June 27, 2013
This
past week we have once again been sidelined with some heavy rains. The southern
locations have been a bit on the dryer side and able to work through some of
the rain fall. The northern locations have been saturated with rain and in some
cases flash flooding. These wet soils have led to many questions about the
survival of crop and nitrogen loss.
First
let’s talk about the survival of corn and beans in the flooded or saturated
areas. There are several factors that corn survival depends on: length of time
flooded, partially submerged or completely submerged, temperature, and stage of
development. Basically, corn in flooded soils depletes its available oxygen in
24 to 48 hours. Continued survival is better with cool temperatures, cloudy
days and clear nights. Also, partially submerged crops can withstand flooding
considerably longer than completely submerged crops. The crops which do survive
may have disease problems later in their lifecycle. Soybeans follow almost all
the same guidelines. They will only be able to survive for 24 hours without
oxygen, and completely submerged fields will suffer a greater loss than
partially saturated fields.
The
next big question is; “What is happening to my nitrogen?” There are a few
different considerations one must look at. First, some areas where apparent
nitrogen loss appears to be showing up may be a lack of oxygen to the corn
roots. Oxygen is needed for normal root development and for efficient uptake of
nutrients by the roots. Oxygen levels will be depleted in flooded soils and foliar
symptoms can be exhibited that may look similar to nutrient deficiencies. Second, we need to look back to when the
nitrogen was applied. Early fall applications will obviously have the greatest
risk of leaching, whereas late spring or side dress nitrogen will be the most
likely to still be available. When the decision comes to rescue apply nitrogen
we will have to take into consideration, timing, cost, current crop condition,
yield potential and application method.
Just
last night we observed some of the first lighting bugs in the area. As most
know when the lighting bugs show up we should start to be concerned with the
root worm and corn borer hatch. According to Iowa State University the corn
rootworm egg hatch typically occurs from late May to the middle of June, with
the average hatching date around June 6th. Egg development is driven
by soil temperature, which is measured by degree days. Iowa State research
suggest about 50% of egg hatch occurs between 684-767 accumulated degree days
based on a soil temperature of 52 degrees. The map below shows the heat unit
accumulation at Iowa State rootworm stations on 6-14-13. On an average summer
day in northeast Iowa we accumulate approximately 18 heat units. This leaves us
roughly 13 days from 6-14-13 until the hatch will begin.
Thanks for reading and if there are
any questions about the topics discussed above please stop and ask one of the
agronomists at a location near you.
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