Calls for Research Targeted At Increasing On-Farm Irrigated Soybean Yields

Since 1970, statewide average corn yields reported by USDA-NASS for Nebraska have increased annually at a rate of 2.1 (irrigated) and 2.0 (rainfed) bu/ac, while the annual yield gains for soybean have been a respective 0.6 and 0.4 bu/ac (Figure 1).

There are two main reasons why corn yields are higher than for soybean. First, corn has a C4 photosynthetic mechanism, which makes it more efficient than C3 crops such as soybean. Second, there is a marked difference in grain composition: corn grain is mostly composed of carbohydrates while soybean seed is rich in oil and protein. There is also an additional growth penalty in soybean due to the carbohydrates that are used to sustain the nodules that capture atmospheric nitrogen and convert it into N available for plant growth.

Dividing the corn yield by the soybean yield in each year provides the corn-to-soybean (C/S) yield ratio , which is shown in Figure 2 for irrigated and rainfed crops. The C/S ratio has been relatively stable since 1970 for irrigated fields and since 1985 for rainfed fields. The C/S yield ratio in Nebraska irrigated systems is typically about 3.2, but it is only 2.8 for dryland cropping systems.

A possible explanation for the lower C/S yield ratio in dryland fields is that soybean has a wider flowering and grain-setting window than corn, which gives soybean plants more opportunity to compensate for the impact of periods of water stress. Also, the soybean pod- and seed-filling period takes place in August under relatively cooler and wetter conditions, compared with the hotter and dry conditions of July when pollination and kernel setting of corn occurred.

The 3.2 and 2.8 C/S yield ratios can be used to benchmark corn and soybean yields in a given region or year. If the C/S ratio under a given water regime departs significantly from the value indicated in Figure 2, this indicates sub-optimal management and/or unfavorable soil/weather conditions for one of the crops.

Figure 3 shows the average C/S yield ratio for each county in Nebraska, based on corn and soybean yields from the last 10 crop seasons (2004-2013). The figure shows interesting spatial patterns. First, the rainfed C/S yield ratio decreases from east (3.2 to 3.4) to west (<2.8) following the gradient in high to low rainfall. The observed pattern suggests that soybean performs relatively better than corn in low-rainfall rainfed environments. This observation is also consistent with the very low rainfed corn/soybean yield ratio of 2.3 observed in the 2012 drought year. Second, the irrigated C/S yield ratios in northeast Nebraska (>3.4) are consistently higher than the state benchmark value of 3.2. The northeast region includes Douglas, Burt, Washington, Cuming, Stanton, Madison, Pierce, Antelope, Knox, Cedar, Wayne and Dakota counties.

Figure 4 shows time trends in irrigated C/S yield ratio in northeast Nebraska since 2001 and also the statewide average C/S yield ratio trend. It can be observed that the irrigated C/S yield ratio in northeast Nebraska is consistently greater than the statewide irrigated C/S ratio across years.

The high irrigated C/S yield ratio in northeast Nebraska might suggest that

• a) irrigated corn performs relatively better in this region than in the rest of the state, or
• b) irrigated soybean performs relatively worse than in the rest of the state, or
• c) both.

To discern the reason for the high C/S yield ratio in northeast Nebraska, we looked at irrigated and rainfed corn and soybean yield trends since 2001 and compared them against the statewide average yield trends (Figure 5). We found that irrigated soybean yields in northeast Nebraska were consistently lower than the state irrigated soybean yield average. In a few words, the high irrigated C/S yield ratio in northeast Nebraska was explained by the lower irrigated soybean yields and not by greater-than-average irrigated corn yields. Possible factors that might explain the low irrigated soybean yields include incidence of soybean cyst nematode (SCN), high soil pH, and late planting dates, but the exact cause(s) is(are) unknown. This calls for additional on-farm research targeted at identifying the causes for the inferior yield performance in irrigated soybean fields in northeast Nebraska.

Patricio Grassini, Assistant Professor and Cropping-System Specialist, UNL Agronomy and Horticulture
James Specht, Professor Emeritus, UNL Agronomy and Horticulture
Bill Kranz, UNL Professor Biological Systems Engineering and Extension Irrigation Specialist
George Graef, Professor, UNL Agronomy and Horticulture
Charles Shapiro, Extension Specialist and Professor, UNL Agronomy and Horticulture/Haskell Agricultural Laboratory
Loren Giesler, Professor, UNL Plant Pathology