When should we Start Irrigating Corn to Enhance Yield Potential?

The Midsouth has started transitioning into drier weather and corn is approaching tassel, prompting decisions of when to start irrigating corn. This dramatic environmental shift is challenging to our corn crop because it is also making huge physiological strides as well. However, inadequate soil moisture is not nearly as limiting as usually perceived – considering we always have abundant moisture virtually all spring. Thus, don’t be fooled by vegetative wilting into initiating irrigation without first checking your soil moisture and looking at the big picture. Premature irrigation is not only unnecessary, but also can be quite detrimental to corn growth and productivity in our high rainfall environment in the long term. In fact, if you have ever had dryland “corners” out-yield irrigated fields, this is probably the reason why.

The primary factor we should use to trigger initial corn irrigation is when soil moisture becomes limiting. It is quite normal to see corn leaves start rolling or wilting the first week or more we go without rain and temperatures climb into the 90’s, despite plentiful moisture in the soil. Thus, leaf wilt is a poor or unreliable indicator of genuine drought stress in our environment. Accordingly, the key factor to determine crop needs is to make a conscientious effort to evaluate soil moisture throughout the root zone. We should evaluate soil moisture availability using simple traditional tools, such as a shovel, probe or auger, or state-of-the-art soil moisture sensors to determine whether the crop needs irrigation and will respond in a manner that will ultimately enhance crop productivity. Our extensive experience with moisture sensors, usually show plentiful soil moisture present this time of year, especially six inches and deeper in the profile. It is very important to allow plants an opportunity tap into this moisture and encourage root development which will enhance plant health and productivity.

Our corn crop ranges considerably in age, depending on planting date, but most are in rapid vegetative growth stages. Not only do plants rapidly attain height during these stages, but root growth and depth increase tremendously as well, given favorable environmental conditions. Premature and unnecessary irrigation, which is common in our region, will surely retard and delay corn root development. This is important because corn plants develop about 75% of their root mass during late vegetative stages. Our soil moisture sensor data from numerous Mississippi fields confirm these findings and show root activity may progress to 36-inches deep or more in highly productive corn fields, if soil compaction and soil saturation do not restrict growth. Premature or excessive irrigation/rainfall will also promote nitrogen loss and instigate other plant nutrition issues associated with saturated soil. These issues ultimately reduce corn yield potential.

Since corn’s water demand and sensitivity to stress increase with plant size during vegetative stages (from emergence until tassel), plant growth stage also plays a very important role in irrigation, but there is not a definitive growth stage when irrigation should commence. Although corn at early vegetative stages is quite prone to wilt, it is very tolerant of water deficit, particularly prior to V9 (about 45 inches tall). Thus, there is little justification to expect yield loss resulting from water deficiency during early vegetative stages, especially when we have plentiful moisture in the soil profile. Therefore, we believe irrigation should be scheduled very conservatively until shortly prior to tassel.

A conservative irrigation strategy is warranted during vegetative stages in our high rainfall environment.

We can discuss ad-nauseam when ear size determination begins, but the first and only corn grain yield component determined prior to tassel is the number of kernel rows per ear.  If early season drought stress regularly limits corn yield potential during mid-vegetative stages, we would see considerably fewer kernel rows per ear on dryland compared to irrigated corn grown in otherwise similar culture – and this rarely, if ever occurs in Mississippi. In fact, data collected from our MSU Extension Corn Hybrid Demonstration Program show corn grown in dryland plots usually have similar or even slightly higher kernel row number than the same hybrids grown in irrigated plots. In other words, we are not normally giving up any corn yield potential associated with drought stress prior to tassel. Conversely, substantial soil saturation prior to tassel is a genuine issue and certainly will reduce corn yield potential when we overdo irrigation or receive abundant rainfall.

Historical research shows corn kernel row number does not differ depending upon cropping culture in Mississippi.

Keep in mind that the traditionally highest irrigated corn yields in the world are produced in areas with annual rainfall about three times less than ours. This shows corn is better suited to a lot drier conditions than what we are accustomed to. In fact, a former world record corn yield producer from the Corn Belt emphasized his yields would suffer if seasonal rainfall exceeded 30 inches. Accordingly, we should recognize there are negative effects associated with overabundant moisture and adapt our corn management accordingly. After all, we commonly grow our row crops on raised beds to help relieve issues with overabundant moisture.

As corn approaches the critical tassel and early reproductive stages, corn irrigation should be scheduled much more generously to fully support increasing crop needs and avoid moisture deficit or surplus. A good rule of thumb is corn at V10 growth stage, which is normally 55-60 inches tall, is 2 weeks from tassel. Therefore, this transition between irrigation strategies should occur shortly after the V10 growth stage. This strategy of using a conservative strategy during vegetative stages, followed by generous irrigation during reproductive stages is supported by long-term corn irrigation studies conducted by Kansas State University and Pioneer Hi-Bred International, Inc.

The key findings of this research include:

• Corn is much more resilient to early-season water deficit than expected.
• Initial irrigation can be safely delayed when soil moisture reserves are ample at planting and deficits are rectified prior to tassel and throughout reproductive development.
• Soil moisture in the profile was the best indicator for scheduling the first irrigation.