Disease Risks when Corn follows Corn
Paul Vincelli, Plant Pathology
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.
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.
Options in Continuous Corn:
partial resistance to gray leaf spot.
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
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.
Options in Continuous Corn: If more than 2 to 3% of ears have
Diplodia ear rot, then management steps are necessary for the next
tolerance to Diplodia
Stalk Rot and Top Dieback
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.
in Continuous Corn:
tolerance to anthracnose
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.
in Continuous Corn:
treatments with that target Pythium, such as metalaxyl or
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.
in Continuous Corn:
resistance to NLB
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
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Corn on corn favors foliar diseases.
Field scouting this year will identify potential problems for next
Management includes hybrids with tolerance or resistance to
Penalty When Corn Follows Corn
John H. Grove, Plant and Soil
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.
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
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.
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.
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
More N When Corn Follows Corn
John H. Grove, Plant and Soil
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.
Response to N
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.
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
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
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
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
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
was grown after corn:
the first 80
lb N/acre was always profitable (100% = 6 out of 6 years)
to 120 lb N /acre was profitable 67% of the time
to 160 lb N/acre was profitable 33% of the time
to 200 lb N/acre was profitable 67% of the time
up to 160
lbs N/acre was profitable most of the time
to 200 lbs N/acre was rarely profitable (16% of the time)
N/acre was profitable 60% of the time
rates were profitable 20 to 40% of the time.
table at the end of this article.
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
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.
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Really Handle More Corn Acres?
Tom Miller, County Extension
Agent, Ballard County
Chad Lee, Plant and Soil
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.
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 are planting
close to 50 or 60% of their acres to corn. Shifting additional
acres to corn most likely require corn to follow corn. Crop
rotation is one reason that Kentucky farmers have had much fewer
insect, disease and weed control problems then some of our
neighboring states that have a higher percentage of corn on corn.
successfully grow corn after corn in Kentucky but there will be a
yield drag. In research published by many states the average seems
to be anywhere from a 5 to 15% yield loss in corn following corn
compared to corn following soybeans. There are a few more risks
involved. In one season of corn following corn you probably will
not see major shifts in insect populations or weed populations.
You may see a big difference in disease problems even in the first
year. Even with issues such as yield drag and increased disease
problems, the economics of corn on corn look good.
management challenges to raising more corn will come from planting
and harvesting. Kentucky has experienced great corn yields over
the past several years, due in part to the great job of timely
planting. Last year (2006) all the corn was planted within 10 days
to 2 weeks. Increasing corn in your rotation will lead to a more
intense planting season, will lengthen the planting season and may
force some corn acres to be planted at a less optimal time, or
under less optimal conditions. Later planting will almost surely
result in yield reductions.
planting will be more intense, harvesting more corn provides even
more challenges. Harvesting 150 bushels of corn compared to 50
bushels of soybean causes more wear on the combine, more trucks,
more elevator dumps, more time in the field, more drying time,
more bin capacity, etc. More corn will lengthen the harvest window
for corn, possibly allowing more losses to occur from stalk
problems and weather damage. Even though soybeans are not
competitive with corn in the current price outlook, harvesting
standing soybeans may be worth more than harvesting lodged corn
late in the season.
workload demands of growing more corn will likely keep some acres
in soybeans. We may be approaching an era when more farmers grow
two years of corn followed by one year of soybeans. If that
happens just realize it will not happen overnight. More corn will
take more input money, more machinery and more labor. Your
challenge is determine if the economics and logistics of raising
more corn is the correct decision in your operation. If you decide
to grow more corn, then careful planning for the additional
investments and additional workloads are keys to successfully
managing more corn.
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Economics favor corn.
Logistics of more corn might favor keeping soybeans in the
Corn on Corn
Ric Bessin, Entomology
corn increases management requirements of insect pests,
particularly corn rootworm. Traditionally, corn rootworm has been
managed very effectively in Kentucky with crop rotation, and can
still be managed very effectively in our state with crop rotation.
There have been reports at some meetings of rootworm problems in
some river bottom fields in the Green River area this past year.
field in corn two or three years only increases the risk of
rootworm damage slightly. But as fields are kept in continuous
corn for longer periods of time the risk increases. Growers may
begin to notice lodged corn in mid to late June as an early sign
of economic damage. Once the symptoms of rootworm damage appear,
there are no rescue treatments. Treatments for corn rootworm must
be used before signs of damage appear.
corn rootworm beetles typically lay eggs during the summer. Those
eggs hatch the following spring. The larvae from those eggs feed
on the roots of the next corn crop. Northern and Western corn
rootworms follow this pattern. They are the most common species of
corn rootworm in Kentucky.
corn rootworm (aka the spotted cucumber beetle) is much less
common in Kentucky and does not follow the same pattern as
Northern and Western corn rootworms. Southern corn rootworms
overwinter as an adult and lays eggs in the spring and on rare
occasions has caused economic damage to corn. Southern corn
rootworm can be a problem in first year corn. But this insect has
a wide host range, has been here a long time and rarely causes
noticeable damage to corn.
photos of corn rootworm adults, larvae, root damage, and lodged
corn on the corn insect picture sheet on the KY IPM website
Confused with Corn Rootworm
soil insect problems are often confused with corn rootworms,
wireworm and grape colaspis. The classic sign of rootworm damage
is lodged corn ("goose-necked") in the late whorl or early tassel
stage. Wireworms cause early stand loss and deadhearts of
developing seedlings. Grape colaspis is uncommon and most often
occurs in corn following red clover. This insect is a root feeder
and can kill large numbers of plants in a field.
Corn Rootworm in Continuous Corn
Kentucky that have been in corn for 2 or 3 years typically do not
have problems with corn rootworm. Field scouting is the best
method to determine if control measures are needed for the next
season of corn. If corn rootworm adult beetles were observed in
the field during mid summer, then management strategies are needed
for next season.
If you are
planting continuous corn and you counted an average of at least
one beetle per plant the previous summer, then there are three
rootworm seed treatment (Poncho 1250, Prescribe, or Cruiser RW)
insecticide at planting (Force, Aztec, Fortress, Lorsban,
Capture, Regent, etc. [see ENT-16 for rates and a complete list
Bt-rootworm corn hybrids (YieldGard RW or Herculex RW). To date,
very few Bt-rootworm hybrids have been tested in the University
of Kentucky corn hybrid performance trials. Most have not
weekly for the adult beetles beginning in late June through early
August to assess how well your management options are working.
rootworm treatments will cost between $15 and $20 per acre.
Specialized equipment is needed on planters for soil insecticides
applied at-planting. Seed treatments and Bt rootworm corn. Seed
treatments and Bt rootworm hybrids don’t require special
equipment, but do require early booking of seed. Ordering either
of these may be too late this year. To date, most Bt-rootworm corn
hybrids have not performed well in the University of Kentucky corn
hybrid performance trials.
widespread preventive treatments for corn rootworms in continuous
corn are not recommended unless there is evidence of a need on a
field by field basis. If a field shows signs of high rootworm risk
for the coming season, consider using rotation to manage the
problem. Growers wanting to grow continuous corn may benefit by
keeping fields in corn for only 2 or 3 years then rotating to a
non-corn crop to manage rootworms.
plants per location and record insects found per plant. Select
locations randomly so that they will be representative of the
entire field. A minimum of 2 locations is per field. A field of
100 acres would need 9 scouting locations. Don't survey along
field margins unless specifically directed to do so. Don't limit
surveys to one side or end of a field. The economic threshold for
rootworm control the following year would be an average of one
beetle or more per plant. For a complete description of rootworm
monitoring guidelines and procedures, see IPM-2, "Kentucky
Integrated Crop Management Manual for Corn," which is available on
line on the KY IPM website.
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Corn rootworm becomes a problem in continuous corn, but rarely in
Crop rotation is the best treatment.
Insecticides, applied at planting, on the seed or in the biotech
trait are other forms of management.
Potassium Levels Declining in Some Areas of Kentucky
Greg Schwab, Plant and Soil
The goal of
soil testing and nutrient management is to avoid yield-limiting
nutrient deficiencies. In Kentucky, we use a slow
build-up/maintenance approach for soils testing low or very low in
potassium (K). An additional amount of fertilizer is recommended
to build levels toward the medium range (200-300 lbs/a). Since
native soil test levels are generally medium to low, we expect a
slow gradual increase in soil test K which should eventually reach
equilibrium within the medium range.
analysis of samples submitted to UK soil testing labs (Lexington
and Princeton) show there is a downward trend in soil test K for
two of our largest grain producing regions. The only samples
considered for this analysis were those having corn, soybean,
wheat, or wheat-soybean as the intended crop (approximately
220,000 samples), and the median value was determined for each
year in each region. The median value is the midpoint in each data
set (50% samples above and 50% below).
Soil test K
in most regions of the state were relatively stable, but in the
Purchase and Western Pennyroyal Regions of the state, median soil
test K levels are decreasing at a rate of 4.12 and 3.42 lbs/a per
year, respectively (Fig 1). Furthermore, the percentage of samples
testing below 200 steadily increased between 1990 and 2006. There
are many factors that may be contributing to these trends:
needed K application caused by:
amounts of K removal (several very high yielding years)
applications rates (high fertilizer prices; greater % of rented
frequent soil testing
Recommendations are inadequate for the sampling protocol
fertilizing using a pre-blended products (9-18-9, 10-10-10, …)
of the cause, producers in these regions should closely monitor
soil test K levels. If soil samples are collected this year, be
sure to follow the recommendations. If you do not intend to sample
fields this year and they tested below 250 lbs k/acre previously,
you should apply either the previous K recommendation or removal
rates (0.35, 1.1, and 0.3 lbs K2O/bu for corn, soybean,
and wheat, respectively) whichever is higher. In this time of high
commodity prices, it is essential to prevent yield limiting
study on phosphorus over the same time period indicated little to
no change in soil test P values.
Soil test K values have been declining in major corn areas of
Closely monitor soil test K values.
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