When Should we Start Irrigating our Corn?

The first week or two we go without rain and temperatures climb well into the 80’s, we are likely to see corn leaves start rolling or wilting. However, evaluating soils normally clearly reveal more than ample moisture levels just a few inches deep. After all, we have been too wet all spring. Thus, leaf wilting is not a reliable indicator of soil moisture and crop needs during early vegetative stages in our environment. Our moist spring weather significantly limits corn root depth until soils begin to dry, so we may have a lag period where vegetative growth overextends the root development, particularly when higher temperatures persist near the onset of rapid growth stages.

These conditions prompt much conversation about the proper time to initiate irrigation for corn. The simple answer is that irrigation should commence whenever soil moisture becomes limiting. However, we need to do more than see leaf wilting in order to accurately assess soil moisture. We should evaluate soil moisture availability using anything from simple methods to sophisticated soil moisture sensors to determine whether the crop actually needs more moisture and will respond in a manner that will ultimately enhance crop productivity. Approximately 75% of corn root growth occurs during these later vegetative stages, and unnecessary irrigation will definitely retard or delay its development. Of course, needless irrigation 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 increases 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 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 hybrids grown in dryland plots have a slightly higher kernel row number than the same hybrids grown in irrigated plots. In other words, we are certainly not generally 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. Thus, 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 must acknowledge there are negative effects on row crops associated with overabundant moisture.  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.