Crop residue inputs in the US Corn Belt have reached unprecedented levels. Historically, crop residue
retention was important to maintain soil organic matter and reduce erosion. However, at current levels
of residue production, partial residue harvest can improve productivity, profitability and environmental
performance. Since 1960, Iowa corn yields have increased at a rate of ~2 bushels/acre/year and Iowa
soybean yields have increased by ~0.5 bushels/acre/year. Because the production of grain and non-grain
portions of the plant have increased in tandem, residue production has increased at approximately 100
and 38 lbs dry matter/acre/year for corn and soybeans, respectively. As a result, harvest of 1/3 of current
corn residue levels would leave more residue on the field than was produced in crop year 1985.
Corn and soybean residue harvest can increase yield and N use efficiency of the following corn crop
(bushels/pound of N) without reducing soil organic matter levels. However, research about residue
harvest is piecemeal and results are inconsistent. In poorly drained soils of the northern Corn Belt, residue
harvest in corn following corn appears to consistently increase the yield of the following corn crop and
reduce the optimum N fertilizer rate. Similarly, soybean residue harvest in this region can increase N use
efficiency of the following corn crop (lbs N/bushel). However, in the drier western Corn Belt, residue
harvest appears to be detrimental to the following crop.
Residue harvest can also decrease environmental N losses. A multi-site study from Pennsylvania to
Nebraska found that, on average, corn residue harvest reduced nitrous oxide emissions by 6%. In the
wettest sites, the reduction was 17%. Although no study to our knowledge has measured the effect of
residue harvest on nitrate leaching, residue harvest consistently increases evapotranspiration, which is
known to reduce drainage and nitrate leaching.
These benefits may occur without reducing soil organic matter. A study performed across four Iowa
locations indicates that carbon inputs of approximately 2.8 tons dry matter/acre are sufficient to maintain
soil carbon stocks. Most Iowa corn and soybean cropping systems are producing much more residue.

In this proposal, we will test the following hypotheses through literature review and process modeling:
i) Residue harvest in continuous corn systems in the US Corn Belt can increase grain yield, reduce
the N fertilizer input required to achieve that grain yield, and reduce environmental N losses to
nitrate and nitrous oxide.
ii) Soybean residue harvest in corn-soybean rotation can increase corn grain yield, reduce the N
fertilizer input required to achieve that grain yield, and reduce N losses to nitrate and nitrous
oxide. These potential benefits will be most likely to occur in systems with reduced tillage and
cover crops.

To test these hypotheses, we will perform a meta-analysis based on published data from field
experiments that measure corn yield and the agronomic optimum N rate across different levels of crop
residue harvest. Specifically, we will search the peer-reviewed scientific literature for research that has
examined the effect of crop residue harvest on grain yield, N fertilizer requirements, and environmental
N losses in the following crop. Our search will encompass the northern Corn Belt (PA to NE) and Canada.
In addition, we will provide a brief review and summary of literature from other regions of the world to
provide context for our hypotheses and a need for further research about these questions in the United
States. Although work in other regions of the world has explored similar questions, the cropping systems
are considerably different with respect to crop rotation, crop genetics, soil type, soil management, and
drainage infrastructure. Hence, we cannot assume that results from these studies are transferable to
corn and soybean production systems. Importantly, we note that a likely outcome is that there is not
enough published literature for a thorough quantitative assessment in the northern Corn Belt.

To further assess the impact of corn and soybean residue harvest on environmental variables, such as
nitrous oxide emissions, nitrate leaching, and soil organic carbon change, we will use the Agricultural
Production Systems Simulator model (APSIM) that has been previously evaluated with field-measured
data across 15 sites in four states of the Corn Belt (Fig. 1). For continuous corn, we will simulate a corn
crop following i) business-as-usual management including local practices and, ii) a combination of three
different levels of residue removal and five different N fertilizer rates. We will perform the simulations
for 30 years (period 1990-2020) to account for the weather variability of the region. For corn-soybean
rotation, we will simulate the effect of two soybean residue levels (0 and 90% removal) on the optimum
N rate for the following corn crop. We will integrate these scenarios with cover crops and reduced tillage
to mitigate the effect of soybean residue harvest on erosion and soil organic carbon loss. We will
calibrate the model with soybean residue harvest data from our current experiments in Ames and a study published in the 1990s from Wisconsin. In addition, we will leverage studies from the Iowa N
Initiative to collect additional data from corn following soybean with soybean residue harvest.

The above meta-analysis and modeling work will lead to two primary products: a publicly available
report as soon as the work is completed and later a peer-reviewed publication. The report and
publication will blend results from the meta-analysis with the results from the process modeling work to
identify consistencies and inconsistencies. This work will be conducted through Iowa State University
and pay the salary and benefits of a post-doctoral researcher.

i) determine the combinations genetics, environment, and management that are mostly likely to
achieve, and maximize, the results described above;
ii) prompt the Department of Energy to reduce the carbon intensity score of biofuels owing to
reductions in N fertilizer and nitrous oxide associated with residue harvest;
iii) demonstrate the world-leading efficiency of Iowa crop production to grain purchasers and other
organizations with an interest in environmental outcomes.