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4/29/2013 - 5/5/2013

Don't Underestimate the Value of Starter Fertilizer for Corn Planted Late

By Antonio P. Mallarino, Department of Agronomy

The weather changes rapidly and unexpectedly in Iowa. We went from worrying about planting in dry soil to late planting in cold soil. Therefore, farmers and crop consultants are wondering about using starter fertilizer to speed up early corn growth and increase yield.


When is starter fertilizer beneficial?

The placement of small amounts of nutrients in bands beside and below the seeds or in the seed furrow increases the concentration of nutrients where seedling roots grow. Common starter fertilizers are N-P or N-P-K products or mixtures, although other nutrients often are added. Research in Iowa and the north central region has shown that corn growth in response to starter fertilizer usually is more frequent in conditions that reduce early root growth or activity, the concentration in the soil of simple nutrient compounds that plants can absorb, or the nutrient diffusion through soil to the root.  The same research shows, however, that starter fertilizer effects on grain yield are not as consistent. Yield responses are likely to occur with cool and wet soils and with reduced tillage because high residue cover keeps soils cooler and wetter in spring. Inconsistent yield responses to starter fertilizer often cannot be clearly explained and are attributed to complex interactions between initial soil conditions, planting dates and climate changes after planting.

Phosphorus is a relatively immobile nutrient; its diffusion to the root surface and uptake is limited by cold soil temperature, and it is critical for plants during early growth. Therefore, it is not surprising that corn early growth response to starter mixtures often is explained by P, sometimes even in high testing soils. Nitrogen often explains corn response to starter, especially when the primary N rate is not applied pre-plant in spring , and in continuous corn. Both in-furrow application and side planter attachments could be used, but available Iowa research can't answer the question if the low "safe" N rates that can be applied in-furrow actually help much if N deficiency is serious. Potassium seldom increases early corn growth in soils with adequate soil-test K levels, so it is desirable in a starter mixture when soil K levels across a field are low or with high small-scale variation and many low-testing small areas.


What about corn planted late?

We seldom think of a need for starter fertilizer when corn is planted late, even with full-season hybrids, because soils usually are warm and plants emerge and grow quickly. We assume that it may advance maturity and grain drying in the fall, but that will not affect yield much.

But this year corn will be planted late and in wet, cold soil, mainly in central and northern Iowa, where the season is shorter. Unfortunately, we have not had research under these conditions in Iowa. Wisconsin research from a few years ago by Dr. Larry Bundy and collaborators demonstrated, however, that starter application was as likely to increase corn yield with early planting dates as with late planting dates and full-season hybrids. We should not blindly extrapolate these results to Iowa because most of the research sites were north of most Iowa fields, but they should apply well to the northern tiers of Iowa counties.


  1. Do not underestimate the value of starter for corn planted late, mainly in northern Iowa cold soils and with a shorter season.
  2. Starter P will provide the most benefit with low or optimum soil test P levels and low pre-plant broadcast application rates. But do not expect much yield response if the two-year P rate for the corn-soybean rotation was applied last fall or this spring.
  3. Starter N may also be beneficial, mainly without high N rates applied pre-plant or near planting in the spring and with corn on corn.


Antonio Mallarino is professor of agronomy with research and extension responsibilities in soil fertility and nutrient management. He can be reached at or by calling (515) 294-6200.

Bean Leaf Beetle Survival Just Above Average

By Erin Hodgson and Adam Sisson, Department of Entomology

Bean leaf beetle adults are susceptible to cold weather and will die when the temperature falls below -10°C. However, they have adapted to winter by protecting themselves in leaf litter. An overwintering survival model was developed by Lam and Pedigo from Iowa State University in 2000, and is helpful for predicting winter mortality based on accumulating subfreezing temperatures. Predicted mortality rates ranged from 40-90 percent for the 2012-2013 winter (Fig. 1). The northern third of Iowa did experience a colder winter, and more than 80 percent of beetles were not predicted to survive.

Figure 1. Predicted overwintering mortality of bean leaf beetle based on accumulated subfreezing temperatures during the winter (October 1, 2012 – April 15, 2013).


The average mortality rate over the last 24 years in cemtral Iowa is 71 percent. The 2012-2013 winter had slightly better predicted survivorship than average (Fig. 2). It is important to remember insulating snow cover can influence the survivorship of bean leaf beetle. The recent cold weather could also influence spring activity in alfalfa and later in soybean.

Figure 2. Predicted bean leaf beetle mortality by year for central Iowa. The red line indicates the average mortality rate.


Overwintering adults are strongly attracted to soybean and will move into fields with newly emerging plants (Fig. 3). First-emerging fields should be monitored this month, especially in southern Iowa. Other fields of concern include food-grade soybean and seed fields where reductions in yield and seed quality can be significant. Bean leaf beetle is easily disturbed and will drop from plants and seek shelter in soil cracks or under debris. Sampling early in the season requires you to be sneaky to estimate actual densities. Although overwintering beetles rarely cause economic damage, their presence may be an indicator of building first and second generations later in the season.

To learn more about managing bean leaf beetle and bean pod mottle virus, click here.

Figure 3. Overwintering bean leaf beetles can defoliate young soybean plants and vector bean pod mottle virus. 


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact her at or 515-294-2847. Adam Sisson is an Integrated Pest Management extension specialist; contact him at or 515-294-5899.

2013 Alfalfa Weevil Hatch Delayed

By Erin Hodgson and Adam Sisson, Department of Entomology

Alfalfa weevils develop based on temperature or accumulating degree days. This cool spring has delayed all insect development, including alfalfa weevil. Scouting in alfalfa should begin at approximately 200 degree days for areas south of Interstate 80, and 250 degree days north of Interstate 80. Based on accumulated temperatures since January, weevils are active in southern Iowa now (Fig. 1).

Figure 1. Accumulated growing degree days (base 48°F) in Iowa from January 1 through April 30, 2013. Map is updated daily at Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.


Alfalfa weevil is an important defoliating pest in alfalfa. Heavy infestations can reduce tonnage and forage quality. Adults can feed on plants, but the larvae typically cause the majority of damage. Newly hatched larvae can be found feeding on terminal leaves, leaving newly expanded leaves skeletonized. Gradually maturing larvae (Fig. 2) move down the plant and begin feeding between leaf veins. Adults (Fig. 3) eat along the leaf margin, leaving irregular notches. A heavily infested field will look frosted or silver (Fig. 4).

Figure 2. Alfalfa weevil larvae have a dark head and pale green body with a white stripe down the back. Fully grown larvae are about 5/16 inches long. Photo by Clemson Cooperative Extension Slide Series,


Figure 3. Alfalfa weevil adults have an elongated snout and elbowed antennae. Their wings and body are mottled or brown in color. Photo by Clemson University,


Figure 4. Heavily defoliated alfalfa fields appear frosted from a distance. Photo by Whitney Cranshaw, Colorado State University,


To initially detect alfalfa weevil larvae in the spring, use a sweep net to sample. After finding larvae, collect six alfalfa stems from five locations throughout the field. Take each stem and shake into a bucket to dislodge larvae from the plant. Average the number of larvae per stem and plant height to determine if a foliar insecticide is warranted (Table 1). Remember, cutting alfalfa is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting within a few days.

Table 1. Economic threshold of alfalfa weevil, based on the average number of larvae in a 30-stem sample.


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at or phone 515-294-2847. Adam Sisson is an Integrated Pest Management program assistant. Sisson can be contacted by email at or by calling 515-294-5899.

Weather Forecast Delays Corn Planting, but Late Planting Does Not Foretell Lower Yields

By Warren Pierson and Roger Elmore, Department of Agronomy

Average recommended planting dates for corn in Iowa lie between April 12 and May 18 in order to achieve 98-100 percent yield potential, but this varies a bit on the specific location (Abendroth and Elmore, 2010).  Currently, Iowa soil temperatures range from approximately 57-63°F (Iowa Environmental Mesonet, 2013). While these temperatures are often considered good for planting because 50°F is the temperature necessary for corn seed germination, the weather forecasted is not ideal for a corn seed to germinate and begin emergence. According to NOAA, Story County highs May 2 and 3, 2013, will be in the low 40s°F with lows in the mid 30s with potential rain, sleet and snow.  Last year, on April 26, Iowa soil temperatures ranged from 58-64°F; however, by April 29 soil temperatures dropped to 49-53°F across the state. This swing in temperatures, coupled with rain at that time, resulted in many reports of lower relative yields for corn planted between April 22-26 than corn planted later. Fluctuations in soil temperatures are related to deformed mesocotyl growth, which can result in “corkscrewed corn” (Nielsen, 2012).

According to USDA-NASS as of April 28, 2013, approximately 2 percent of Iowa corn acres was planted.  This is somewhat similar to 2011: by April 24, only 3 percent was planted and, by May 1, only 8 percent was planted.  Within two weeks after that though, approximately 84 percent of Iowa’s corn acres (11.9 million acres) was planted, bringing the total planted to 92 percent.  Even with this later than usual planting corn yield was only 1 percent above trend line in 2011. 

One way to look at yields at different planting dates is to compare state average yields – as reported in deviations from the trend line - to the percent of the crop planted at different times during the spring.  This is the way Bob Nielsen - The Planting Date Conundrum for Corn - and Emerson Nafziger - Planting Delays and Corn Prospects - looked at corn yield in their recent articles. For example, in Iowa the trend line for acres planted by April 30, compared to yield relative to trend line, is actually negative (Figure 1). This suggests that delayed planting tends to increase yield potential - although the trend is a weak one.  The negative trend for acres planted by April 30 is likely due to the negative effect of 2012 and the three years of near trend line yields with 60-70 percent of corn acres planted. What is clear is that earlier planting does not always guarantee high yields, and late planting does not always foretell low yields. 

Figure 1. Deviation from Iowa corn 30–year yield trend compared to percent of corn acres planted by April 30. 1993 was removed as an outlier with 0 percent of acres planted by April 30 and a yield deviating from trend line by -39 percent. Data adapted from USDA-NASSThe trend line in this figure relates the percent of corn acres planted in Iowa by April 30 to the deviation from trend yield. The trend line in this figure shows that having more acres planted by April 30 does not necessarily relate to high yields, and perhaps even slightly lower yield.  (larger image)


Although the trend line of corn planted by May 15 was positive, two years were behind and still yielded above trend lin. In 1984 and 2008, only 35 and 46 percent of the crop was planted by May 15, respectively; yields were above trend line both years by 3 percent (Figure 2). However, in 1991 and 1995, only 33 and 30 percent of the crop was planted by May 15, respectively. In these two years in which planting proceeded slowly, yields deviated from trend by -6 percent and -9 percent (Figure 2).

Figure 2. Deviation from Iowa corn 30–year yield trend compared to percent of corn acres planted by May 15. 1993 was removed as an outlier with 7 percent of acres planted by April 30 and a yield deviating from trend line by -39 percent. Data adapted from USDA-NASS. The trend line in this figure relates the percent of corn acres planted in Iowa by May 15 to the deviation from trend yield. The trend line in this figure shows that more acres planted by May 15 is related to slightly higher yield. (larger image)


The relationship between acres planted by May 30 and yield is positive; with more acres planted by May 30 we see better yield. However, in 1991 and 1995, only 70 and 80 percent of corn was planted by May 30 and yields were below trend line by only -6 percent and -9 percent  (Figure 3). 

Figure 3. Deviation from Iowa corn 30–year yield trend compared to percent of corn acres planted by May 15. The yield of 1993 was removed as an outlier with 65 percent of acres planted by April 30 and a yield deviating from trend line by -39 percent. Data adapted from USDA-NASS. The trend line in this figure relates the percent of corn acres planted in Iowa by May 30 to the deviation from trend yield.  The trend line in this figure shows that more acres planted by May 30 is related to higher yields. (larger image)


Nielsen found similar trends in Indiana and suggested that farmers be patient and not “mud in” corn. The effects of “mudding in” corn are likely more negative than the effects of a late planting date for corn.  Elmore also recommended patience in an Iowa Farmer Today Crop Watch Blog posted on April 29, 2013.

Having planted 1.2 million acres per day in critical windows of recent years, Iowa farmers are now equipped to plant many acres per day than ever before. Waiting for soil temperatures to rise above 50°F and warmer weather in the forecast is most favorable for corn growth, development and yield (Elmore, 2013).  Delaying planting until conditions improve will encourage more uniform and faster emergence,  greater emergence percentage and more rapid growth and development.  Planting date is only one of many yield factors; weather conditions the rest of the season, management and genetics will likely be more substantial yield factors. 

While our normal recommendation – based on multiple years and locations of data - is to plant early to maximize yield potential, keep in mind that soil conditions and forecasted weather affect planting recommendations. Early planting does not ensure high yields just as planting late does not foretell low yields.


Warren Pierson is a graduate research assistant. He can be reached at or (515) 294-1360. Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. He can be contacted by e-mail at or (515) 294-6655.

This article was published originally on 5/6/2013 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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