This blog entry is long so forgive me, but it is important to me to explain the importance of irrigating in our current conditions. I certainly appreciate, Dr. Wes Porter, UGA irrigation specialist and ag engineer weighing in on the topic by adding his expertise. Collaboration from the UGA Corn Team is always appreciated and frequently needed.

Our earliest planted corn has reached silking and pushing into R1 stage.  This means the corn is getting near the peak water use stage and is at the period where it needs over two inches per week.  The next 18 to 21 days is critical to corn yield at this stage, and our current weather patterns suggest it will be easy to get behind in irrigating if we are not paying attention.  Actually, paying attention to water needs is important even if your corn is much younger such as a V7-V9 or just now coming up.  As you know, corn is very responsive to irrigation.  Our current growing degree unit accumulations in south-Georgia closely mirroring 2012 & 2015 which were very good years for irrigated corn.  Unfortunately, dryland yields suffered.  We are getting plenty of sunlight pushing that crop to grow fast, which is very good for corn.

The past few weeks temperatures have been moderated in such order that mean temperatures are lower than normal.  Night time temperatures have certainly kept early morning and late evening times down right pleasant!!  Even with these cooler night time temperatures the growing degree units are still building rapidly.  I am seeing 35 to 38 GDU’s per day in some cases. Humidity levels are low and solar radiation in areas are high.  Keep in mind that actually this is ideal production weather for a crop like corn, extremely warm day and night time temperatures will hinder growth, so these cooler nights have only helped the crop to grow and develop at a rapid pace.  All of this together indicates that corn water use is probably higher than would normally be expected for these temperature and conditions.

The roots of a corn plant that is at VT or beyond are pulling water from the subsoil particularly if the top 8 inches are dry. Due to this if we are not careful, we can easily get behind.  We need to keep in mind that most of our irrigation systems apply water at such a high intensity that we cannot accomplish a full profile refill.  This means that due to the recent lack of rainfall our deeper moisture is starting to be used by the plant and we are not able to fully refill it with irrigation.  Thus, it is imperative that you keep this in mind and know that we are really only able to irrigate and manage the top 8 to 12 inches effectively meaning that we will use water much more rapidly than it may seem especially with the cooler temperatures.  In most sandy loam soils, we are able to store approximately one inch of water per foot of soil depth, if we only managed this part of the rooting zone we would need to irrigate every two to three days during peak water use for corn if we are not receiving any rainfall.

I found this illustration from a University of Nebraska publication. While our soils are certainly different, Nebraska soil types are close enough (sandy loams, silt loams and types that drain similar to ours) that I don’t mind using some of their great info.  The important part is to remember that corn does not extract water uniformly throughout the profile. Generally, more water is extracted from shallow depths and less from deeper depths. If water is applied to the soil surface, the typical extraction pattern follows the 4-3-2-1 rule: 40 percent of the water comes from the top 1/4 of the root zone, 30 percent comes from the second 1/4 and so on. The 4-3-2-1 rule is illustrated in this figure. IF some of you have moisture sensors, please check them daily and make sure you are keeping up with the water demand. I spoke to a consultant yesterday and he was frustrated that growers weren’t paying attention to the reports of greater and faster water use.  I am seeing corn growing very fast and creating much higher demands for water than it appears.

Let’s look at this current pattern and try to understand what is going on.  Water removed from the soil by evaporation from the soil surface and transpiration by the plant is known as evapotranspiration (ET).  For corn, evaporation can account for 20 to 30 percent of the growing season ET. Transpiration is the last part of a continuous water pathway from the soil, into the corn roots, through the plant stalks and out through leaf surfaces and into the atmosphere. I suggest that 70 to 80 percent of crop water use results from plant transpiration. The amount of daily corn water use varies with atmospheric conditions: humidity, solar radiation, air temperature and wind speed. High daily temperatures, low humidity, clear skies and high wind speed will result in a high ET demand. High humidity, cloudy skies and low wind speed will result in lower ET demand.

The water use table I referenced in my entry on May 7^th can be found in the 2020 production guide except I added the GDU’s to the earlier blog entry to give you a more complete understanding of our corn growth stages.  To get a better perspective of corn water use, the atmospheric demand must be adjusted for the growth stage to estimate crop water use on a daily basis. Currently our high atmospheric demand has resulted in a near peak crop water use in our earliest planted fields in south-Georgia because the corn roots are fully developed and the plant leaf area is sufficient to transpire water at very high rates. The same demand will result in a little lower ET in corn that is much younger and in north Georgia because the corn plant is small with a limited root zone and little leaf area to transpire water.

Currently, our evapotranspiration rates are close to .20-.24 inches per day which will increase as temperatures rise.  Again, keep in mind that most of the water use and refill capacity is occurring in the top portion of the soil and root zone.  This means that we can access approximately 1 to 1.5 inches of water (assuming a 12-18 inch rooting depth at this time).  It is important to note that of the water storage that only about 50% of it is plant available.  Thus, we will need irrigation every 2 to 4 days depending on total amount applied, total rooting depth and crop water use.  If you don’t keep up with your irrigation applications it is very easy to fall behind during this time of year, especially moving into peak water use with no significant rainfall predicted in the near future.  Though mean temperatures are lower than normal, wind speeds have been fairly high and humidity rates have been low (45% to 55%).  This combination sets the stage for greater crop demand. As temperatures rise and prospect for rains diminish, corn water use will rise rapidly.  I encourage you to pay much closer attention to a crop that responds very well to water. Please don’t get behind.