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Key Aspects for High Yielding Corn

Corn fields in Alabama are currently at a rapid growth rate period which suggests the plants’ water requirements are increasing. The elevated ambient temperatures and the lack of rain experienced in most areas of the state over the last 20 days have forced corn farmers to start irrigation much earlier than usual. Click here to see the current drought monitor conditions in Alabama.(https://droughtmonitor.unl.edu/CurrentMap/StateDroughtMonitor.aspx?AL).

Currently, our team has several on-farm demonstrations of best irrigation management practices across the state. Table 1 summarizes the 2019 growing conditions of two fields in north Alabama, one in central Alabama, and one in southwest Alabama. The most southern site is in Samson, and it has received irrigation since early May.

Table 1. Basic growing conditions of the 2019 on-farm demonstration sites of best irrigation practices.

Growing Conditions by 05/28/2019Town Creek, ALTanner, ALShorter, ALSamson, AL
Planting Date3/27/194/1/194/2/193/24/19
Growth Stage by 05/27/19V10V10V12-13V12-13
GGD by 05/27/191009101811131155
Mean Temp. MIN (0524-0527) (degrees F)67.566.266.766.1
Mean Temp. MAX (0524-0527) (degrees F)91.789.49597

Irrigation Considerations

Irrigation decisions are not always easy tasks for farmers. They must consider a number of factors.

  • How much water is applied
  • How much  water a soil can hold
  • What is the soil infiltration rate
  • How long an irrigation system must run to apply a specific rate
  • What are plant water requirements at a specific growth state

Fine-textured soils, such as clay and silt loam soils, have higher plant available water than sandy soils, which could allow farmers to space their irrigation events a little bit more. However, the current hot and dry conditions are forcing farmers to irrigate more often in order to keep up with plant water use.

Differences in soil type and terrain elevation within a field could result in differences in water availability and plant water uptake; however, most farmers apply a single irrigation rate. Tables 2a and 2b provide information about recent values of crop water use, plant available water, and irrigation rates applied over different zones of our 2019 demonstration fields.

Table 2a. Status of crop water use, plant available rate and irrigation amounts of two North Alabama sites.

Growing conditions by 05/28/2019Town Creek Zone ATown Creek Zone BTanner Zone ATanner Zone B
Soil texture - 6" (% Sand, % Clay, % Silt)10.4%, 68.8%, 20.8%40.4%, 22.8%, 36.8%12.0%, 32.8%, 55.2%25.6%, 16.4%, 58.0%
Soil texture - 12" (% Sand, % Clay, % Silt)16.8%, 26.8%, 56.4%40.8%, 22.8%, 36.4%21.6%, 55.2%, 23.2%23.2%, 26.4%, 50.4%
Soil textural classes Clay (6"), Silty Clay Loam (12") Loam (6", 12"), Clay (24")Silt Clay Loam (6"), Clay (12", 24")Silt Loam (2ft)
Predominat zone type - SSURGOAbernathy -Emory Silt LoamDecatur Silty Clay LoamDecatur Silt LoamDickson Silt Loam
Irrigation Conditions: Cummulative irrigation by May 28th (in)0.80.750.320.72
Crop water use (in) - Average May 25th to May 28th (in/day)0.2600.1100.080.12
Plant available water - PAW up to 2ft (in)4.012.672.994.78
Irrigation rate by 05/28/20190.80.750.320.72
Crop water use by 05/30/2019 (in/day)0.2000.3300.15-
Irrigation rate by 05/30/2019----


Table 2b. Status of crop water use, plant available rate and irrigation amounts of fields in Central and Southwest Alabama.

Growing conditions by 05/28/2019Shorter Zone AShorter Zone BSamson Zone ASamson Zone B
Soil texture - 6" (% Sand, % Clay, % Silt)80.0%, 14.4%, 5.6%16.8%, 49.2%, 34.0%47.2%, 30.4%, 22.4%91.6%, 4.0%, 4.4%
Soil texture - 12" (% Sand, % Clay, % Silt)74.0%, 18.4%, 7.6%18.4%, 59.6%, 22.0%46.8%, 34.4%, 18.8%91.2%, 4.4%, 4.4%
Soil textural classes Sandy Loam (6", 12"), Loamy Sand (24")Clay (2ft)Sandy Clay Loam (2ft)Sand (2ft)
Predominate zone type - SSURGOCahaba Sandy LoamAltavista Silt LoamEunola Sandy LoamAlpin Sand
Cummulative irrigation by May 28th (in)2.852.21.814.17
Crop water use (in) - Average May 25th to May 28th (in/day)
Plant available water - PAW up to 2ft (in)2.733.514.342.32
Irrigation rate by 05/28/2019--0.330.63
Crop water use by 05/30/2019 (in/day)0.360.240.440.56
Irrigation rate by 05/30/20191.250.750.870.9

Observations from On-Farm Demonstrations

Across these fields, the zones with the lowest plant available water corresponded with the highest crop water use values. If soil moisture is insufficient to meet crop water demand, then irrigation should be applied to meet those needs. Table 2b shows that the corn growing in Samson has the highest crop water use among the other fields. At this location, the crop was at the V12-V13 growth stage the week of May 25, and the crop water use was 0.21 inches/day on the sandy soil area and 0.15 inches/day on the sandy clay loam soil. By May 30, the first tassels emerged and now the crop water use has increased to 0.56 inches/day on the sandy soil areas of the field. The rapid increase on crop water use, in this case, is a combination of the high water demand because the crop is approaching tasseling, the elevated temperatures that are increasing plant transpiration, and plant water available which is soil type related.

An increase in crop water use is also being observed on the field located in Shorter now that the crop is approaching tasseling.  The crop water use by May 30 was 0.36 inches/day and 0.16 inches/day more compared to the previous week (Table 2b).

The cornfields in north Alabama are growing under different soil types and weather conditions. The accumulation of growing degree days (GDD) is lower compared to central and south Alabama which explains why those fields were at the V10 growth stage on May 27, compared to V13 growth stage in southwest Alabama. Nonetheless, an increase in crop water use has been observed in those fields in the last week of May. The first irrigation event at the Town Creek cornfield was on May 27 and by June 1, the crop water use was 0.2 inches/day (Table 2a).

The differences outlined above and presented on Tables 2a and 2b suggest that irrigation management is site-specific and under- or over-irrigation could have significant impacts on crop growth and final yield. If drought conditions progress, it will become more critical that farmers apply the right rate at the right time, and at right location within each field if variable rate irrigation is available. If farmers over-irrigate now, there is a risk they might run out of water by the time of silking and grain filling period, when the crop has the highest water demand.

Possible Consequences of Over-Irrigation and Under-Irrigation

Last year, we conducted a study in Shorter to evaluate the impact of three irrigation rates on corn yield. The test covered two soil types, a clay loam to clay soil and a sandy loam soil. We had a fully irrigated treatment, in which the soil was kept at field capacity, and the other two treatments corresponded to 66% and 33% of the full irrigation rate.  Figure 1 shows the yield results from the three different irrigation rates. Under the conditions of the clay soil, it was obvious that both over- and under- irrigation impacted corn yield. A reduction of water by 33%, 66% of the full irrigation rate applied, resulted in a higher yield compared with full irrigation which suggests that this treatment could perhaps have prevented poor soil aeration or oxygen deficiency in the root zone. Under the conditions of the sandy loam soil type, the full irrigated and the 66% of the full irrigation rate treatments showed similar results. However, the 66% of the full irrigation rate treatment showed higher irrigation water use efficiency, suggesting that the farmer could have reduced the irrigation rate by 33%. Under irrigation, 33% of the full irrigation rate treatment, resulted on yield losses across all soil types; however was severely penalized, 20 bu/ac yield loss, under Sandy Loam soil conditions.

Corn Yield Differences

Figure 1. 2018 yield differences among three irrigation rates applied under two soil types. Shorter, Alabama.


Irrigation Scheduling Tools

Irrigation scheduling, determination of the right rate or the actual crop water use that should be replenished by irrigation, and right time of application can be accomplished using different methods. The actual crop water use can be determined directly or indirectly by measuring changes in soil water content. Regardless of the method, it is important to understand the crop water use rate by growth stage and the factors that might influence it (e.g., weather conditions and soil water availability).

The use of soil sensors for irrigation scheduling is gaining attention from farmers and consultants. The main advantage of this method is that crop water use can be determined directly from soil sensor readings from several soil depths, which can lead to a more precise and accurate determination of plant water uptake. Most of the sensors available in the market measure either volumetric water content (cm3/cm3) or soil water tension (kPa).

Figure 2 shows the changes in volumetric water content at two soil depths from a Sandy Loam soil in Shorter this summer. The staircase behavior of the soil water content is an indication of plant water uptake. Minimum changes in water occur during the night (small plateau sections on the soil water curve) and the drop in water content from one night to the other corresponds to the daily water use. Higher values of plant water use were observed the last week of May compared to the second week of May (May 11 to 16) and it is observed by steeper slopes during the day period. When small or none changes in soil water are observed, especially in shallow soil layers (May 21 to 24), the crop could be experiencing water stress due to the lack of water available in the soil for the plants (Dashed oval area on Figure 2).

Changes in Volumetric Water Content

Figure 2. Changes in soil water content as indicators of plant water use and soil water movement.

Irrigation scheduling using evapotranspiration is the most widely used method. This method relates crop water use with evapotranspiration (ET). The ET value can be estimated using weather data; however, it should be multiplied by a crop coefficient (Kc) to adjust the reference ET value for local conditions and crop growth stage. Then, the crop coefficients are used to predict water use rates at different growth stages.

Figure 3 shows how corn crop coefficients change between growth stages and also the differences between Kc determination methods. The Kc values from different methods on figure 3 represent the conditions of  a corn irrigation study conducted in Shorter, Alabama in 2018 using the weather data specific to that location. Most methods agreed that the period of highest crop water use is around silking and continue until the dent stage. The methods differed on Kc values towards the end of the growing season with some methods suggesting much lower values of crop water use as the crop approaches black layer. The AU-Kc method corresponds to Kc values calculated using measured changes on soil water content in 2018. The differences between this method and the others could be due to the fact that the AU-Kc values were site-season specific and the values provided by the other methods are the result of long-term average values which sometimes do not provide an accurate estimate of the current year’s crop water use rate.

Changes in Crop Coefficients

Figure 3. Changes in crop coefficients (Kc) according to plant growth stages and determination methods.


Irrigation Scheduling Apps

Currently, there are several smartphone applications for irrigation scheduling using the crop evapotranspiration method. The University of Georgia in collaboration with the University of Florida have developed smart irrigation apps for the major row crops in the Southeast (https://smartirrigationapps.org/cotton-app/). The Cotton App has been tested for several years in south Georgia and the Corn App was released last year. The USDA National Peanut Laboratory developed the Irrigator Pro irrigation scheduling tool many years ago, and it is now also available as a phone application (https://irrigatorpro.org/#/main). Although Irrigator Pro provides irrigation scheduling information for corn, cotton, and peanut, peanut farmers have experienced good results from its use.

The information provided on this report is intended to increase awareness of the importance of irrigation rate and timing, especially in times of drought. Differences might exist between the irrigation prescriptions provided by some irrigation scheduling methods but those prescriptions will be much more accurate than the traditional “feel the soil” method. The Alabama Extension Precision Agriculture team is currently evaluating several irrigation methods and has a network of soil sensors distributed across multiple counties in Alabama and can assist you with the adoption of best irrigation management practices.

Authors: Brenda Ortiz, Associate Professor and Extension Specialist, Bruno Lena, Luca Bondesan, Guilherme Morata, Auburn University Department of Crop, Soil and Environmental Sciences


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