I went to bed last night hoping this
article would not have to be written, but that whopping 0.05 hundredths (or less) of rain we received across much of Southern WI last night forced the
issue. According to data from the 2012 Weather Summary for UW ARS - Arlington and Marshfield
WI Locations (compiled by Dr. Joe Lauer) we are tracking over 4
inches behind the 30 year norm at Arlington (precipitation from April 1 to today). A similar trend
was occurring at our Marshfield location until recent rainfalls brought us back
to near 30 year norms. The implications of this water deficit on soybean are as
follows.
In soybean there are two growth
periods for which soil moisture is critical for optimum growth and development:
at planting and during the reproductive stages from bloom through pod
fill. The time period from stand
establishment to bloom is not as critical.
Drought stress during this time period will often shorten internodes;
however yield loss rarely occurs. Luckily most of the WI soybean crop is in the
vegetative growth phases though bloom is beginning. As stated above depleted soil moisture at planting can significantly impact the soybean crop. Therefore if this dry weather pattern continues growers should be very cautions about planting double cropping soybean into dry soil.
In Wisconsin the main reproductive
growth in soybean occurs from early July to mid-September. Soybean in this stage use about 1/4 to 1/3
inches of water per day. Lack of
sufficient water can cause flowers and young pods to abort reducing the number
of seeds per plant (Image 1). Also,
soybean plants reduce the size of their leaf pore openings to reduce the loss
of water vapor. This also reduces the
intake of carbon dioxide and the manufacturing of photosynthates which slows
plant growth. When normal soil moisture
returns, normal growth is resumed. This
ability to reduce metabolic activity allows plants to tolerate dry spells
without dying or harming their ability to resume growth when normal moisture
returns.
Image 1. Pod abortion in soybean caused by drought stress.
If a drought does develop and severely affects podset and seed fill, and if livestock feed is needed, soybeans can be harvested
as a forage for ensiling. Highest
protein and yields are obtained from soybean harvested at the R6 to R7 growth
stage. Harvesting soybeans for forage
between the R1 and R5 stage will result in very high quality silage, but dry
matter yields will be reduced significantly.
Forage quality will be reduced from R5 soybean forward if a conditioning
process is used during harvest.
Conditioning will cause significant seed shattering.
References:
Managing Drought-Stressed Soybeans
in the Southeast. North Carolina
Cooperative Extension Service. 1999.
Virginia Soybean Update. Virginia
Agricultural Experiment Station. Volume 2, No. 4, July 1999.
