When should we initiate Irrigation to Enhance Corn Yield?

The Midsouth has started transitioning into summer weather and corn is growing quickly, prompting decisions of when to start irrigating corn. This dramatic environmental shift is challenging to our corn crop because it is making huge physiological strides as well. However, inadequate moisture for vegetative corn during May is not nearly as limiting as usually perceived – considering we nearly always start with a full soil profile of moisture. 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 fields or 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 high rainfall 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. Soil moisture sensor data from Mississippi fields confirm these findings and show root activity progress 36-inches deep or more in highly productive corn fields, if 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 an 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.

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 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 similar culture – and this rarely, if ever occurs in Mississippi. In fact, 10 years of data collected in the MSU Extension Corn Hybrid Demonstration Program show corn grown in dryland plots has similar kernel row number as irrigated plots. In other words, we are not sacrificing any corn yield associated with drought stress prior to tassel. Conversely, substantial soil saturation prior to tassel is a genuine issue and certainly can reduce corn yield when we overdo irrigation or receive abundant rainfall.

Keep in mind that the highest irrigated corn yields in the world are traditionally produced in areas with annual rainfall about three times less than us. This shows corn is better suited to a lot drier conditions than what we are accustomed to. In fact, a farmer who once produced world record corn yield stressed his yields would suffer if seasonal rainfall exceeded 30 inches. Accordingly, we should recognize there are negative effects associated with overabundant moisture and manage 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, irrigation should be scheduled much more generously to fully support increasing crop needs and avoid moisture deficit. A good rule of thumb is corn at V10 growth stage, which is normally 55-60 inches tall, is about two and a half weeks from tassel. Therefore, this transition between irrigation strategies should occur about a week or more 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.