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Late planted crops need late fall

By Jean Caspers-Simmet

Date Modified: 07/15/2013 9:57 AM

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NASHUA, Iowa — Extension field agronomist Brian Lang provided farmers with tools to stage late-planted corn and soybeans at last week's field day on the Nashua farm.

Lang said it takes 1,250 growing degree days to go from R1 to R6 or physiological maturity.

"I'm interested in when corn silks," Lang said. "If it silks in July, we should be in good shape. Silking in August can be problematic."

While the average frost date for Nashua is Oct. 15, killing frosts have occurred in mid-September, Lang said. There also have been long falls with a late frost, which was the case in 2010 and 2011 when an ISU late planting corn study showed that corn planted in June yielded 70 percent of normal. Longer term research shows June planted corn is more likely to yield 50 percent to 55 percent of normal.

For May 1 to June 25, Nashua has accumulated 752 GDD, 51 behind normal.

Lang staged corn planted at Nashua April 30 at V7 toV8. Corn planted May 15 is at V5 to V6 and corn planted June 3 is at V4.

"You can kind of track development based on growing degree days," Lang said. "Knowing the stage of development is very helpful."

Yellow corn does not necessarily mean that there is no nitrogen left in the soil, Lang said. Farmers need to dig plants and look at roots. Yellow corn could be due to restricted root growth from soils that are too wet, too cool, compacted, or from seedling diseases. It could be due to a slower rate of mineralization of nitrogen or sulfur due to cool soils. The nitrogen that was available could be gone due to leaching or volatilization. Yellowing could be caused by potassium deficiency from restricted root growth from cool, wet soils.

"As soils dry, soil oxygen levels increase, leaves grow and improve in color, sugars become more available to the roots as well as the tops and this will further improve root uptake of nutrients," Lang said.

By V5, the corn crop has roots extending to the middle of rows, so injecting nitrogen between the rows should work, Lang said. Broadcast urea will need some rainfall.

For flooded corn, Lang said plants that are completely submerged are at higher risk. Those plants will be depleted of oxygen in 24 to 48 hours. Plants partially submerged will continue to photosynthesize and move oxygen. Young corn can survive up to four days of ponding under cooler temperatures. Rainfall is needed to wash mud deposits off leaves. Lengthy periods of wet soil conditions favor development of seedling blight diseases.

Lang said not much data exists on soybeans planted in June or July. Studies conducted from 1994 to 1996 show northern Iowa soybeans planted in mid-June produced 61 percent of potential yield, dropping to 33 percent for early July planting.

Northeast Research Farm Superintendent Ken Pecinovsky said he has had fairly good yields on soybeans planted in late June, but soybeans planted into July have yielded poorly and often don't make it to maturity.

In 2008 soybeans planted June 11 at Nashua yielded 18 percent less than soybeans planted in early May for 1.9 maturity group and 16 percent less for a 3.1 maturity group.

The rate of soybean development is directly related to temperatures, Lang said. Late-planted soybeans shorten the time between vegetative stages. Early maturing varieties develop fewer leaves and progress through stages at a faster rate than late-maturing varieties. Deficiency of nutrients, moisture or other stresses may lengthen the time between vegetative stages and shorten the time between reproductive stages.

Shortening day length and warm temperatures control soybean flowering, Lang said. Soybeans must reach at least V1 before they can be induced to flower.

Lang staged soybeans planted April 30 at Nashua at V4 to R1 on June 25. Beans planted May 16 were at V3 and the June 3 planting was at V1. Beans planted June 17 were at VE to VC.

Because flooding conditions deplete rhizobia populations, Lang said to add rhizobia inoculum to seed for late planted or replanted soybeans. Also consider seed treatments for pythium and phytophthora.

Producers should consider planting soybeans in narrower rows to close the canopy sooner and increase the seeding rate 5 percent to 10 percent.