Farmers knew crop rotations improved both crop and animal production long before scientists studied them. Recently, Thomas Sinclair, an agronomist from North Carolina State University, suggested that this 18th-century understanding formed the basis of the second green revolution.
Green Revolution 2.0 was developed in Britain, 1700-1859, with the four-field Norfolk rotation: turnips (for winter animal feed), barley, clover and wheat. Yield increased to the point where there was enough feed for horses to work the fields releasing people for jobs other than agriculture. According to Sinclair, even earlier in history ancient Sumerians (3500 to 2334 B.C.) improved yields dramatically by irrigating with nitrogen-rich Euphrates River water: Green Revolution 1.0. Eventually irrigation increased soil salinity, and crop productivity plummeted. This forced the Sumerians farther north.
Green Revolution 3.0 began in the 1950s and is the one with which we are most familiar. Manufactured nitrogen coupled with hybrids and other management factors came together to ignite yield trends in corn as well as other crops. This explosion occurred first in developed countries and then in some developing countries.
Corn following corn today
Total Iowa corn and soybean acreage has remained relatively constant since 1996 with between 22 million and 23 million acres (see chart). However, the amount devoted to either has varied considerably from year to year since 1996.
By determining the difference between corn and soybean acres in any given year, we can estimate the amount of corn following corn. I assume that corn and soybeans compete for the same acres; therefore, a reduction in one results in an increase in the other. Based on that assumption, since 1996, corn-following-corn acreage ranged from about 6% in 2001 (700,000 acres) to almost 40% in 2007 or more than 5.5 million acres!
Markets, science and tradition drove these trends. With this much of the Iowa land base in corn-following-corn production, we need to re-examine what we know of ecological systems and why crop rotations are important. Are 18th-century European management practices still relevant in the 21st-century Iowa?
Some things don’t change
Rotating corn and soybeans, even in the 21st century, usually results in maximum yield of both crops. In the very best of years, corn following corn yields as well as corn following soybean. In the worst of years, lower yields occur when corn follows corn. Iowa State research shows when corn follows corn, yields range from almost the same to 28% less than corn that followed soybeans. When averaging across all years in the dataset, the average reduction is 14%. These penalties do not disappear with multiple years of continuous corn, but instead may be masked by overall yield increases due to better genetics and management practices. Crop rotation continues as an important tool to maximize yield and profitability today.
Reasons behind lower yields
Reasons for lower yields when corn follows corn are multifaceted including allelopathy, autotoxicity, residue breakdown products, organic acids, nitrogen immobilization, lower soil temperatures especially with no-till in poorly drained soils, reduced plant stand, increased plant-to-plant variability and slower early-season vegetative growth.
Allelopathy is the suppression of growth of one plant species by another plant species, due to release of toxic substances called allelochemicals. Many crops are allelopathic when grown with other crops or when they are grown sequentially. Autotoxicity is a specific type of allelopathy. Autotoxicity occurs when the allelochemicals released from a specific crop affect that same crop planted at a later time. Corn is one of several autotoxic crops.
Nitrogen immobilization, low soil inorganic N, and inadequate N to meet early corn N needs can all result in poorer performance of corn following corn. However, the beneficial effects of starter fertilizer (nitrogen and phosphorus) are more visible with corn following corn rather than for corn following soybeans. This is especially true in areas with large residue accumulation resulting from reduced tillage systems, and with wet and cool soils.
Continuous corn can impact soil biological diversity by limiting microbial biomass for nutrient cycling. With the exception of no-till systems, corn following corn typically requires one more tillage pass than corn following soybeans. More intensive tillage affects soil’s physical, chemical and biological condition, and opens land to more erosion.
No ‘silver bullet’ answer
The risk of disease in corn-following-corn fields is greater than in fields that have been rotated to nonhost crops. Continuous cornfields, in particular those with crop residue left on the surface, are more prone to seedling diseases due to higher inoculum pressure and cooler, wetter soils.
Insects are another possible yield thief for corn following corn. Corn rootworms can adapt to corn rotation with soybeans, but are even more serious pests when corn is grown continuously. Transgenic rootworm, or RW, resistant hybrids are not the complete answer; yields of RW hybrids in corn following corn lag behind those of RW hybrids in corn following soybeans.
We’ve found no silver bullet to eliminate the effect of continuous corn. We understand that yields of corn following corn are not always penalized; some farmers are convinced they’ve beaten the system.
Certainly, markets and production costs may shout that corn following corn is more profitable in the short term. Yet the best scientific data attests that corn following itself normally reduces yield. Obviously, the performance of corn in a continuous corn system is complex and multifaceted. Even with three centuries of tradition and science, many questions remain.
Elmore is the Iowa State University Extension corn agronomist.
See www.agronext.iastate.edu/corn for more information.
ACREAGE MIX: Iowa corn and soybean acreage (scale on the left vertical axis) is shown from 1981 to 2010. Small black circles in the lower part of the chart represent an estimate of the percentage of corn following corn (C>C, scale on the right axis). Source: Data adapted from USDA-NASS
This article published in the February, 2011 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2011.