Corn & Soybean News

Disease Risks when Corn follows Corn

Paul Vincelli, Plant Pathology

Crop rotation is one of the most fundamental disease control practices available. Continuous corn, especially in a no tillage or conservation tillage situation, provides a continuous food source for pathogens. Several diseases can be more active under continuous corn, particularly those caused by pathogens that survive in crop residue or in the soil.

Gray Leaf Spot

This disease is a significant threat wherever corn is grown after corn. The fungus survives between growing seasons in corn residue. From there, it is spread by wind and rain to leaves of the new corn crop. Tillage practices can reduce levels of inoculum, but rotation is a key management practice. In the absence of crop rotation, susceptibility to gray leaf spot should be carefully considered when selecting a hybrid.

Management Options in Continuous Corn:

v      Hybrids with partial resistance to gray leaf spot.

Diplodia Ear Rot

Although most corn fields do not experience much damage from this disease, Diplodia ear rot can occasionally cause severe epidemics, causing rot on as many as 50-75% of the ears in a field. The fungus that causes the disease only attacks corn, and it survives between seasons in residue of corn stalks, cobs, and fallen kernels. Thus, continuous corn production–especially under conservation tillage-allows the pathogen to build up to potentially destructive levels.

Scout for Diplodia. Pull back the husks of 50 to 100 plants selected at random as you walk the field before harvest. Look for white, cottony mold growth between the kernels, which usually progresses upwards from the base of the ear. Suspect samples can be confirmed through your county Extension agent.  

Management Options in Continuous Corn:  If more than 2 to 3% of ears have Diplodia ear rot, then management steps are necessary for the next season.

v      Hybrids with tolerance to Diplodia

v      Tillage

Anthracnose Stalk Rot and Top Dieback

The anthracnose fungus survives in corn residue, and causes a variety of symptoms including leaf spotting and blighting. The leaf blight phase only affects plants at the one- to four-leaf stages, but fields should be scouted later in the season for the lower stalk rot phase and “top dieback” (upper stalk rot) phase, which can affect yield and/or harvestability. There was quite a bit of anthracnose top dieback in 2006, so I suspect inoculum levels are rather high in many fields.

Management in Continuous Corn:

v      Hybrids with tolerance to anthracnose

Pythium Seedling Diseases

Pythium microbes commonly present in agricultural soils can cause a variety of symptoms such as seed decay, pre-emergence damping off, and post-emergence damping off. However, Pythium can also infect root hairs and young rootlets, causing reduced vigor of developing plants. Pythium diseases can significantly reduce stand, vigor, and yield in continuous cropping situations.

Management in Continuous Corn:

v      Seed treatments with that target Pythium, such as metalaxyl or mefanoxam

Leaf Blights

The fungi that cause northern leaf blight (NLB) and southern leaf blight (SLB) survive in corn residue. NLB has re-emerged in the past several years as a serious limitation to yield in fields in Kentucky where susceptible hybrids are grown. SLB generally occurs at low levels in Kentucky, because past breeding efforts have led to high levels of resistance in most of the hybrids currently available.

Management in Continuous Corn:

v      Hybrids with resistance to NLB

Stalk Rots

Crop rotation has little to no direct impact on the severity of stalk rots in corn. Stalk rot incidence is influenced by high plant populations, excessive nitrogen, leaf diseases, and other factors. However, increasing continuous corn acreage could result in lengthening of the harvest season, allowing some fields of corn to stand longer before harvest. Late harvests could result in occasional increases in lodging risk and marginal increases in mycotoxin risk.

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·    Corn on corn favors foliar diseases.

·    Field scouting this year will identify potential problems for next year.

·    Management includes hybrids with tolerance or resistance to diseases.

The Yield Penalty When Corn Follows Corn

John H. Grove, Plant and Soil Sciences

Recently, there has been publicity that the yield loss associated with growing corn after corn “goes away” in fields where management is optimized for continuous corn production. This publicity is directly contradicted by UK field research, which indicates that corn grown after corn incurs a “yield penalty” that a grower should factor into his or her economic analysis.

Yields in continuous corn fields will increase over time. However, corn yields in continuous corn fields will continue to be less than corn following soybean over time. The figure directly below summarizes 18 years (1989 to 2006 production seasons) of corn rotation research, comparing corn after corn to corn after wheat/doublecrop soybean. The study began in 1983, so these yields are well after the “initiation period”. No-till corn was planted on a Maury silt loam, and was managed optimally for soil fertility, weed and insect control, but leaving disease resistance and plant available water as potential limiting factors. Corn after corn and corn after wheat/doublecrop soybean yields were graphed against season-average yields (average of the two rotation treatments). Simple linear models were fitted to the rotation by season responses.

The figure illustrates a striking separation in corn yields due to rotation. Corn after corn yield equaled or exceeded that of corn after wheat/doublecrop soybean in only 3 of the 18 seasons (1994, 1998, and 2003). There was no discernable pattern to these results, indicating that there was generally no improvement in corn after corn yield, relative to that of corn after wheat/doublecrop soybean, with better production seasons.

The yield increases over time in both rotations are likely due to a combination of improved genetics and management. Similar to the research study, growers are experiencing yield increases in their fields due to improved genetics and management. The yield increases over time are mistaken as a “removal the rotation effect”. Based on the data from this long-term field trial, corn yields would benefit from being in rotation.

Not only does corn following corn yield less than corn following soybeans, the yield differences are even greater in more productive years. In this study, the yield from corn following soybeans increased over corn following corn by 8 bu/acre for each 100 bu/acre in yield potential, as the season-average yield increased. This results in predicted differences of 16.5, 20.5 and 24.5 bu/acre at season-average yield levels of 100, 150 and 200 bu/acre. The diverging linear models indicate that the yield benefit to rotation will rise as yields rise with improved corn genetics and crop/soil management. This suggests that, in the future, the “rotation effect” will be result in larger yield differences in the presence of better varieties, better management, and excellent corn growing conditions.

Finally, while the agronomics clearly demonstrate a yield advantage to corn following soybeans, the economics may not show an advantage, When putting the pencil to the two systems, be sure to include the proper yield differences between the two rotations.

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·    The yield penalty for corn on corn exists even when yields increase over time.

·    The yield penalty for corn on corn is higher in high-yield environments.

Very Little More N When Corn Follows Corn

John H. Grove, Plant and Soil Sciences

The high corn prices have many farmers less concerned about saving money on fertilizer nitrogen (N) rates. University of Kentucky’s recommendations for N rates include factors such as soil drainage, tillage, fertilizer N timing, nitrification inhibitor use, winter annual legume cover crops and previous crop. However, UK does not give a N credit to a previous crop of soybean, wheat or grain sorghum. Land grant universities in northern states give a substantial N credit, usually 1 lb of N per bushel of previous soybean yield, against the fertilizer N rate for corn after corn. Further, many grain growers believe that the yield loss associated with growing corn after corn can be overcome with a greater fertilizer N rate.

Corn Yield Response to N

Figure 1 illustrates corn yield response to N from UK research in 2004 and 2005. No-till corn was grown after corn, doublecrop soybean, and grass/clover hay on a Maury silt loam. Rates of fertilizer N ranged from 0 to 200 lb N/acre, using ammonium nitrate, and were applied just after planting. Prior doublecrop soybean yield averaged 44 and 50 bu/acre in 2003 and 2004, respectively.

Yields for corn after corn were less than yields for corn after wheat/doublecrop soybean, regardless of N rate used. No N rate adjustment was able to compensate for the “yield penalty” associated with continuous corn.

 Nitrogen required for maximum yield was slightly higher for corn after corn (about 25 lb N/acre) than for corn after soybeans. Nitrogen requirements for both systems were within the range of UK recommendations.  (Models were fitted to corn yield-fertilizer N rate relationships. The solid vertical lines indicate the N rate where each model found no further yield response. Vertical dashed lines represent the range in UK fertilizer N rates recommended) for no-till corn following another grain crop on well-drained soil.)

If an “N” credit was used, then corn after wheat/doublecrop soybeans would be under-fertilized or corn after corn would be over-fertilized.

Will Extra N Pay?

Each farmer has heard a neighbor at the coffee chop boast about applying high rates of N and getting high yields of corn. Once in a while he may actually be right. But, is he right often enough to pay for the extra N?

Corn yield responses to N were analyzed over 6 years of data to determine how often a high rate of N pays for itself.  Assuming N prices of $0.35/lb of N and $4.10/bushel of corn the following conclusions were made.

When corn was grown after corn:

v      the first 80 lb N/acre was always profitable (100% = 6 out of 6 years)

v      increasing to 120 lb N /acre was profitable 67% of the time

v      increasing to 160 lb N/acre was profitable 33% of the time

v      increasing to 200 lb N/acre was profitable 67% of the time

Corn after wheat/doublecrop soybean:

v      up to 160 lbs N/acre was profitable most of the time

v      increasing to 200 lbs N/acre was rarely profitable (16% of the time)

Corn after grass/clover hay

v      80 lbs N/acre was profitable 60% of the time

v      Higher N rates were profitable 20 to 40% of the time.

The complete table at the end of this article.

The probability of profitable return was influenced much more by the rate of N applied than the price ratio of N to corn. At intermediate N application rates, even when the “average yield increment” would always pay for the additional N applied, there were often occasional years when that was not the case. On the other hand, the data also illustrate that there are always going to be a few years/situations where the highest rate of N is profitable. “Rare, but true” – the neighboring grower boasting at the coffee shop about one year’s economic response to high fertilizer N application rates, in one cornfield, may be telling the truth.

No N Credit

No N credit is given to soybeans in Kentucky. The mild winters in Kentucky allow crop residue to decompose.  As soybean residues decompose, N is lost, leaving little for an N credit next spring. Corn residues typically decompose such that very little residue is left to tie-up fertilizer N in the spring. Kentucky corn growers need to adjust their fertilizer N rate to the higher end of the recommended range when corn follows corn.

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·    No N credit is given to corn following soybeans in Kentucky.

·    Use the higher rate of N in the recommended rate range from AGR-1 for corn on corn.

Can You Really Handle More Corn Acres?

Tom Miller, County Extension Agent, Ballard County

Chad Lee, Plant and Soil Sciences

With $4 corn and soybeans at $7.50, the economics indicate corn is the better choice for 2007. Assuming a corn/soybean yield ratio of 3.3, soybeans need to trade near $10 to be economically competitive with corn at $4. However, deciding to grow more corn includes more than economics… it also includes changes in logistics.

Farmers in most of the grain-producing areas of Kentucky have been fairly stable in their crop rotation pattern for the last several years. Many farmers are somewhere between 40 to 50% corn with the remaining acres being in soybeans or wheat/double crop soybeans. In counties where less wheat is grown many farmers