Vol. 1 No. 19

[Home][ISU Extension][IA State University]

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue
Carroll, IA 51401
Office: 712-792-2364; Cell: 712-249-6025; Fax: 712-792-2366
Email: vagts@iastate.edu

Northwest IA Crop Update, August 27, 2001

In this issue

New Web Site

Estimate soybean yields

Corn stalk rot

Insect pests: Bean Leaf Beetle, Soybean Aphid and Alfalfa Webworm

New Crops Web site http://www.extension.iastate.edu/carroll/crops/

Introduction

Areas across west central and northwest Iowa continued to receive scattered showers over the last week. Accumulations ranged from 4+ inches to only sprinkles. These late season rains will benefit the soybean and alfalfa crops the most as well as pastures. The additional moisture will help to add test weight to the corn crop as it progresses towards physiological maturity. Alfalfa fields are greening up very well following the rains and we need to continue to monitor webbing and feeding from the alfalfa webworm. This pest has been causing problems in Missouri, Kansas, Nebraska and western Iowa. I’ve been mentioning the Soybean aphid and its westward progression occasionally in this newsletter. Last week I found the first aphids in a soybean field in Carroll County.

Crop Development

Corn development, although slower, is progressing nicely towards physiological maturity. Physiological maturity is the point in which maximum dry weight accumulation has occurred in the grain. This can often be determined by the formation of the black layer at the base of the corn grain kernel. Harvest of corn for silage has begun in many areas; concerns over high nitrate concentrations have been lessened due to the additional rainfall.

Estimate your soybean yield potential. Although yield estimates are most accurate when made within 3 weeks of harvest, I will provide the formula now so that you have time to prepare for the procedure.

Estimate the number of plants per acre (measure an area 1/1000 of an acre and count the number of plants within the marked area.)
Count the number of pods on ten randomly selected plants within the marked area and calculated the average number of pods/plant

Calculate pods per acre by multiplying plants/acre by pods/plant
Calculate seeds per acre by multiplying pods per acre by an estimate of 2.5 seeds/pod
Calculate pounds per acre by dividing seeds/acre by an estimate of 2900 seeds/pound
Estimate yield by dividing pounds/acre by 60 lbs. per bushel.

The formula is: [(plants/acre) x (pods/plant) x (2.5 seeds/pod)] / (2900*60) = Estimated Yield bu/acre

This formula uses several estimations and therefore may be variable depending on the final number of seeds per pod and seed weight.

(Source: Corn and Soybean Field Guide, 1997; Purdue University Coop Extension Service)

Crop Disease

With the many stresses that the corn crop endured this year, stalk rot may be a problem in many cornfields. Particularly in areas that were very wet early in the season then became very dry in July and August. Extensive rootworm injury may compound the stalk rot problem where it occurs. Last week’s Integrated Crop Management newsletter addresses stalk rot detail, I’ve touched on the highlights here.

The majority of stalk rot problems will appear in September and October, but it is not too early to begin checking for potential problems.
While scouting for stalk rot, look for visible symptoms and test stalk firmness by pinching the lower internodes with thumb and forefinger.
Healthy stalks are firm and can't be compressed. If a stalk can be compressed or feels soft, it is rotted and is a good candidate for lodging.
Different hybrids and fields with different tillage, rotation, or fertilization histories should be scouted separately.

Insect Pests.

Second generation Bean Leaf Beetle economic thresholds are given in Table 1. Most damage from 2^nd generation BLB will be from feeding on the pod, look for this when scouting.

Table 1. Bean leaf beetle economic thresholds in reproductive-stage soybeans.*

Crop value
($/bushel)

Treatment cost per acre (insecticide + application)

$7

$8

$9

$10

$11

$12

$13

$14

$15

beetles per foot of row

$5.00

5.5

6.3

7.1

7.9

8.7

9.5

10.3

11.0

11.8

$6.00

4.6

5.2

5.9

6.5

7.2

7.8

8.5

9.2

9.9

beetles per sweep

$5.00

3.5

4.0

4.5

5.0

6.5

7.2

7.7

8.3

8.7

$6.00

2.9

3.3

3.7

4.1

5.4

6.0

6.4

6.9

7.3

Over the last several weeks, the soybean aphid has been found in Webster, Calhoun and B.V. Counties. Late last week I found the soybean aphid in Carroll County as well. As I’ve mentioned previously, I would not expect any yield problems from the aphid this year due to its late arrival, but I would expect that we will be finding this potentially damaging pest much earlier in soybeans next year. Look for the aphid on the underside of new soybean trifoliate leaves.

Information on the Alfalfa Webworm (garden webworm) can be obtained at ISU’s Integrated Crop Management newsletter http://www.ipm.iastate.edu/ipm/icm/2001/8-20-2001/webworms.html

Prepare for fall Harvest

Even though days are still warm and most of the crops are green, harvest is just around the corner. You should begin to prepare for the coming harvest, transport, conditioning and storage of the crop. Preparing a thorough list of all facilities and machinery that need to be cleaned as well as checked for annual maintenance should be the first step. If specialty crops have been produced where I.D. preservation is necessary, be sure that all facilities are ready and procedures have been developed.

This page last updated on 02/03/04

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