CMEG Crop Management Extension Group

Field Crop News

Website Address: http://fcn.agronomy.psu.edu/

June 3, 2008    Vol. 08:13

IN THIS ISSUE:

Weather Outlook — Paul Knight, Pennsylvania State Climatologist

A heat wave is coming! During the past few weeks (except for last time), a short, but intense heat wave was heralded for the first week of June. The change is that it will not be short, but will still be somewhat intense and it will occur from June 6–11.

The first volley of more humid air will arrive during Tuesday night and Wednesday morning with noisy thunderstorms dropping close to an inch of rain in most places and perhaps as much as 2 to 3 inches near the I–80 corridor. Temperatures will be held down by the clouds and showers on Wednesday, but the dew—points will climb into the 60’s (rather uncomfortable levels). Partial sunshine will return on Thursday along with a more typical scattering of afternoon thundershowers — mainly over the mountains.

The period from Friday through Monday will turn sultry with hazy sun, high humidity and readings reaching 85–90F in the mountains and 90–95 in the valleys. Each afternoon, a few thunderstorms will boil up on the higher terrain, but less than 25% of the area will have rain. A front approaching from the northwest will trigger more widespread thunderstorms later Tuesday and again on Wednesday. More than half of the state should receive some rain each day. Some relief is likely on either Thursday or Friday (June 12–13) as drier air pushes in from the northwest.

The overall outlook for a near to slightly below seasonal summer temperatures is still expected, though the ‘cool’ month is more likely to be July (which may also turn rather wet).

Check out the predicted Growing Degree Days (base 55) for Pennsylvania for the next two weeks at: http://climate.met.psu.edu/data/regional_gdd.php

Evaluating Alfalfa Stands — Marvin Hall, PSU Forage Specialist

The sporadic and heavy spring rains have hurt some new alfalfa plantings enough that producers are trying to decide if they should try to thicken the stand or if it is even worth keeping. Unfortunately this is not an easy decision.

The magic number of plants that indicates when it is time to rotate out of an established alfalfa stand is 4 – 5 plants per square foot. However, plant density in new alfalfa seedings should be a minimum of 15 plants per square foot. This greater density is needed because the plants have not developed large crowns yet and will consequently have fewer stems per (directly related to yield) plant than older plants.

Unfortunately, trying to thicken up (overseed more alfalfa seed) a new stand that has fewer than 15 plants per square foot is rarely successful because the existing plants provide too much competition for the emerging alfalfa plants. Cool-season forage grasses such as orchardgrass or timothy can be used to thicken up these thin alfalfa stands because the grass seedling can compete with the existing alfalfa plants.

Post—Emergence Herbicide Application Timing in Corn — Bill Curran, PSU Weed Science

Timely post control of weeds in corn is critical for maximum yield. Corn is less resilient than soybeans and early season weed competition can have lasting impacts. In general, timely POST applications of herbicides to corn should be between the V2 and V4 stages of corn growth when most annual weeds are less than 4 inches tall. In addition, corn tolerance to herbicides is almost always better when the corn is V4 or less. Applications beyond V4 increase the risk of herbicide injury because of warmer air temperatures and higher humidity (associated with mid June timings) and when the corn is beginning to develop reproductive tissues (silks and ears). A study we conducted a few years ago showed how weed density and application timing show the benefit of residual activity for POST weed control in corn.

A two—year experiment repeated at five locations across the Northeast (PA, NJ, and DE) evaluated the impact of weed density and time of glyphosate application on weed control and corn grain yield using a single postemergence application (Myers et al. 2004, Weed Technol. 19:102–107). In conventional till corn, three weed densities, designed to reduce corn yields by 10, 25, and 50%, were established across the locations, using forage sorghum as a surrogate weed. At each weed density, a single application of glyphosate at 0.75 lb ae/acre (22 fl. oz. Roundup Weathermax) was applied to glyphosate—resistant corn at the V2, V4, V6, and V8 growth stages. The forage sorghum was over seeded again after each herbicide application to simulate an additional flush. At the low (3 plants/m2) and medium (14 plants/m2) weed densities, the V4 through V8 applications provided nearly complete weed control and yields equivalent to the weed—free treatment. Without a residual herbicide, weed biomass and the potential for weed seed production from subsequent weed emergence made the V2 timing less effective. At high weed densities (66 plants/m2), the V4 followed next by the V6 timing provided the most effective weed control, while maintaining corn yield. Weed competition from subsequent weed emergence in the V2 application and the duration of weed competition in the V8 timing reduced yield on average by 12 and 15%, respectively. This research shows that single postemergence applications can be successful, but weed density and herbicide timing are key elements. Our experience suggests that early POST applications (V2) should include a residual herbicide in the mixture and that mid POST timings (V3/V4) are most consistent when they follow a soil residual program. Applications beyond the V4 and certainly V6 stage greatly increase the risk for yield reduction and herbicide injury. Some common POST herbicides and restrictions are provided in Table 1.

Table 1.  Selected common POST herbicide timing restrictions for field corn.
Herbicide Max corn size/time for over the top application Comments
Atrazine 12 inches tall Maximum corn height for atrazine  - water quality restriction
Dicamba (Banvel or Clarity) 1 pt up to 8 inches or 0.5 pt up to 36 inches tall Use 8 fl oz/A or less and best applied to corn less than 12 inches tall
Basis Up to V2 or 6 inches Do not apply to corn > 6 inches tall
Callisto Up to 30 inches (V8) Remember atrazine restriction
Distinct Up to 24 inches Status is the safened Distinct
Glyphosate Up to 30 inches (V8) Roundup Ready hybrids only
Harmony GT Up to 16 inches (V5) -
Hornet Up to 20 inches (V6) -
Impact Up to 45 days prior to harvest Both field and sweet corn
Laudis Up to V8 V7 for sweet corn
Liberty Up to 24 inches (V7) Liberty Link hybrids only
Northstar Up to 20 inches (V6) Dicamba + Beacon
Option Up to 16 inches (V5) -
Permit Through lay-by -
Resolve Up to 12 inches (V5) -
Resolve Q Up to 20 inches (V6) New product with safener
Spirit Up to 20 inches (V6) -
Steadfast Up to 20 inches (V6) Less for ATZ
Status Up to 36 inches (V10) Distinct with safener
Yukon Up to 36 inches Dicamba + Permit

Options for Controlling Glyphosate Resistant Corn When Replanting Corn — Bill Curran, Weed Science

Last week Mark Loux at Ohio State had a good article in their C.O.R.N. newsletter about killing glyphosate resistant corn in a (no-till) replant situation. Here is the link: http://corn.osu.edu/story.php?setissueID=230&storyID=1435

Soybean Seed Size and Planting Depth — Del Voight, Grain Crops

A study I am involved in is looking at the effect of seeding rates on soybean growth and development. The size in this seed lot was 3,300 seeds per pound as compared to 2,600 seeds per pound in a normal year. Seed size varies among varieties but also depends on the environment where the seed is grown. With the long delayed emergence this season seed size may have made a difference in the length of time a plant takes to emerge and produce its own energy from sunlight.

Published studies from the University of Kentucky have shown seeds of a same variety will have the same yield potential. However, conditions in that three year study were ideal for germination. They concluded that as long as germination and vigor were tested, no differences existed related to size.

There are two main considerations that need to be addressed with small seed sizes. A small soybean seed planted in the ground cannot derive the energy it needs for growth from photosynthesis. The seed is, therefore, totally dependent on its reserve of the energy produced by the parent plant. The more energy the seed contains, the longer the seedling can continue to grow without becoming photosynthetically self-sufficient. In practice, this means that a large seed with a considerable amount of stored energy can usually be sown at a greater depth than a small seed with a limited energy reserve. Therefore, seeding depth is critical because of the small seed-size. Optimum seeding-depth is 1 to 1.25 inches. Over the years with the incidence of seed corn maggot and small seed sizes I have seen more problems due to planting too deep.

Rain on Drying Hay — Marvin Hall, PSU Forage Specialist

Lots of hay got rained on this past week and there has been some question about how this affects the hay. This graph will help explain why this is a “depends” type of question.

Summary

Potato Leafhoppers Arrive In Pennsylvania — Paul Craig, Dauphin County Educator

This past weekend marked the first reports of the annual migration of the Potato Leafhoppers (PLH) into PA. The PLH does not survive the winters in most alfalfa producing areas but is transported north by weather events each May and June. Once the PLH arrives is seeks out a multitude of plants to feed on and rapidly builds up populations. Among favored host plants are: alfalfa, potatoes, soybeans, maple trees and many ornamental plants. The PLH does not feed on any forage grasses.

Upon arrival the female PLH begin to lay eggs. Females lay 3 - 7 eggs per day for up to 50 days. Eggs hatch into nymphs which mature into adults in about 3 weeks. As a result populations of PLH can quickly increase and can result in significant injury to alfalfa from mid June through early August. Typically populations of PLH rapidly drop off suddenly in mid-August.

The adult PLH is lime green color, wedge shaped approximately 1/8 to 3/16 of an inch long. Nymphs closely resemble the adult but are smaller, 1/16 to 1/8 inch long, slightly paler in color but they cannot fly. Nymphs move in a characteristic diagonal direction. Both the nymph and adult feed by sucking cell contents from the alfalfa. This feeding results in blockage of the “piping” system of the plant and produces stunted growth and under high population levels the typical hopper burn on leaf tips.

Harvesting alfalfa disrupts the life cycle of PLH. Harvesting removes eggs which are laid in the stems and leaves of alfalfa. Nymphs cannot fly and frequently die after mowing and adults move to other feeding sources nearby. As the alfalfa regrows the adults move back into the alfalfa stand and lay eggs again.

Too often many acres of alfalfa are not treated for PLH infestations until hopper burn is observed. By then significant yield and quality is lost. In addition, many acres of alfalfa are treated when insect levels are below threshold levels. Using a sweep net to monitor population levels is the best way to monitor insect levels and to determine when controls are necessary.

To monitor for PLH take the sweep net and make 20 sweeps at 5 different locations in each field. Sweep approximately 4 inches below the tops of the alfalfa, taking a step or two between each sweep. Quickly grab the net to trap the insects and slowly unfold the net to allow insects to escape a few at a time. Count all of the PLH adults that fly out and look on the sides of the net for the nymphs. Keep track of all adults and nymphs. Repeat at 4 other locations across the field and sum up total counts. Divide total count by 100 to determine number of PLH per sweep. (Ex. 60/100 sweeps = 0.6 PLH/sweep)

Compare PLH/sweep to the height of the alfalfa. If the number per sweep exceeds the average height of the alfalfa, treatment is necessary. (Ex. 0.6 PLH/sweep in 6 inch alfalfa would need treatment)

Labeled insecticides provide excellent control but typically only protect the stand for 1 to 2 weeks. Be certain to check the pre—harvest interval following applications. To provide best control ensure you are applying sufficient carrier, 20 gal/acres minimum with proper boom height and sufficient pressure to provide coverage throughout the crop. Refer to the 2007/08 Agronomy Guide for labeled products and rates.

Most alfalfa in the southern parts of PA was harvested by mid May. In most situations these earlier harvested fields will be ready for second cut before these recent arrivals have the opportunity to reach threshold levels. However third cutting and all new seedings will need to be closely observed and scouted.

Cutworm, Armyworm, Cereal Leaf Beetle, and Seed Corn Maggots Too! — John Tooker, PSU Entomology Specialist

Reports indicate that numerous insect pests are feeding away and causing headaches for growers. True armyworm and black cutworm are both active with cutworm causing damage in the northern part of Pennsylvania and true armyworm active in the south. Black cutworm can severely damage late plantings of corn whereas true armyworm can cause problems in corn fields adjacent to recently harvested small grains. Adult females of both species lay their eggs on weeds and/or grasses along field margins or within fields. When weeds are killed with herbicides or small grains are harvested, cutworms and armyworms, respectively, can move into corn fields. Cutworm damage is fairly distinctive with older larvae cutting corn plants off at the soil, but it is uncommon for cutworms to approach numbers requiring chemical treatments. Nevertheless, thresholds are available (http://fcn.agronomy.psu.edu/2008/fcn0809.cfm#g). Armyworms tend to feed at night along the margins of corn leaves, avoiding midribs. During the day, larvae hide in leaf sheaths or in the soil or leaf litter. Armyworms can warrant treatment should infestations reach 25% of plants in a field. If perennially a problem, both pests can be controlled to a degree by using certain lines of transgenic corn. Herculex I and Herculex Xtra offer protection against black cutworm and appear to have some activity again true armyworm. Del Voight (Lebanon Co. Cooperative Extension) recently saw infestations where armyworms had stormed out of recently harvested rye fields into adjacent corn and Herculex lines had only around 10% defoliation whereas non—Herculex lines had upwards of 75% defoliation.

Cereal leaf beetle has been reported to be a problem in oats. This is another insect species that begins the growing season by feeding on weedy species (grasses) and then moves into small grains, where adult females lay their eggs. This pest species can be a problem in conventionally managed fields where good populations of natural enemies (mostly parasitoid wasps) do not have a chance to build—up. If you see adults, it is likely too late to manage this pest, but if scouting reveals one or more larvae per stem, treatment may be warranted. Be aware that this pest can be patchily distributed so thorough scouting is necessary and localized treatment is an option.

Lastly, seed corn maggot has forced some growers to replant their corn this spring. This pest can be a problem in wet and/or reduced—tillage fields with high organic matter and if soil insecticides or seed treatments were not used. If you find a large number of corn or soybean seeds infested with maggots, replanting should be considered. Scouting numerous portions of fields will help determine if full or partial replanting would be better. Soil insecticides or seed treatments may not be necessary for replanting if fields have dried out a bit and the seeds will not be sitting in the ground for long. Once seeds have germinated and plants are established, this pest is not typically a concern for corn and soybean growers. If this pest is regularly troublesome, soil insecticides or seed treatments can be considered for future years.

Upcoming Events—use the link to Calendar of Events (below) for contact information.

June 26 Leguminous Cover Crop Twilight Meeting 7–8 pm, PSU Agronomy Farm Contact Beth Futrick at 814–355–6817 for more information.
July 1 Weed Twilight Tour—Landisville
July 2 No—tilling for Success Field Day—Landisville
July 10 Rock Springs Agronomy Weed Tour
July 22 and 23 Agronomic Field Diagnostic Clinics — Rock Springs Agronomy Farm

Contributors: County Extension Educators: Paul Craig (Dauphin), Andrew Frankenfield (Montgomery), Kevin Fry (Armstrong), Mena Hautau (Berks), Mark Madden (Sullivan) University Park: Bill Curran, Sjoerd Duiker, Marvin Hall, Greg Roth, Ron Hoover, John Tooker, Del Voight

Editor: John Rowehl (York)

Upcoming Events

Real time pest and heat unit activity: http://psu.zedxinc.com/cgi-bin/site.cgi?location=2&user=psu#

Calendar of Events: http://www.events.psu.edu/cgi-bin/cal/webevent.cgi?cmd=opencal&cal=cal209&

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