After a very wet spring, many of us have been experiencing high temperatures and a break in rainfall. Although we are busy harvesting wheat and planting wheat-beans, it is time to consider irrigating corn and full-season soybeans. With the 90-degree plus temperatures we have been having, crop water use (leaf transpiration + soil evaporation) for corn and soybeans has been upwards of 3/10^ths inch per day, regardless of planting date. However, the amount of water in the root zone is very different between crops and planting dates due to differences in rooting depth. For example, corn planted in early April has almost an inch more water in the root zone than corn planted in mid-May. For a young crop, the root system is rapidly developing and gaining an inch or more of depth per day, giving it access to more soil stored water. Roots need soil moisture to grow and our primarily concern at this stage is ensuring that a young crop does not run out of soil moisture before it can develop a deep root system.

When should I start irrigating?

For soybeans, holding off irrigation until R1 usually will not hurt yields. In some cases, delayed irrigation of soybeans has shown to increase yields. However, if it remains hot and dry into next week, I would certainly consider irrigating to prevent soil moisture from getting too far depleted and/or to help a recently planted stand get established.

Corn is a different situation. Early-planted corn, between V9 and V12, is already in the critical period of rapid growth. Even corn planted in mid-May should be at V9 by the end of next week. I would begin irrigating both as soon as possible and not stop until the ¾ milk line. Few irrigation systems can meet the maximum water usage of corn around silking which can exceed 2.5”/wk. So, it is important not to fall too far behind.

Cool temperatures, excessive rainfall, and flooding have really delayed cotton growth and development for those of us who planted in early to mid-May. In most cases, there is still plenty of time before cotton reaches the critical fruiting period. Unless you need to irrigate to help a late-planted cotton stand get established, I would hold off irrigating for now and re-evaluate the conditions at the end of next week.

The latest forecast shows lower temperatures (which could reduce crop water use to about 2/10^th” per day) as well as a good chance for rainfall on Sun. and especially Mon. If the rains pass us by, I would irrigate corn as soon as possible and soybeans as soon as they begin flowering.

Critical periods for corn water use

* Irrigation not recommended past ¾ milk-line.

Critical periods for soybean water use

* Rainfall is usually adequate until R1

Critical periods for cotton water use