Yield Differences between Irrigated
and Dryland Corn in Missouri

The cost of the investment of a new irrigation system and the expenses required to operate it must basically be offset by yield increases. The actual yield increase of irrigated corn over dryland corn in Missouri varies from year to year and by region. In areas that have deep, water-holding soils the yield increases will generally be smaller than those areas that have either sandy soils or restricted rooting depths.

Sources of information that help estimate the average corn yield increase in Missouri from irrigation include 1) annual mail-in surveys and 2) UMC conducted variety trials that include both dryland and irrigated corn. When making evaluations using information from earlier sources, it is important to remember that hybrid improvement has been steadily occurring. Therefore, it may be better to look at yield increase of irrigated over dryland as a percentage, rather than as a differential amount on data from the 1970s and 1980s.

Mail-in Surveys

County-based and campus-based staff mail annual questionnaires to farmers each November or December inquiring about yields on both dryland and irrigated corn. The survey, begun in the early 1970s, incorporates data from areas in the central, northwest and southwest parts of the state. A Bootheel irrigation survey by Van Ayers, Regional Extension Agricultural Engineer, is also conducted annually. Approximately 80 farmers have responded to the surveys each year, with several reporting on multiple irrigation systems and fields.

Figure 1. Difference between Irrigated and Dryland Corn Yields Based on Mail-in Surveys

Figure 1 shows the yield increase of irrigated over dryland for the Central region (since 1978) and for the Bootheel (since 1987). The 1997 increase should be larger than normal due to dry conditions during the growing season. Average yield increase from the surveys was 41.7 and 45.5 bushels per acre for the Central and the Bootheel regions, respectively. This represents a 41.7% and a 41.5% increase over dryland for the Central and Bootheel regions, respectively. Using only 1987 to 1996 data, the Bootheel has a 5.3-bushel higher level of increase than does the Central region. The reason that higher increases may occur in the Bootheel is that it has many soils that do not store water well because they are either sandy or possess a hardpan.

One caveat on using this survey data is that results may be biased. For example, irrigation systems may have been installed on better fields, and consequently the results reflect more factors than just the presence or absence of irrigation.

Hybrid Trials

The corn hybrid trials performed by UMC can provide information on the impact of irrigation. Published data from these tests goes back to 1937. Normally about ten tests of mid- to full-season types of hybrids are done each year at locations around the state. Approximately half the sites are irrigated. In addition, the Columbia and Portageville locations have had both irrigated and dryland variety trials, in which all conditions except irrigation were kept the same.

Irrigation information can be gathered in three ways from these hybrid trials:

• The Columbia and Portageville locations, obviously, provide information since at each site one hybrid test is irrigated and the other is dryland.
• Taken collectively, the irrigated locations can be compared to the non-irrigated locations. However, the results from this may be somewhat skewed since results may reflect other factors, such as location (e.g., irrigated test locations are generally located further south than the dryland tests).
• The irrigated test results and the non-irrigated test results can be made relative by comparing them to the average county yields received that year.

Table 1 presents some of the data gleaned about the impact of irrigation from evaluation of hybrid trials. One of the benefits of data from a hybrid trial is that literally dozens of hybrids are pooled together. Irrigation tests, looking at the effect of irrigation, generally have only a single hybrid; this single hybrid may tend to be a better dryland hybrid than an irrigated hybrid, or vice versa. Another nice thing about hybrid tests is irrigated hybrids tend to have less variation, as a whole, than dryland hybrids. One important item to note is that the yield differences in the paired tests (Columbia and Portageville sites) shows a very high amount of increase in yield from irrigated to dryland (48.2 bu/acre). This also represents a very high percent increase (49.3%).

Table 1
Yields of Irrigated and Dryland Corn with Associated Yield Increase Based on UMC Hybrid Trials (after Minor et al., 1996)

                     Test Site    Paired-site    Relative 
                       Yield*       Yields**     Yields*** 
                     (bu./ac.)     (bu./ac.)        (%)    
-----------------------------------------------------------
Irrigated              173.8         146.0          188
Dryland                136.3          97.8          146
Yield Increase          37.5          48.2
Yield Increase, 
  % over Dryland        27.5%         49.3%          29.5%
-----------------------------------------------------------    

Conclusions

The yield increase in corn production in Missouri due to irrigation varies from year to year. It appears that the average annual yield increase ranges from 40 to 45 bushels per acre. If the soil has a problem storing water (e.g., sandy, on a slope, or has a restricting rootzone), the yield increase will be greater. Deep soils with good water-holding characteristic will likely have less yield increase.

An irrigation system can be viewed as an insurance policy to guarantee that some favorable level of yield will be made. In the years when rainfall is scarce during planting or tasseling, an irrigation system can greatly increase yields, and change a disaster into a profitable year.

The new yield monitoring technology that is rapidly being adopted, will help provide information on yield differences between dryland and irrigated. A field equipped with a center pivot irrigation system will be an obvious location to glean this information since the outside corners are dryland areas, while the circle itself is irrigated. These yield monitors will be a rich source of information in the future since they provide region-specific data on what yield increases might be, based on local soils, farming practices, etc.

A final avenue available to gather information as to what the yield increase may be is from the experiences of local neighbors who have adopted irrigation. These farmers can provide information on yield increases they experienced after adoption, plus data on economic factors that might be overlooked, such as costs for additional fertilizer.

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