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Safely Delaying the First Irrigation of Corn

 

Safely Delaying the First Irrigation of Corn


By Tom Doerge, Agronomy Research Scientist

Key Findings of CMRA¹ Research Project

  • The vegetative stage of corn is the least sensitive to water stress and judiciously delaying the first irrigation may offer an opportunity to conserve water and maintain profitability.
  • A nine year field study was conducted in northwestern Kansas to evaluate the effects of delaying the first irrigation on corn grain yield and its physiological components.
  • Corn proved to be more resilient to early-season water stress than expected if water stress was corrected by silking time and minimized for the remainder of the season.
  • In only three of the nine years did delaying irrigation significantly depress yields, and then only for delays of four weeks or longer after the normal first irrigation date.
  • The available soil water in the top four feet of the soil profile during July (July ASW) gave the best indication of when to schedule the first irrigation.
  • Growers may be able to delay the first irrigation as late as tasseling in years of lower evaporative demand (i.e., moderate temperatures and humid conditions) provided soil water reserves at planting are ample and their irrigation system has the capacity to rapidly correct soil water deficits.
  • Fields with limited capacity wells (<0.25 inches/day), sandy soils and very low winter recharge of water in the soil profile should begin irrigation at the V4 to V6 stages.

Introduction

The corn vegetative stage prior to tasseling is considered the least sensitive stage of the crop to water stress. Because water is costly and may be limited in availability, delaying the onset of irrigation water applications during vegetative growth could be advantageous if the resulting yield reductions are not too severe. Such irrigation "macromanagement" could conserve a precious natural resource while maintaining grower profitability. The complex interactions between water availability and cost, irrigation system constraints, corn yield potential and drought tolerance of modern hybrids combine to dictate a tolerable level of vegetative-period water stress. However, a renewed understanding of the biological effects of vegetative-period water stress on corn production is warranted because of advances in plant breeding and an ever-improving array of commercial irrigation system solutions.

Irrigation delayed until July 10 Irrigation started June 5
Figure 1. Delays in the first irrigation date of corn decreased plant height but only occasionally reduced grain yields.

Therefore a study was needed to evaluate the impact of water stress events throughout the vegetative stage on corn yield components such as the number of ears/plant, kernels/ear and kernel weight.

Primary Research Objective

The primary objective of this study was to examine the effects of delaying the first irrigation during the vegetative and early reproductive periods on corn grain yield. Pertinent factors were yield and yield components as affected by irrigation dates, total water use, evaporative demand and critical levels of soil water. Dr. Freddie Lamm of Kansas State University led this study.

Study Description

Location and Irrigation Treatments - The study was conducted over nine years at the KSU Northwest Research-Extension Center near Colby, Kansas on a deep, well-drained Keith silt loam soil. In all years the same field site with a subsurface drip irrigation (SDI) system was used. The study had six irrigation treatments as main plots and Pioneer® brand corn hybrids as the split-plot treatments (3162 grown the first five years with 32B33 and 33B50 grown the final four years). All treatments were replicated three times in a randomized complete block (RCB) design. Irrigation was scheduled as needed by a climate-based water budget except for the specific treatment delays as shown in Table 1.

Table 1. Irrigation delay treatments applied to corn hybrids during 1999-2007 field experiments with average dates of the first irrigation and total seasonal water application amounts.

Averages across
1999-2007
Delay of first irrigation after normal date (weeks)
0 1 2 3 4 5
Date of 1st irrigation 12-June 21-June 30-June 6-July 13-July 20-July
Total water applied (in) 15.5 15.1 14.9 14.6 14.6 14.2


The corn silking period typically occurred between July 15 and 20 (Figure 2). In some years, excessive rainfall between two adjacent treatment initiation dates necessitated a one-week delay for the remaining treatments. After the initiation of the first irrigation, SDI was scheduled to provide 0.4 inches/day until calculated soil water deficits were eliminated. A neutron probe was used to monitor soil water content.

Figure 2. Corn is most vulnerable to water stress at silking.

Cultural Practices and Yield Measurements Taken - Corn plots were planted in late April to early May using standard cultural practices for the region. The average plant populations were 32,500 plants/acre for 3162 and 28,500 for 33B50 and 32B33. Corn yield components of crop yield, including plants/area, ears/plant, and kernel weight were measured by hand harvesting a representative sample from each plot. The number of kernels/ear and kernels/acre were then calculated.

Results

Did delaying the first irrigation affect corn yields?
Yes, but not every year. In only three of the nine site-years did delaying the first irrigation significantly lower grain yield for all hybrids grown that year. And yields were not significantly impacted in any year unless the first irrigation was delayed by at least four weeks. In the three years when delayed irrigation curtailed yields, the average yield losses for delaying four and five weeks before making the first water applications were 11 and 42 % respectively.

Is it safe to delay irrigation of corn in the Western Great Plains until mid to late July and save all water inputs until the critical flowering growth stages?
No. These research plots generally started the season with reasonably full silt loam soil profiles and most irrigators do not have adequate irrigation capacity (i.e., gpm/acre) to quickly alleviate severely depleted soil water reserves. In addition, it is difficult to infiltrate large amounts of water into the soil quickly with sprinkler and surface irrigation systems without causing runoff problems. These study results describe the corn plant's innate ability to tolerate vegetative period water stress.

Then how long is it safe to delay the first irrigation?
Growers can safely delay the first irrigation longer into the season when soil water reserves at planting are ample and irrigation system capacities are sufficient to avoid soil water deficits through the critical flowering stages and beyond. Fields with limited capacity wells (<0.25 inches/day) and very low winter recharge should begin irrigation at the V4 to V6 stages. In no cases should the first irrigation be delayed until after the tasseling stage (Figure 3), which typically occurs in about the first week of July in the Western Great Plains. Sandy soils will also require irrigation earlier in the season if precipitation is low.

Figure 3. Relieving crop water stress prior to the critical pollination stage resulted in little or no loss of grain yield in delayed irrigation experiments over nine years in NW Kansas.

Did early-season water stress result in reduced yields when water stress was fully corrected by the flowering stage and for the remainder of the season?
No. Final grain yield was largely determined by the number of kernels/acre with little influence of kernel weight (Figure 4). This indicates there was little or no effect on the grain filling stage imposed by the vegetative and early reproductive period water stress.

What measurement gives the best indication of when to begin irrigations in the early-season?
Kernels/acre (i.e., yield) was found to be reasonably well related to both July water use and the available soil water in the top 4 ft of the soil profile during July or "July available soil water" (July ASW). The July ASW value is the preferred soil water index to use for irrigation scheduling. This is because it is easy to measure and to project into the future, whereas July water use isn't known until the end of the month.

Figure 4. Relative corn grain yield as affected by relative kernels/area in an early season corn water stress study, KSU-NWREC, Colby, Kansas, 1999-2007.

At what July ASW value do irrigations need to be initiated?
The two curves in Figure 5 indicate the minimum levels of July ASW at which to initiate irrigation in years of low and high evaporative demand. During years of low evaporative demand (i.e., mild and humid conditions), water was extracted from the soil profile without much detriment to kernels/area (top line). In this case, relative July ASW can safely be as low as 0.4 (40% of the water holding capacity in the top four feet).

However, severe yield reductions occurred for similar soil water conditions in years with high July evaporative demands (i.e., hotter and windier conditions). Under those conditions, July ASW levels below 0.7 (70% of the water holding capacity in the top four feet) resulted in very pronounced yield losses.

Figure 5. Relative kernels/area as affected by July minimum available soil water in the top 4 ft of soil. The upper and lower lines are manually drawn to illustrate years with smaller and larger July evaporative demand, respectively.

1The Pioneer Crop Management Research Awards (CMRA) Program provides funds for agronomic and precision farming studies by university and USDA cooperators throughout North America. The awards normally extend for three years and address crop management information needs of Pioneer agronomists, sales professionals and customers.

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