When is the Right Time to Start Irrigating your Corn?

As a lot of our March-planted corn progresses through late vegetative stages nearing tassel, most of the Mississippi corn crop has not generally received any supplemental irrigation. This is a bit uncharacteristic, but certainly no cause for alarm. In fact, moderate or sporadic rainfall and negligible soil saturation during corn vegetative stages relative to normal should improve corn root development and potential productivity in our climate. However, as corn plant size increases, so does water demand, so we will likely face decisions on initiating irrigation soon.

This prompts much conversation about the proper time to initiate irrigation for corn. The simple answer is that irrigation should commence whenever soil moisture becomes limiting. We often see corn leaves start rolling or wilting the first week or more we go without rain and temperatures drastically climb into the upper 80’s, despite plentiful moisture in the soil. However, leaf wilt is not a very reliable indicator of genuine drought stress in our environment. Thus, 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 anything from simple methods to state-of-the-art soil moisture sensors to determine whether the crop actually needs more moisture and will respond in a manner that will ultimately enhance crop productivity. Corn root growth and depth increase tremendously during late vegetative stages, as plants develop approximately 75% of root mass. Soil moisture sensors we’ve monitored in numerous Mississippi corn fields confirm root activity progresses from about 12-inches to ultimately 36-inches deep or more during late vegetative stages, if soil compaction and soil saturation do not limit growth. Premature and unnecessary irrigation, which is often the tendency in our region, can definitely retard or delay this corn root development. Of course, excessive irrigation/rainfall and soil saturation will stunt plant growth, reduce corn yield potential and promote nitrogen loss as well.

Since corn’s water demand and sensitivity to stress increase with plant size during vegetative stages (from emergence until tassel), plant growth stage plays a very important role, 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. 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 regularly limits corn yield potential during mid-vegetative stages, we would see considerably fewer kernel rows per ear on dryland corn 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 Demonstration Program show corn grown in dryland plots have a 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, overabundant pre-tassel rainfall (or excessive irrigation) is a genuine issue and certainly will reduce corn yield potential when we get too much of a good thing.

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. So 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 in order 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 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 published on the Pioneer Growing Point Website: Safely Delaying the First Irrigation of Corn.

The key findings from this research include:

1. Corn is much more resilient to early-season water deficit than expected.
2. 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.
3. Soil moisture in the profile was the best indicator for scheduling the first irrigation.