Terminating Irrigation in Corn

Matt Foster, LSU AgCenter Corn Specialist

Article Highlights:

• Dry conditions have been prevalent in most corn producing areas of Louisiana.
• Terminating irrigation too soon during the R5 growth stage can reduce grain yield by up to 20%.
• Once a field reaches blacklayer, irrigation can be terminated.

With much of the corn crop denting (Figure 1), kernels are filling weight and size. Since heat unit accumulation has been higher this year, the crop is moving along at a much faster pace. As the corn crop approaches maturity, one of the most important management decisions you will make is when to terminate irrigation. Factors such as estimated time to maturity and soil moisture should be considered when making this decision. Corn kernels continue to gain weight and size until physiological maturity or blacklayer (R6). Previous research has shown that terminating irrigation too soon during the dent stage (R5) can reduce grain yield by up to 20%.

Staging corn at the R5 growth stage is done by monitoring the milk line (Figure 1). To see the milk line, break a corn ear in half and look at the cross-section of the top half of the ear (the side of kernels opposite the embryo.) The starchy solid interior portion moves from the top of the kernel toward the cob as the kernel matures. Kernels within the R5 growth stage are specifically designated by the progression of the milk line: one-quarter, one-half or three-quarters. Progression of the milk line and time required between each quarter are temperature, moisture, and hybrid dependent. It generally takes around 24 days for the milk line to progress through the entire kernel profile.

Following the progression of the milk line during R5 is a good way to estimate when a field will reach R6. This stage can be identified by a black layer at the tip of the kernel (Figure 1). Once a field reaches blacklayer, irrigation can be terminated since any stress has little effect on grain yield.

Figure 1. FROM LEFT, R5/dent; R5.5/half milk line; R6/physiological maturity or blacklayer. LSU AgCenter Photos.

Soybean Drought and Heat Stress

David Moseley, LSU AgCenter Soybean Specialist

Article Highlights

• The 2022 soybean acres in Louisiana increased 6% from 2021 but total production could decrease due to drought and heat stress.
• Drought and heat stress can cause limited plant growth and development.
• An increase in soybean flower and pod abscission has been noticed across the state.

Based on the USDA-NASS June acreage report, 1.15 million soybean acres were planted in Louisiana in 2022. The number of Louisiana acres planted to soybean is up 6% from 2021. The 2022 planting season progressed one to two weeks ahead of last year which would normally help produce higher yields. However, many acres of the state have had high drought and heat stress which may offset the timely planting progress.

Drought Stress

During the reproductive stages, soybean plants require approximately 0.2 to 0.3 inches of water per day. The soybean acres that were planted in mid-April began to flower in mid-May and develop seed in mid-June. According to the Louisiana Agriclimatic Information System, Chase Louisiana received more than 0.65 inches of rain over a week only twice in May through the end of June. According to the USDA-NASS report, the average percent of topsoil and subsoil that was either very short or short in moisture was 47% and 42%, respectively. The soil moisture improved after the first week of July but declined again by July 10^th, where both the topsoil and subsoil moisture was rated as 66-67% very short or short. The rate of precipitation has not kept up with the water requirement of the plants.

Drought stress in soybean can lead to leaf flipping and clamping. These defense mechanisms help protect the plant but will decrease photosynthesis. Dry soil conditions can also lead to a decrease in nutrient availability and plant uptake. In addition, the temperature of dry soils can increase faster causing additional stress to the plants.

Heat Stress

The average high and low temperatures during May and June in Chase, LA was approximately 92 and 72 degrees Fahrenheit, respectively. The average high and low temperatures were approximately four degrees higher than historical averages pulled from available weather data.

The optimum temperature for soybean is 85 degrees Fahrenheit. Soybean plants will suffer heat stress with temperatures above 85 degrees Fahrenheit, leading to a decrease in photosynthesis and limited growth and development. In addition, the plants will use the energy produced from photosynthesis less efficiently as the nighttime temperatures increase.

Flower and Pod Abscission

A combination of drought and heat will exacerbate the stress on plants leading to less energy produced and more energy spent. Under these conditions, the plants will increasingly abscise flowers and pods. Figure 1 was taken on June 30^th in a field that had not received rain or irrigation in three weeks. The plants were able to retain a few pods, however most of the pods and flowers were lost due to abscission. Soybean pod and flower abscission is normal, even in good environmental conditions, but stress will increase the rate of loss. Soybean plants can compensate for early stress by developing additional flowers (Figure 2). If the stressful conditions subside, the soybean plants can retain the flowers and develop additional pods. The seed development stage is also a critical time as the seed size can decrease if the stressful conditions continue.

Figure 1. Drought and heat stress caused an increase of flower and pod abscission.

Figure 2. Soybean plants were trying to compensate for flower and pod abscission by producing additional flowers and pods on the same nodes that contained previously developed pods. The plants previously retained and developed one pod on a node but all other flowers and pods were lost due to abscission.

LSU AgCenter Specialists