Understanding the Continuous Corn Yield Penalty

by L.F. Gentry, M.L. Ruffo, and F.E. Below

 

 

photo of Alison

Alison Vogel

Graduate Assistant

amvogel2@illinois.edu

Highlights

 

As one of the Seven Wonders of the Corn Yield World, previous crop plays a vital role in maximizing corn grain yields. While many aspects contribute to a farmer’s decision to alter corn or soybean acres in Illinois, continuous corn still constitutes over 20% of corn acres despite the wide acknowledgement of a yield penalty (Figure 1). Previous research conducted by the Crop Physiology Laboratory indicated that the primary agents of yield reduction in continuous corn were nitrogen availability, residue accumulation, and the weather (Gentry et al., 2013). Our current research was designed to determine how to reduce or eliminate these causative factors, and we focused on the effects of enhanced fertility and agronomic management, and hybrid selection for increased corn yields and reduced continuous corn yield penalties (CCYP).

 

 


Introduction

After three consecutive years of disappointing yields in the Corn Belt (2011-13), farmer enthusiasm for continuous corn (CC) began to wane. Escalating corn prices have encouraged many farmers to attempt growing corn continuously (3 or more years) or to move to a corn-corn-soybean rotation as opposed to the traditional corn-soybean (CS) rotation. Although corn can be cropped continuously, it is widely accepted that there is a yield reduction compared to corn rotated with soybean. We refer to the corn yield difference for CC and CS systems as the continuous corn yield penalty (CCYP). Although it varies, the CCYP is generally in the range of 20 to 30 bushels per acre. Despite many years of agronomic research, the cause(s) for the CCYP remains unclear. A common belief among growers, however, is that the CCYP “goes away” over time, so that after 4-6 years of CC, yields increase to the same level as corn following soybean.


Does Nitrogen Fertilizer Alleviate the Continuous Corn Yield Penalty?

We initiated a study in 2005 to investigate 1) how the yield penalty changes with time in CC, 2) under what conditions increasing the N rate reduces the penalty, and 3) what is causing the penalty. The experiment was conducted from 2005 to 2010 in east-central Illinois beginning with corn produced in a 3rd year CC system or a CS rotation at six N fertilizer rates. We calculated the agronomic optimum N rate (AONR) for each rotation as the N rate at which corn yield increases associated with additional N begin to decline. We calculated the yearly CCYP as the corn yield difference between the CS and CC treatments at their respective AONRs. We found that CC treatments required more N fertilizer than the CS treatments to reach the AONR and CC produced lower yields at that N rate (Fig. 1), demonstrating that CC systems require more N fertilizer to produce lower yields than CS rotations. Averaged across all years, yield at the AONR for CC was 167 bushels/acre and for CS was 192 bushels/acre, resulting in a CCYP of 25 bushels/acre; CCYP values ranged yearly from 8.9 to 42.0 bushels/acre.

Continuous corn response to nitrogen fertilizer
Figure 1. Corn yield response to nitrogen fertilizer application rate for each year, 2005-2010, for continuous corn (CC) and corn-soybean rotation (CS). Coefficients of determination (R2-values) provided in legends. As noted in titles, continuous corn treatments were in the 3rd, 5th, and 7th years of continuous production for 2005-06, 2007-08, and 2009-10, respectively. Cross marks indicate the agronomic optimum nitrogen fertilizer application rate (AONR) for each rotation treatment.

To explore the causes of the CCYP, we tested a number of different weather- and yield-related measurements for their relationships with the CCYP. We found that with just three predictors, we could estimate the CCYP with almost 100% accuracy. The predictors were: 1) unfertilized CC yield, 2) years in CC, and 3) the difference between CC and CS delta yields.

 

Delta yield = (Maximum yield under non-N limiting conditions) - (Yield without N fertilizer)

 

The best predictor of the CCYP was unfertilized CC yield; in years when unfertilized CC yields were relatively high, the yield penalty was low, and vice versa. Unfertilized CC yield is an indicator of how much N the soil is supplying to the corn crop either from residual fertilizer N or from decomposition of previous crop residues and other organic matter sources (N mineralization). Unfertilized CC yield is also affected by weather since warm, wet soil conditions increase decomposition rates and make released N more available for crop uptake. The second predictor of the CCYP, years in CC, was also strongly correlated with the CCYP and demonstrated that the CCYP got worse with each additional year in the CC system through the 7th year, when the study was terminated. This conclusion is in contrast to the observations of many farmers in the Corn Belt who claim that corn yields in CC eventually attain the same level as CS rotations. On average, the CCYP in this study increased by 186% from 3rd year CC to 5th year CC and 268% from 3rd year CC to 7th year CC. We believe that yields declined with time in CC as a result of large amounts of accumulated corn residue in these systems. The final predictor of the CCYP, difference in delta yields between CC and CS, is probably a function of weather conditions, particularly during critical growth periods such as ovule determination and grain fill. Some weather conditions, such as drought and heat stress, can disproportionately reduce yields of the CC system relative to the CS system.

 

Intensive Management for Continuous Corn

The yield penalty associated with 11th year CC vs. long- term CS rotation was assessed using either a standard or an intensive management system, with contrasting plant populations (32,000 and 45,000 plants/acre), and eight commercially available hybrids with distinctly different genetic makeups.

 

The standard management system consisted of:

 

The intensive management system consisted of:

Results

Intensive management increased yield in continuous corn by 50 bushels per acre, and in the CS rotation by 32 bushels per acre, suggesting intensive management as a way to mitigate the continuous corn yield penalty.

 

Influence of management level and rotation on final grain yield in 11th year continuous corn and corn following soybean at Urbana, IL in 2014. Values are averaged over two populations and eight hybrids.
  Rotation Yield difference
due to rotation
System Continuous Corn Corn- Soybean  
 
bushels/acre
Standard
167
205
+38
Intensive
217
237
+20
Yield difference
due to management system
+50
+32

 

 

 


Conclusions
This study suggests that the continuous corn yield penalty (CCYP) persists for at least 11 years. However, we found that during very favorable growing seasons, increased N rates can overcome the CCYP. Higher N rates do not reduce the CCYP during average or poor growing seasons, however. This study concludes that the primary causes of the CCYP N availability, corn stover accumulation, and unfavorable weather. Given that weather cannot be controlled and the optimum N fertilizer rate can only be determined after crop harvest, managing corn stover has the greatest potential for reducing the CCYP.