April 1, 2005: Focus on Corn

Nebraska ranked third in the nation in the production of field corn with 1,319,700,000 bushels in 2004, according to the USDA Agricultural Statistics Service Web site. Iowa (2,244,400,000 bushels) ranked first and Illinois (2,088,000,000 bushels) ranked second. This and the next issue of CropWatch will include early season corn production and pest management topics, as well as other production topics. (USDA Photo)

Note: This topic was one of more than 20 presented during the 2005 Extension Program Series, 10 Easy Ways to Boost Profit $20/Acre. Look for the 10-20 icon in future issues for more stories on how to improve your bottom line.

Give proper credit for legumes, cut nitrogen inputs for corn

The primary mechanism for the nitrogen credit from soybeans is reduced immobilization of soil-derived nitrogen relative to decomposing corn residue. Because of the low C:N ratio, soybean crop residues will decompose faster than cereal crops such as corn or wheat stubble. This faster decomposition will release more nitrogen to the soil, which consequently will be available to the new crop in the next year.

University of Nebraska recommendations for corn following soybeans indicates that there can be a credit of 45 pounds of nitrogen per acre unless the soybean yield was less than 30 bushels per acre. For yields less than this, one pound of nitrogen per bushel harvested can be credited to that field.

Nitrogen recommendations for corn following alfalfa can be reduced 120-150 lb on fine textured soils and 70-100 lb on sandy soils, depending on the alfalfa stand. Use the higher end of the range when there are more than four alfalfa plants per square foot and the lower end of the range when there are fewer than 1.5 plants per square foot.

Nebraska research

Nitrogen rate and yield averaged over two years for three methods of soil and legume credit. Method N Rate lbs/ac Yield bu/ac UNL recommendation1 142 219 Actual soil N2 116 219 1 lb/bu credit3 82 214 1UNL assumes 3 ppm soil Nitrate-N, credit soybeans 45 lbs/ac N. 2Soil N based on 36-inch deep sample, credit soybeans 45 lbs/ac N. 3Soil N based on 36-inch deep sample, credit soybeans 1 lb N/bu of previous yield.

From 1988 to 1991 UNL soils specialists studied irrigated corn following soybeans in the South Central District. In total, the research covered 19 site/years. Across all site/years, a significant yield increase due to nitrogen was found only with the first 40 pounds of nitrogen applied per acre on corn following soybeans. The yields averaged 172 bushels per acre with a range of 97 bushels to 221 bushels per acre. The results of this study, along with others, led to the recommendation of a 45 pound nitrogen credit for corn following soybeans.

In 2003 and 2004 under irrigated cropping systems, UNL Cooperative Extension Educators and producers in York, Hamilton and Seward counties compared production using University of Nebraska nitrogen recommendations (assuming 3.0 ppm nitrogen carryover and using 45 pound soybean credit) to two other methods:

1. using actual residual soil nitrogen and crediting 45 pound soybean credit;
2. crediting actual residual soil nitrogen and a credit of 75 pounds nitrogen for the legume credit.

All three recommendations exceeded producer yield goals of 205 bushels per acre. Following the previous soybean crop, residual soil nitrate levels in the nine plots varied from 5.3 to 9.0 ppm and averaged 6.4 ppm, well above the 3.0 ppm assumed in the UNL formula.

In fall 2004 we collected several deep soil samples following irrigated soybeans in the York County area and the residual nitrogen in the three foot profile varied from 3.5 to 9.5 ppm and averaged 6.3 ppm.

Gary Zoubek
Extension Educator

The average yield increase with starter fertilizer was 14 bushels per acre for irrigated corn. With dryland corn, often there was no yield increase. (Many of these fields were affected by drought during this period.) The results confirm the value of using starter fertilizer when soil water is adequate throughout the season.

The response was primarily to nitrogen and phosphorus in starter fertilizer. Including sulfur in the did not result in increased yield. In-furrow and over-the-row placement were as effective as 2x2 placement, and in-furrow placement resulted in higher yield when soil test phosphorus (Bray-P1) was less than 15 ppm. Responses were as likely on bottomland soils as on upland soils.

If fertilizer is placed near the seed, such as with in-furrow placement, the potential for salt damage to the seedlings needs to be considered. A conservative guideline is that the sum of the pounds-per-acre of nitrogen and potassium and one-half the pounds-per-acre of sulfur should not exceed five pounds for in-furrow placement.

Charles Wortmann
Extension Soils Specialist, UNL

In Nebraska, 86% of all farmers surveyed were aware of Asian soybean rust. Of those aware of the disease, only 8% said it was a decision making factor in their soybean planting intentions for 2005. Of those 8% who said it was a factor in making their decision on how many acres to plant, 17% increased their planting intentions, 43% decreased their planting intentions, and 40% ended up making no change at all.

"Awareness by Nebraska producers regarding Asian soybean rust is the third highest in the nation", said Mark Harris, director of USDA's Nebraska Agricultural Statistics service, "This knowledge will provide a solid base for future decisions and actions by producers should they become necessary".

Early detection of a cutworm problem is essential because most cutting occurs within seven days of plant emergence. Every field should be checked for cutworm problems regardless of black cutworm trap counts.

Cutworms damage field corn somewhere in Nebraska each year. Several species of cutworms can attack corn. The severity and the area affected will vary greatly, depending on the species, previous crop history, and weather conditions. Cutworms that attack corn can be divided into two categories based on seasonal life cycles:

1. Black cutworms do not overwinter in Nebraska;
2. Dingy, claybacked, darksided, sandhills and other species overwinter as partially grown larvae in the soil.

Since black cutworms do not overwinter in Nebraska, they are dependent on spring weather conditions, primarily prevailing southerly winds, to bring them into our state. Nebraska is on the western edge of the black cutworm area of influence, and there is little chance of activity west of Hwy 183 in central Nebraska. We don’t have black cutworm problems as often as they do further east in the Corn Belt. The last black cutworm “outbreak” that affected a large portion of eastern Nebraska occurred in 1986. However, because of their cutting habits and the possibility that large numbers can be transported to Nebraska with favorable weather conditions, these cutworms have the most potential for causing a widespread problem.

Black cutworm Darksided cutworm

Black cutworm moth flight can be detected from pheromone and light traps located across the state. The presence of moths in a trap only indicates potential problems and is no guarantee that extensive damage will occur. Trap counts are more useful in alerting growers and consultants as to when to start scouting. Moth flight can begin as early as April and continue through May.

Black cutworm moths prefer to lay eggs in green vegetation or heavy surface residue and seem to prefer legume (alfalfa or soybean) residue over corn residue. Late planted fields where winter annuals have been allowed to grow are also a concern. When weeds are destroyed mechanically or by herbicides, they will transfer feeding to emerging corn. With some late infestations, the corn will be too big for the cutworms to cut. In this case, the cutworms will burrow into the stalk just below the soil surface. The symptom is usually called “dead heart,” since the newer leaves in the whorl will turn brown as the larvae reach the growing point.

This damage may be confused with wireworm damage. Wireworm damage usually occurs early in the season and involves seed damage, although in severe infestations they may bore into the corn plant. Cutworms that overwinter as larvae generally prefer to lay eggs in the fall in green vegetation such as small grain stubble, legumes, rye, and pasture. The eggs hatch and the larvae feed on the vegetation before overwintering. In spring, after the previous crop is removed and the corn emerges, the cutworms will transfer their feeding activity to the corn. Recent experience has been that corn planted into alfalfa just killed in the spring has a larger potential for cutworm problems.

Usually tillage will not have a significant effect on cutworm populations. If fields are tilled before black cutworm migration, egg-laying may be limited in those fields. Cutworms already in the field may suffer some mortality by mechanical action, but there is no guarantee that tillage by itself will eliminate cutworm problems. Many cutworm problems have occurred in fields that have been tilled. Previous vegetation is probably the most important factor in cutworm potential.

It is extremely rare to experience cutworm problems in continuous corn. Corn residue is not a preferred egg-laying site. Potential problems in continuous corn may be the result of a previous year’s late season flush of weeds or an interseeding of a fall cover crop such as rye, which might attract moths laying eggs in the fall.

Managing cutworms in corn

Several options exist for the grower who wants to manage cutworms in corn. Since a vast majority of corn acreage is not affected by cutworms, the most economically sound practice is to scout for cutworm damage as soon as the corn emerges and apply a rescue treatment if necessary. Generally, a rescue treatment should be considered if 5% (1 in 20) plants are damaged, cutting is observed and the worms are one inch or less in length.

Choosing a herbicide strategy to deal with weeds is not always an easy task. There are many options to consider. Factoring in the economics such as herbicide costs, fuel and time, combined with biological and environmental factors makes the job tough enough; however, when you consider that within that environment, you include weed species spectrum, soil type, organic matter, herbicide efficacy, herbicide restrictions, annual precipitation, precipitation at time of application, and the ability to make a timely application, it is just not an easy decision. Even when you consider all these factors, often there’s still no perfect choice.

Corn/weed competition

All weeds are not created equal. Each weed species competes differently with corn, with some species being much more competitive than others. For example, common sunflower has a competitive index of 10 and is much more competitive than lambsquarters, which has a competitive index of about 2. Understanding the differences between each species and its competitive factor is important in determining what weed management strategy will provide the best return on investment.

Since weeds are not created equal we should acknowledge that neither are crops. Each crop differs in its competitive ability as well. Corn is one of the most competitive row crops planted in Nebraska. The relative competitive load necessary to cause a specific yield loss quantifies the competitiveness of a crop. For corn, it would take a competitive load of around 36 per 100 sq ft, to cause a 5% yield loss. (100 sq ft is approximately 40 ft of row in 30 inch rows.) If we recall, sunflower has a competitive index of 10, therefore it would take 3.6 sunflower plants per 100 sq ft (3.6 x 10 = 36) to cause a 5% yield reduction in corn. This is all under the assumption that the weeds emerge at the same time as the crop.

Accurately calculating yield loss, especially when several species of weeds are present in the field, can be very difficult. WeedSOFT, a computer aided weed management decision support tool, can be purchased from the University of Nebraska to supply this information at the click of a button. Using this technology allows for more accurate yield loss analysis, providing better information to make a weed management decision.

Early season weed management

Controlling weeds before they become a problem just makes good sense. As the saying goes “an ounce of prevention is worth a pound of cure” and this is true with weed control in corn. Various techniques are available and, depending on individual circumstances, one may be better than the other. Producers need to determine their seasonal goals before committing to a strategy.

Before we dive into all the available strategies, be sure to refer to Table 1 for an explanation of the terms and acronyms used to describe weed control for preemergent corn.

Table 1. Pre-emergence herbicide terms Acronym Treatment Timing EPP Early Preplant 10-30 days prior to planting PP Preplant 0-10 days prior to planting PPSA Preplant Surface Applied 0-30 days on the surface PPI Preplant Incorporated 0-30 days incorporated PRE Premergence Planting time until crop emerges

Early preplant applications

Early preplant herbicide (EPP) applications 10-30 days before planting offer many advantages to most producers, especially no-till farmers. First, and especially in no-till, early preplant treatments allow producers to burn down winter annuals, including henbit and mustards, and early summer annuals, including giant ragweed, common sunflower and lambs-quarter. This can be important in a year with drought conditions as these early weeds, while not competing directly with the crop, can quickly rob precious soil moisture.

Second, an early preplant treatment reduces most, if not all, weed competition as the crop emerges. Although this early competition may not be the most critical with respect to yield, it can quickly reduce yield as corn enters the 2-leaf stage.

Another advantage is that in years with limited moisture, the herbicide has a greater chance of being activated before the crop emerges. A disadvantage of the early preplant treatment is decreased longevity of the residual activity. Common sense tells you the earlier a herbicide is applied to the soil the earlier it will stop working. Postemergence programs need to be carefully evaluated before making such a decision and some knowledge of the weed history in the field will be helpful.

Preplant applications

Preplant (PP, PPSA, PPI) is similar to early preplant and can use many of the same herbicides. Treatments are typically made 0-10 days before planting. Preplant doesn’t give you the advantage of catching early weeds, but it may provide the needed residual for setting the stage for a good POST treatment.

Preemergence applications

A preemergence treatment (PRE), applied after the crop is planted but before emergence, offers many of the same advantages as preplant applications. An additional advantage is that it allows the producer to increase the longevity of control provided by the herbicide. This works well with conventional till fields. This also provides increased management flexibility later in the season as summer annuals begin to emerge.

It is estimated that annual broadleaf weeds cause an average of more than $2 million loss to winter wheat producers in Nebraska.

Survey your wheat fields for weeds and then select the herbicides or herbicide combinations best suited for the situation. Remember to always check replant options and rotation restrictions. Your herbicide selection may affect crop options next year, the following year or even three or four years later or as soon as this summer if a storm wipes out the crop.

Several herbicides provide excellent broadleaf weed control with minimal wheat injury; however, some varieties are more sensitive to herbicides than others. Injury varies with herbicide, variety, and growth stage. Research has not been conducted on the herbicide sensitivity of many of the varieties presently planted. The following are fundamentals that should be considered before selecting a herbicide treatment:

1. Identify problem weed(s).
2. Spray when weeds are small and actively growing. Spray at the proper winter wheat growth stage for the herbicide used.
3. Use proper spray equipment that is in good condition and not contaminated with previously used herbicides.
4. Calibrate the sprayer to ensure application accuracy.
5. Read and follow directions on the herbicide label.
6. Know your rotational plants to avoid herbicide carryover problems to sensitive crops.
7. Be aware that crop disasters such as winter injury, hail, or disease occur and previously applied residual herbicides may limit recropping options.

Some of these products should be combined to control a wider spectrum of broadleaf weeds in winter wheat. Herbicide combinations are also recommended for managing the potential development of herbicide resistance. Ally, Amber, Finesse, Harmony Extra, and Peak are sulfonylurea herbicides and are ALS-AHAS inhibitors.

See the 2005 Guide for Weed Management in Nebraska (EC 130) for more information.

Bob Klein
Extension Cropping Systems Specialist
West Central REC

Nebraska corn growers expect to plant 8.4 million acres in 2005, up 2% from last year and up 4% from two years ago.

Expected plantings of soybeans, at 4.8 million acres, would be unchanged from last year. This would tie for second highest on record, behind only the 4.95 million acres planted in 2001.

Last fall, winter wheat was sown on 1.8 million acres, down 3% from a year earlier and down 5% percent from two years ago.

Sorghum growers expect to plant 390,000 acres, 29% less than 2004 and 41% less than 2003.

Hay acreage for harvest, at 2.85 million, would be up 2% from last year. Dry edible bean producers intend to plant 160,000 acres, up 33% from a year earlier. Oat planting intentions, at 150,000 acres, are up 7% from 2004. Sunflower planting intentions, at 95,000 acres, are up 70% while sugar beet plantings of 49,000 acres would be down 2% from a year ago.

These acreage estimates are based on surveys conducted the first two weeks of March. This report is intended to assist growers in finalizing their acreage plans for 2005. Actual area planted may vary from that indicated due to final farmer assessment of planting options, the effects of weather, availability of production inputs, and changes in expected prices for 2005 crops.

March 31, 2005 News Release
USDA Nebraska Agricultural Statistics Service

1. visit with their crop insurance agent about requirements and ask any questions they may have;
2. use good farming practices and when necessary, treat for rust based on recommendations from ag experts; and
3. document the advice received and actions taken to combat this disease.

• Grand Island - April 5
• Alliance - April 7
• North Platte - April 12
• Valentine - April 14
• Norfolk - April 15
• Lexington - April 19
• Beatrice - April 22
• Norfolk - April 29

The contact for the Farm Mediation program at the Nebraska Department of Agriculture is Marian Beethe (402-471-6890) or marianjb@agr.state.ne.us

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