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Volume 1, Issue 4

July 24, 2018

This newsletter is an update on current topics and events in agriculture in Boone, Dallas, Jasper, Madison, Marshall, Polk, Story, Tama, and Warren counties.

Meaghan Anderson
Extension Field Agronomist

ISU Extension & Outreach
3109 Old Highway 218 S.
Iowa City, Iowa 52246

Office: 319.337.2145
Cell: 319.331.0058
Twitter: @mjanders1
mjanders@iastate.edu

Upcoming Events

IDALS Apr-Dec 18 Pesticide Testing Schedule

July/August - Farmland Leasing Meetings, Multiple locations

Aug. 8 - Cover Crop Field Day, Roland

Aug. 10 - Iowa State Fair Weed ID Contest

Aug. 21-23 - Iowa Drainage School, Nashua

Aug. 28-30 - Visit us at the Farm Progress Show!

Sept. 13 - Managing Farmland Drainage for Women Landowners, Altoona

Central Iowa Crop Update

Updates from an agronomist and weed nerd in central Iowa.

In this issue:

Upcoming Cover Crop Field Day - August 8
Growing season update
Scout now for corn rootworm beetles and root feeding
Soybean insects - defoliators, aphids
New tissue testing recommendations from ISU for P and K

August 8 Cover Crop Field Day - Roland, IA

ISU Extension and Outreach, Practical Farmers of Iowa, and Iowa Learning Farms will be hosting a cover crop field day from 5:30 - 7:30 p.m. on August 8th to share information on incorporating cover crops into farm operations, including following seed corn, cost share opportunities available for farmers interested in implementing cover crops or other nitrogen-management practices, and information on local water quality data. We will offer 1.5 hours of soil and water CCA CEUs available to attendees. The workshop will begin at the Roland Community Center at 5:30 p.m. with a free, catered meal and will end on a nearby farm. Please RSVP to Iowa Learning Farms by calling 515-294-5429 or via email at ilf@iastate.edu. Learn more about this event here.

Growing season update

As I'm sure you know, our crops are still way ahead of what would be considered 'normal' with the late planting dates we had this year. I put in an April 23 date to start the growing degree day (GDD) departure calculator here (but you can change the starting date and see where your crop is estimated to be) and the state is 150-400 GDD ahead of an average year. The National Weather Service is still predicting a warmer long-term outlook, but the temperatures are down for the immediate forecast, which will provide the crops some welcome relief. We are in need of continued timely rains through much of the central, eastern, and southern portions of the state to keep our crops going through the reproductive stages as some areas that are not currently in drought are at risk for a flash drought with continued heat and lack of rain.

Most corn is in the R3 (milk) growth stage now, with some behind and some ahead depending on planting date and geographic area. Pollination appears to have gone very well for most fields I've checked, though some field edges may have spotty pollination at ear tips due to insect feeding on silks. During R2 to R3, we can lose yield via abortion of kernels that were pollinated during R1. This article from Bob Nielsen (Purdue) has a lot of great information on the grain fill stages of corn that might be useful to you. When corn reaches the R4 growth stage and beyond, yield reductions come from lack of filling kernels (small kernel size). I've included some information below about some of the upcoming reproductive stages in corn.

Stage	Description of stage	Comments	Time to next stage
R2	Blister (clear liquid in developing kernels)	Maximum vegetative dry matter. Minimal grain dry matter.	About 8 days to R3
R3	Milk (white liquid in developing kernels)	Outside of kernel is yellow. Starch accumulation is increasing.	About 6 days to R4
R4	Dough	Starch accumulation increasing. Kernel moisture starts decreasing.	About 7 days to R5 (beginning dent stage)

Most soybeans I've seen recently are either in the early pod (R3) or full pod (R4).

Stage	Description of stage	Comments	Time to next stage
R3	A pod at least 3/16-inch long at one of the four uppermost nodes of the main stem with a fully-developed leaf	Most popular stage for foliar fungicide applications other than for white mold control. In some cases, R1+R3 stage applications for white mold control	About 9 days to R4 stage
R4	A pod at least ¾-inch long at one of the four uppermost nodes of the main stem with a fully-developed leaf	Beginning of the most crucial period of plant development in terms of stress influencing seed yield. Rapid and steady dry weight accumulation by pods.	About 9 days to R5 stage
R5	Seeds are 1/8-inch long in the pod at one of the four uppermost nodes on the main stem with a fully developed leaf.	By R5.5 stage, plants obtain maximum height, leaf area, and node number. Rapid and steady seed dry weight accumulation.	about 15 days to R6 stage

I continue to receive phone calls about various herbicide injury in soybeans, which has primarily been related to dicamba drift, tank contamination, or other plant growth regulator herbicide movement. We wrote an article several weeks ago about other causes of injury that may be mistaken for dicamba drift, which you can read here. Please don't hesitate to call if you have an issue and are uncertain of the cause or would like a 2nd opinion.

Scout now for corn rootworm beetles and root feeding

Western and northern corn rootworm beetles seem to be a constant issue we battle across parts of the state each year. Several fields across the state have already been reported to myself or colleagues as having potential Bt trait failures in managing western corn rootworms. Next week, I'll be checking a field with a suspected trait failure in Dallas County. Now is a great time to be out checking your fields for the presence of high levels of beetles, as well as digging roots to check for root feeding. I've included some notes about their identification below, but you can also click this link to look at photos.

Northern corn rootworm beetles are typically the smallest of the rootworm beetles at only about 1/4" long. They are typically one color with no apparent markings and can range from a yellow/light green to a darker green. NCR produces one generation per year, but some NCR populations can survive as eggs in the soil for multiple years. This is known as extended diapause, allowing them to adapt to corn-soybean rotations in Iowa.

Western corn rootworm beetles are slightly larger than NCR beetles and have distinctive black markings on their forewings. Typically the black markings are three obvious lines, but sometimes the black looks more solid (usually on males). WCR females can lay eggs in other crops aside from corn, but this variant is rare in Iowa and more common in the eastern corn belt. WCR is notorious for its development of resistance to Bt traits used to manage it in continuous corn fields.

This farmer's first suspicions of an issue came from the root-lodged corn in his field and the abundance of western corn rootworm beetles on the leaves, silks, and flying around the field. Roots showed browning and obvious feeding under close inspection.

Learn more about approaches to manage corn rootworms here and use the interactive node injury scale to look at the effects of progressive rootworm larvae feeding on corn roots. You can also compare the feeding you see to that on the scale to determine severity.

Soybean insects - defoliators, aphids

Now that corn is done pollinating, concerns with insect issues return in soybeans as Japanese beetles defoliate some upper canopy leaves and questions about "where are the aphids" come up.

In general, defoliators like Japanese beetle need to remove about 20% of the leaf tissue on an entire plant (and across the whole area) to require treatment. Feeding is typically worse along field edges or waterways, so it's important to scout the whole field or area to determine the severity of the issue. This article from the University of Nebraska-Lincoln has a nice description on scouting procedure for defoliation in fields. Soybeans are remarkably good at compensating for leaf tissue loss. I've included a photo below of what 20% defoliation might look like on a soybean leaflet, but this link has some nice photos to gauge your eyes. As some areas are getting drier, it is even more important to wait until this 20% defoliation threshold to spray for Japanese beetle due to the potential risk of flare ups of twospotted spider mites. Cooler temperatures will mediate this risk, but I expect we'll warm back up again.

Adult Japanese Beetles are metallic bronze and green with white tufts around the side of their abdomen.

Example of 20% defoliation on two soybean leaflets. Photo credit: U of W-Madison.

ISU Extension Entomologist, Erin Hodgson has a good article on Japanese beetle thresholds and management in corn and soybean, which can be found here. Be careful not to confuse Japanese Beetles with another similar looking beetle called a chafer.

On the soybean aphid front, we were hearing crickets until late last week when great crop scouts spotted them in several areas across the state, including central Iowa. Scouting for soybean aphids can be tricky, especially if populations are very low, but keeping an eye out for the brightly-colored (red/orange) Asian lady beetles can be helpful. They're avid predators of soybean aphids. If aphids are found, it's important to monitor populations and not treat too soon. The economic threshold for aphids is 250 per plant, but late-season accumulation of aphids (after R5) does not impact yield as significantly as earlier infestation does. Read more on management recommendations and Erin Hodgson's 2017 insecticide evaluations here.

New tissue testing recommendations from ISU for P and K

Tissue testing for nutrients is always a hot topic this time of year when suspicions of deficiencies show up in early reproductive stages. Until recently, Iowa State University had no reliable calibration to suggest what tissue testing results meant. Recently, Antonio Mallarino and John Sawyer were able to conduct more research to provide interpretations for what the phosphorus (P) and potassium (K) results mean in an Iowa crop field. They've also provided detailed information regarding how plants should be sampled at different growth stages.

Tissue testing should be used as a complementary tool to soil tests and can be useful in assessing the supply of these nutrients in the crop plant. Read more about this new publication here and download it for free from our Extension Store here.

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Iowa State University Extension and Outreach · 3109 Old Highway 218 South · Iowa City, IA 52246 · USA