Soybean:  Could “Imbibitional Chilling” result in
reduced plant populations? 

As often happens in the spring, daily temperatures can vary above and below long time historical averages. This wild weather ride can pose challenges to farmers wishing to balance timely planting with planting under optimal conditions. One potential effect of planting soybean into cold soils is the uptake of cold water which may damage cotyledonary and axis tissues. This phenomena has been called “Imbibitional Chilling”. The effect of this phenomena can be reduced plant populations, poor plant health, and perhaps yield reductions. 

Before seeds will germinate they must take in water, a process called imbibition or rehydration. Seeds have been in a dry condition during storage. They become metabolically active within a brief period of time following planting and water uptake.  Because imbibition is immediate, the temperature AT THE HOUR of planting and 6 to 24 hours after is the crucial time and can have great impact on seed health, probably greater impact than later soil temperatures. Seeds become far less sensitive to cooler temperatures after this initial imbibition. 

If the initial water imbibed by soybean seed is too cold it can disrupt cell membranes causing leakage of cell contents. An end result can be reduced germination. “Leaky” cells can make a seedling more susceptible to pre-emergent damping off, especially since some pathogens use the leaking cell contents as a source of energy. Cold temperatures also reduce hypocotyl elongation, which can affect successful seedling emergence.  

Soybeans are more vulnerable to this kind of injury when seed is very dry or the seed coat is cracked. Drier seed is more susceptible since it absorbs water more rapidly. A thin or cracked seed coat does not moderate the absorption rate of water as successfully as a sound seed coat can.

Seeds placed into adequately warm soils and imbibed water over the initial 6 to 24 hrs are more tolerant of soils which cool later. The risk of damage is less since the seed will have imbibed a large percentage of its water before cooling and the cell membranes will have had a chance to hydrate normally. 

Horst Bohner, a soybean specialist with Ontario Ministry of Agriculture and Food, reported on a field day demonstration designed to illustrate the potential impact of cold soils at the time of planting. His treatments were: 1 - seed planted and maintained at 77 F; 2 - seed planted at 44.6 F for 24 hrs, then held at 77 F; 3 - seed planted at 77 F and after 8 hrs cooled to 46.6 F for 4 days before being warmed to 77  F. The outcome was reduced emergence in the cold treatment. To read his comments, follow the link provided:

http://www.omafra.gov.on.ca/english/crops/field/news/croptalk/2003/ct_0903a7.htm

The message is that planting soybeans into cold soils may be very harmful to seeds and drastically affect emergence. Soil ideally should be at least 50 F at planting and for the following 6 to 24 hours since it is difficult to predict the exact length of time for initial imbibition. It may be as short as only a few hours or up to a full day. Planting which begins early in the day or late in the day may place seed at risk because of cold soil temperatures (Table 1). 

Consider comparing stands with early morning planting vs mid-day vs late evening planted soybeans to evaluate the impact this phenomena may have had in your fields. If there are differences in stand, this may be one possible explanation.

Phillip Glogoza, Regional Extension Educator  - Crops