End of Season Irrigation Decisions

In North America, we are finally winding down from summer into fall. In western Iowa and eastern Nebraska, the weather is cooling enough that it actually feels like football season! The summer was, as usual, filled with lots of extreme and odd weather events in various locations. However, there was also a sense of “same old, same old” to it. We all know that summer will be hot, and at some point(s) there is a significant potential for crop stress due to water shortage. Even in this area, you did not have to drive very far from a flooded Missouri River Valley to see significant drought problems.

At this time of the season, many farmers have stopped irrigating; others are still running. The question is, when should we stop? The balance to strike is to make sure the crop has enough water available to finish its development and maximize yield, but still leave plenty of room in the soil to store any off-season precipitation that comes from winter snow and rain.

The University of Nebraska has an excellent extension publication, Neb Guide G1871, titled “Predicting the Last Irrigation of the Season”. It focuses on conditions typical to Nebraska, but the strategy is consistent with any crop growing area. You assess the current growth stage of the crop, estimate the time (and amount of water) needed to finish its development, and subtract the available water in the root zone. This difference is the additional water needed from rain and/or irrigation to finish the crop.

For example (in Nebraska), with a corn crop past full dent (3/4 milk line), full maturity is approximately 7 days away. Water use to reach maturity is approximately 1.0”. If your crop is in this condition, and you have at least an inch of available water in the root zone, no problem. However, if your corn crop is barely at full dent (1/2 milk line), there are still about 13 days to maturity, with a water use expected to be 2.25”. If you have already been backing off irrigation and the soil is bone dry, it might be wise to consider a little irrigation. In soybeans, once the leaves start to yellow, there are about 10 days left to maturity, and the crop will need about 1.9” of water.

Of course, different areas of a field could be in different conditions, due to soil, crop, and topographic factors. Having the ability to use precision management techniques may be useful. While we all want to maximize yield, we also shouldn’t underestimate the value of having good, dry conditions heading into harvest. Compaction, rutting, and machinery problems will be lower if we don’t incur the extra costs of watering too much at the end of the season.

Have a great, safe and profitable harvest!