soil profile.

“Weeds have a deeper and more vigorous root system (than corn),” she points out. “They’re able to access water at deeper depths more quickly than the primary crop. They deplete the soil profile of essential stored water, which often will be needed to get a crop through later periods of high evapotranspiration demand.”

For corn, that’s usually during tasseling and pollination. In cotton, that period falls during flowering.

“If weeds use that water early in the season – and we don’t have sufficient precipitation or irrigation to refill the soil profile – then we wind up with a water deficit during peak demand,” Bell says. “That, in turn, leads to significant yield reductions.”

#2. Better kill weeds early with a preplant application

So, it was no surprise that best yields came from plots in which weeds were controlled early with a preplant herbicide application.

“If there is anything a farmer can do or is going to do, the priority is making sure that preplant application goes out,” Bell notes. “Keep the weeds down at the beginning of the season so we can put resources, including water, toward the crop.

“We sometimes see producers miss their preplant herbicide applications and not control weeds until later in the season. But our research validates that they have lost a significant portion of their yield potential to early weed pressure.”

#3. Leaves are solar panels

If the crop starts out behind due to the impact of weeds, it typically stays behind, according to Bell.

“At those early stages, we’re already determining the girth of the ear,” she emphasizes. “We also saw that early weed pressure significantly affected leaf area. The leaf is like a solar panel that allows the plant to grow in size and assimilate carbohydrates for yield. As we move throughout the season, our ear length also was affected by weed pressure.”

In plots where weeds were controlled, corn yielded 160 bushels per acre compared to 60 bushels per acre in weedy checks.

The higher yielding plants “also canopied faster, which improves light interception by the leaves and suppresses weed growth,” Bell adds.

#4. Weed control improves water use efficiency

Bell points out that as groundwater becomes increasingly limited, producers in the semi-arid environment of the Panhandle are often unable to meet the crop’s full water demands. Timely weed management can actually save water because weeds are not consuming the crop’s water and nutrients.

#5. Solid return on herbicide costs

One concern Bell hears from producers is the cost effectiveness of herbicides. “At low commodity prices, they are concerned about spending an additional $20, $30 or $40 an acre on herbicides. But when they see how much water is used as well as how much yield is lost to poor weed control, they recognize there is a return on the herbicide investment, even at low commodity prices.”

To put that another way, an extra 50 to 100 bushels an acre should more than offset the cost of an effective herbicide program.

A related perspective

An interesting explanation of the interaction between water, weeds and crops comes from Matt Hagny, an independent Kansas consultant with Pinnacle Crop Technologies.

“Crops may ‘sense’ competition from weeds and shift growth away from roots and toward aboveground vegetation to try and get ahead of whatever is competing with it,” he says. “This retardation of root growth persists all season, which adversely affects crop water efficiency.”

Hagny suggests that weeds in the row are the worst “because of the aforementioned light absorption phenomenon, especially if those weeds emerge at the same time or ahead of the crop. There are additional complexities if the weeds engage in chemical warfare, such as allelopathy or phytotoxicity, which can actively suppress the crop. Many weed species excel at this.”

As Hagny adds, “The science tends to show that every gallon of water transpired by weeds is a gallon taken from the crop. That’s an oversimplification, but roughly true.