Crop Update for SE Iowa by Jim Fawcett
CROP UPDATE 4/9/2013
Iowa State University Extension Information for Southeast Iowa
By Jim Fawcett, ISU Extension Field Agronomist
3109 Old HWY 218 South
Iowa City, IA 52246
SUBSOIL MOISTURE LEVELS & TEMPERATURES
This is certainly been quite a change from last March and early April, and I wouldn’t expect to see corn planters going any time soon. Last year we had soil temperatures above 50F by now, but they have been mainly in the low to mid 40s the past few days http://extension.agron.iastate.edu/NPKnowledge/soiltemphistory.html. We have been fortunate in SE Iowa to have gotten more rainfall than other parts of the state, and parts of the area are no longer considered to be in a drought according to the drought monitor http://droughtmonitor.unl.edu/DM_state.htm?IA,MW. Tiles are again running at the research farm near Crawfordsville, indicating that the subsoil moisture is now full. We also pulled some soil samples to a 5-foot depth last week in 3 locations in eastern Iowa (Harper, Cedar Rapids Airport, and Keystone). Although those samples have not been analyzed yet, we’re guessing that there is about 7-9 inches of plant available moisture in the top 5 foot at those locations. This is slightly below normal, but with this week’s rainfall, those locations should also be getting close to field capacity (10-12 inches). I wouldn’t be making any major changes in cropping plans due to a fear of drought, but it is always a good idea to carry crop insurance, because the weather can change in a hurry.
One thing that we learned from the drought of 1988 was that less nitrogen was needed for the 1989 corn crop because of more carryover nitrate. There were even fields in 1989 where the optimum rate of nitrogen was zero on soybean ground because so much less nitrate was lost through leaching and de-nitrification. However 1989 was also a dry year, which reduced nitrate losses in that year also. If this wet week continues, we cannot expect to have as much carryover nitrate as what we saw in 1989. Rather than just guess how much nitrate has carried over, it would be best to pull some soil samples this spring shortly before the nitrogen application is made to see if nitrogen rates can be cut. Samples need to be pulled to at least a 2-foot depth, taking at least 15 cores per sample. For more information, see the fact sheet at http://www.extension.iastate.edu/sites/www.extension.iastate.edu/files/www/040113EstimatingNCarryover.pdf.
If soil samples are not taken, I would at least error on the low side of the recommended nitrogen rates this year. We went through several wet springs and summers prior to the drought of 2012 where it paid to error on the high side. This is a year where we can hopefully save some money and also reduce future problems with high levels of nitrate in the water. A tool that can be used to help select nitrogen rates can be found at http://extension.agron.iastate.edu/soilfertility/nrate.aspx.
From Brian Lang:
ALFALFA & SMALL GRAINS
Evaluating Over-Wintering Alfalfa
I do not see any significant problems with the over-wintering alfalfa other than from the ‘frozen pond areas’ in some fields. Alfalfa can only survive for about 3 weeks under thick ice sheets. However, also consider that older stands eventually decline due to disease, wheel traffic damage and inherent aging issues. So alfalfa stands, particularly older stands should still be evaluated for stand density and root health. Dr. Barnhart, ISU Extension Forage Specialist, recently posted information on stand evaluation of alfalfa and other forages in the ICM News at: http://www.extension.iastate.edu/CropNews/2013/0318barnhart.htm An alternative evaluation to plant counts is stem counts. Once alfalfa plants reach 6 to 8 inches tall, we could count stems per square foot for a better estimate of stand potential. The UW publication explains the stem count method at: http://learningstore.uwex.edu/assets/pdfs/A3620.pdf Alfalfa stands averaging 55 or more stems per square foot are excellent stands.
Control Winter Annuals in Alfalfa Now
Every May I get phone calls on outbreaks of weeds in alfalfa fields like Shepherds purse, Field pennycress, and/or Pepperweed. However, by that time there isn't much of anything that we can do about it. These are among the most common winter annual weeds that show up in alfalfa fields. They are not very palatability or high in quality. Winter annuals germinate in the fall, develop a rosette-type growth like this Shepherds purse plant http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/shepherds_purse/rosette_3_750.jpg The plants overwinter as the rosette, and then bolt (rapid growth of upright stems) in early spring producing seed for next fall. Once these plants bolt, herbicides are not very effective. The time to control these weeds is while alfalfa is still dormant, using herbicides such as Pursuit or Velpar. Velpar is the better choice for dandelion control. The only way to know if these weeds are present is to scout the fields. However, realize that most winter annual weed situations are not heavy enough to significantly interfere with yield and quality of the overall forage, so the herbicides are not usually recommended. If the alfalfa field is a marginal stand and with weed problems, rotating to corn may be the best option.
Planting Small Grains and Alfalfa
Plant small grains as soon as spring soil conditions permit. For alfalfa we prefer to wait until soil temperatures are in the 40’s (usually April). Small grains germinate at soil temperatures in the mid- to high 30’s, whereas alfalfa germinates at soil temperatures in the high 40’s. On average, planting after mid-April, oats and wheat lose an average of 10% yield per week, and planting after May 1 they lose about 15% yield per week compared to a mid-April planting. Seeding rates for a pure small grain stands should be about 30 seeds per square foot, which is about 3 bu/ac for oats, 2 bu/ac for barley, and 1.7 bu/ac for wheat and triticale. As a companion crop with alfalfa, reduce small grain seeding rates by one-third to one-half to reduce the competition on the alfalfa seedlings. Oats are seeded at about 1 inch deep, and alfalfa and other forage legumes and grasses at about ¼-inch to ½-inch deep followed by press wheels or a cultipacker. More details on alfalfa establishment can be found starting on page 10 in the Alfalfa Management Guide at: https://www.agronomy.org/files/publications/alfalfa-management-guide.pdf
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