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Managing delayed-growth crops

By Staff | Jul 5, 2013

MIKE OWEN, an ISU Extension weed management specialist, talks to one of several farmers on June 25 who toured the ISU research farm in Kanawha. He spoke about developing long-term approaches to controlling resistant weeds.

KANAWHA – Crop management in a wet, late-planted season, plus integrated weed and pest management options were prime topics for about 100 farmers on June 25 at the Iowa State University Extension’s summer field day at its Northern Research Farm in Kanawha.

Under skies that went from cloudy to threatening rain to sunny, the hayrack seminar was able to complete its stops that covered the subjects of current crop development, herbicide resistance, corn root worms and soybean aphids and nutrient reduction strategy.

Extension field agronomists Paul Kassel and Mark Johnson presented information on the problems encountered by farmers with the wet spring and late planting.

A corn plant was uprooted to show its progress at V6 stage when nodal roots grow rapidly in search of nutrients and water.

They said if yellowing was a problem in a corn field, additional nitrogen should be applied when the corn was at stage V8 at a rate of 50 to 60 pounds per acre.

DAVE RUEBER, left, superintendent of the ISU?research farm in Kanawha is assisted by an unidentified employee in charting the normal and actual rainfall amounts recorded this year at the farm.

Data from 1995-96 from the Kanawha and Nashua research farms showed soybeans planted on June 20 had a reduced yield of 25 percent. After July 1, soybeans will produce 33 percent of normal yield.

Kassel and Johnson advised farmers who planted soybeans from June 15 to July 1 to use a late group I variety.

Resistant weeds

Extension weed specialist Mike Owen talked about herbicide resistance in weeds.

He said herbicide resistance has been present in weeds even before the introduction of glyphosate in the mid-1970s.

A sample of 800 weeds were collected that showed some herbicide resistance.

Owen said 66 percent were resistant to glyphosate, 65 percent were resistant to atrazine, and 6 percent were resistant to all five groups of herbicides.

Owen’s suggestions to herbicide resistant weeds is a soil-applied pre-emergence herbicide to supplement post-emerge spraying.

Owen advised farmers to create a herbicide program five years in advance taking into account the weed problems, crop rotations and the diversity of chemistry available for weed control.

Owen told farmers to get away from using the “same-old, same-old” in their herbicide program.

He also suggested cultivation should be part of a weed management, program, not cultivating all acres necessarily, just where it would give the best results.

Owen encouraged farmers to use site-specific management to get maximum control of weed problems.

A fall cover crop is part of weed management and herbicide label restrictions may not be applicable on a cover crop as it is not intended to be harvested, said Owen.


Entomologist Erin Hodgson said the cool conditions of this spring have made early season pests that include seed corn maggot and slugs more plentiful.

Corn root worms are currently hatching, but the wet soils will decrease their numbers.

Hatching usually takes place around June 6 which makes them five weeks behind, said Hodgson.

CRW feed on the root tips and can enter larger roots interfering with nutrient uptake and destroying the root structure that holds the plant upright.

Crop rotation is the best control for CRW, she said, making it a bigger problem under continuous corn conditions.

Hodgson has been watching the soybean aphid population, which has the biggest population since 2007. Michigan is currently reporting high numbers of soybean aphids.

Soybean aphids can move from state to state, said Hodgson.

Seed treatment for soybean aphids lasts 45 days. Afterward, migratory aphids will be a problem.

An effective method of controlling soybean aphids is planting host plant resistant seed, said Hodgson.

Another problem in spraying for soybean aphids is an increasing resistance of spider mites to insecticides.

Japanese beetles are becoming a bigger problem for both corn and soybeans, Hodgson said.


Matt Helmers, an Extension agricultural engineer, who has an active role in the Iowa Nutrient Reduction Strategy, said one of the ways to reduce nitrogen runoff is through controlled drainage where the flow of water from field tile is slowed by using a cube-shaped device Helmers had at his feet.

The INRS is designed to reduce nitrogen and phosphorus from entering the state’s surface waters.

Other methods to reduce nutrient runoff and leaching, Helmers said, includes changing the rate of application, nitrogen inhibitors, reduced tillage, cover crops, creating wetlands and installing bioreactors.

A study on nitrogen and herbicides on drainage water has been conducted near Gilmore City since 1989 and is one of the longest running, continuously active studies in the United States.

Land use is also a possibility to reduce runoff by growing small grains and alfalfa, switch grass as a biomass, and targeting highly erodible land.

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