The early harvest this year in Iowa has created opportunities to complete fall fieldwork and manure applications much earlier than usual. However, applying manure before soils have cooled to below 50°F can be a costly decision.

“Pushing manure application to later in the fall or waiting until spring can help to prevent nitrogen loss and better match nutrient availability with nutrient demand by crops,” says Brian Dougherty, an Iowa State University Extension ag engineer in northeast Iowa.

A research trial at the ISU Northeast Research and Demonstration Farm near Nashua found significant yield reductions when fall manure was applied when soil temperatures exceeded 50 degrees. In a corn-soybean rotation, late-fall-applied manure (soils less than 50 degrees) averaged 40 bushels per acre greater corn yield than early fall manure over a three-year period from 2016 to 2018.

Weather conditions in the fall 2018 prevented the early-to-late fall manure comparison for 2019. However, in a late-fall-to-spring comparison, the spring manure treatment had an 18 bushel-per-acre yield advantage.

Research: Better to wait

Similarly, a late-fall-to-spring comparison in continuous corn showed a 38-bushel-per-acre yield advantage (three-year average) for the spring-applied manure. Research conducted at other locations has also reported yield advantages with delaying manure application timing, as shown in the table below.

“There is always a possibility of poor weather conditions if manure application is delayed,” Dougherty says, “but this needs to be compared with the risk of nitrogen loss and lower yields when manure is applied too early in the fall.”

He suggests reading an article on the ISU post Improving crop yields and water quality with manure management for more information, including the effects of a cereal rye cover crop with early-fall applied manure.

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