NE Irrigated & Rainfed Yield Trends

• Irrigated soybean yields are rising at an annual rate of 0.622 bu/ac.
• Rainfed soybean yields are rising slower, at a rate of 0.364 bu/ac yr.
• USA yields (mostly rainfed) are rising at a rate of 0.441 bu/ac yr.
• Large season-to-season fluctuations occur in rainfed soybean yields, due to unexpected intra- & inter-seasonal variance in the amount and distribution of rainfall events. 
• Irrigated soybean yields are predictably high each year, thereby making productivity-improving iinputs less risky and thus more profitable for the irrigated iproducers.
• Such investments are a likely reason why the yield difference between irrigated and rainfed production is widening, on average, by about 1/4 bu/ac per year.

Excel Chart 1: NE Soy Yield Trends
Data Source: National Agricultural Statistics Service

NE Irrigated & Rainfed Acreage Trends

• In 2008, 2.22 million irrigated acres (46%) and 2.64 million rainfed acres (54%) of soybeans were grown in Nebraska.
• Irrigated acreage, which in 1976 was less than 7%, rose to 30% in the next 15 years, and in the late 1990s spurted to a 2006 high of 48%.
• Rainfed acreage followed a similar trend, but the late 1990s spurt was not as large.  
• The 2007 dip in irrigated and rainfed soybean acreage was due to high corn prices. 

Excel Chart 2: NE Soy Acreage Trends
Data Source: National Agricultural Statistics Service

Yield Improvement in Soybean

Many producers (and even some scientists) think that soybean yield improvement is not keeping pace with corn yield improvement.  This perception arises because the yield trends of both crops are typically compared in absolute terms, as shown in the below chart:

Excel Chart 3: NE Irrigated Corn & Soy Yield Trends (absolute terms)
Data Source: National Agricultural Statistics Service

However, any argument about the pace of yield improvement requires that the yield trends of the two crops be compared in relative terms, because of these substantive physiological differences between corn and soybean:

• The photosynthetic CO2 acquistion system in corn (C-4) is much more efficient (though it requires more light energy) than the one in soybean (C-3), which makes corn intrinsically more productive.
• Corn seed is comprised primarily of carbohyrate (75%), with minimal protein (10%) and oil (5%), whereas,
• Soybean seed is high in protein (40%) and oil (20%) relative to carbohydrate (35%).
• Seed protein and oil require more photosynthetic energy to produce than carbohydrate, and thus plants producing such seed typically yield less total seed dry matter.
• To generate the amino nitrogen needed for plant proteins, both corn and soybean assimilate the nitrate absorbed from the soil by roots. 
• However, soybean also uses nitrogen (N2) fixation (occurs in soybean root nodules) to generate about half of its amino nitrogen needs,
• But, the energetic cost (to the plant) for N2 fixation is slightly greater than the energetic cost of nitrate aborption and reduction.  
• These physiological advantages for corn give rise to an intrinsic 3- to 4-fold productivity advantage to corn over soybean.   
• In Nebraska's irrigated production systems, where crop water stress is not an issue, this intrinsic productivity difference translates into a corn/soybean yield ratio of 3.24 / 1.00.

In the below chart, the corn yield units on the left axis and soybean yield units on the right axis have been coordinately adjusted to reflect the typical 3.24 corn/soybean yield ratio that has prevailed in the last quarter century in Nebraska irrrigated production fields.  The negligible difference between the two trend lines indicates that in relative terms, soybean yields have improved at a pace nearly equivalent to the pace of improvement in corn yields. 

Excel Chart 4: NE Irrigated Corn & Soy Yield Trends (relative terms)
Data Source: National Agricultural Statistics Service

Actually, as the below chart shows, the corn/soybean yield ratio in Nebraska's irrigated production systems has not changed much over the past 37 years, and was a projected 3.24 / 1.00 in 2008.  In Nebraska's rainfed production systems, the ratio was a projected 2.83 / 1.00.  

Excel Chart 5: NE Corn / Soy Yield Ratio Statisics
Data Source: National Agricultural Statistics Service