Wednesday, September 26, 2012

2012 Area Soybean Conferences: December 11-13

First Announcement: The Wisconsin Soybean Association, Soybean Marketing Board, and Cooperative Extension are hosting the 2012 WI Area Soybean Conferences. They will be held on December 11-13 in Holmen, Ripon, and Janesville, WI. Topical areas include:

  • The 2012 Drought Impact on the 2013 Soybean Crop
  • Assessing Soybean ROI. Where Should I Spend my Money for 2013
  • New Soil Fertility Recommendations for Soybean
  • Do Soybean Leaf Tissue Tests Tell you the Real Story
  • The National Soybean Sustainability Initiative and Why You Should Care
  • Soybean Disease Considerations for 2013
  • Marketing Tips for 2013
  • New products
  • Membership
 Registration information will be available November 1st.

Wednesday, September 12, 2012

A review of glyphosate use for preharvest weed control

From Vince M. Davis – Extension Weed Scientist

Late-season weed escapes are very prevalent in corn and soybean fields this year. Poor residual herbicide activation and poor postemergence herbicide efficacy is part of the reason. Additionally, and perhaps more importantly, poor crop canopy development and late-season rainfall encouraged large weed flushes much later this year. In some cases, weed densities are quite high and a preharvest herbicide application may help limit seed production of some annual weed species and improve the efficiency of harvest operations. This will be particularly true where thick grassy weeds or lots of large broadleaf weeds like giant ragweed, common lambsquarters, and velvetleaf are in soybean fields.

We’ve already received a couple questions regarding preharvest applications of glyphosate in soybean. First, there are a couple other herbicides besides glyphosate, like paraquat and carfentrazone, which could also be used in this manner. Paraquat (Gramoxone Inteon) and carfentrazone  (Aim) are both contact herbicides. The advantages of these products versus glyphosate will be faster desiccation of weeds. However, proper nozzle selection to deliver appropriate droplet sizes and higher carrier volumes will be very important. Additionally, carfentrazone has a much narrower weed spectrum than glyphosate or paraquat. For more details regarding the preharvest use of carfentrazone or gramoxone products, please read and follow the directions accordingly. However, we get many more questions about how to use glyphosate as a preharvest aid so I will try to elaborate a little further.

There are too many glyphosate formulations for me to know them all, so the most important message is to read and follow the herbicide label for the specific product you are using. But, I’ll give you some ‘cliff notes’ that are important to know. Unlike paraquat and carfentrazone mentioned above, glyphosate is a systemic herbicide. A systemic herbicide translocates to the growing point once it is inside the plant. This movement typically happens with the phloem of the plant (i.e. with the ‘food’ for the plant), and we call this movement ‘source to sink’. In many ways, this movement is an advantage over contact herbicides because it is the new growth of the plant you most want to terminate. I went through that very brief lesson on herbicide action to make a very important point when it comes to using glyphosate as a preharvest aid, and that is; appropriate timing is very important.

Preharvest applications can be made in corn, but I know it is more common in soybean so I will focus on that crop. It is illegal to spray glyphosate after full bloom (R2) until soybean pods have lost all their green color. Why, because between R2 and R8 the soybean plant is developing seed. As the seeds develop, they are a ‘sink’.  Technically, the R8 growth stage is the final growth stage call ‘full maturity’.  Full maturity is defined as 95% of the pods having lost green color and is usually 5 to 10 days before the field is ready to harvest. However, the glyphosate label reads (as I’ve already stated) that preharvest applications should be made after ALL pods have lost green color. Also note, you should not make preharvest applications to beans used for seed because a reduction in germination or vigor may occur.  Applications made too early in pod maturity run greater risk of glyphosate being translocated into the seed tissue resulting in illegal residues in the seed.

On the flip side, there is a required preharvest interval for grain of 14 days between a glyphosate application and harvest of glyphosate-resistant (Roundup Ready and Roundup Ready 2 Yield) soybean.  Moreover, because glyphosate often takes some time to ‘work’ it may take near those 14 days before complete activity will be achieved.  So, in order to have time for the weeds to desiccate to the maximize amount, and to avoid excessive shattering from delayed harvest, it is imperative to scout fields closely in the final days of maturity to time the application correctly. This will be an even greater challenge in fields that reach maturity very un-uniformly this year due to the variable field moisture conditions.

Now for some good news, because of the translocation effect of glyphosate is from source to sink, in annual weed species that are setting seed glyphosate may help to significantly reduce the number of viable seeds following the preharvest application, versus no application at all. At this point in the season the application can’t reduce biomass, or save the production of yield, but could limit the size of future weed problems. Moreover, a glyphosate application will have much greater efficacy on any biennial and perennial weed species like thistles, quackgrass and common dandelion because their active ‘sink’ in the fall is the root system. So, a preharvest application will also reduce populations of those weeds more than an application of a contact herbicide.

How much glyphosate and adjuvant to use at a preharvest application? Rates and surfactants should be added according to the label for each specific glyphosate product. In the case of Roundup PowerMax, no additional surfactant is needed. The rate for a preharvest application on Roundup Ready and Roundup Ready 2 Yield soybean is 22 fluid oz/a.

Maximum Application Rates of Roundup PowerMAX on glyphosate-resistant soybean
Combined total per year for all applications 5.3 quarts per acre
Total of all Preplant, At-Planting, Preemergence applications 3.3 quarts per acre
Total of all in-crop applications from cracking through flowering (R2 state soybeans) 64 fluid ounces per acre
Maximum preharvest application rate 22 fluid ounces per acre

Hopefully this helps with making properly timed and legal preharvest glyphosate applications where deemed beneficial.

Tuesday, September 11, 2012

Drought Induced Shatter

The drought of 2012 continues to introduce new issues even as we progress into R7 (beginning maturity) soybean.  Reports out of SW WI and verified at our Lancaster research site show drought induced shattering localized in the upper pods (Images 1 and 2 below). This issue is mainly occurring in the early maturity group soybeans and is being noted in other states. I have two running hypothesis for this phenomenon. The first hypothesis that Emerson Nafziger (University of IL), Seth Naeve (University of MN) and I agree on is (and I will quote Dr. Nafziger)...

"Preharvest shatter is an issue here with early-maturing beans – the ones that were already at R7 before it rained a lot the first days of September. Those would not have added any more weight to speak of, but they might have swelled with the rain. Dew has been heavy the last week or so as well, and rewetting and drying has occurred on a daily cycle. We’ve seen before that drought conditions during pod development simply result in weak pod sutures, and when pods (and beans, maybe) rewet after they’ve dried, they simply open more easily."

My second hypothesis is that this is a drought induced mechanism of survival or seed dispersal. This is less plausible given "that soybeans have had their natural tendency to shatter beaten out of them as much as possible by breeders". However given the severity of the drought coupled with shatter timing (pre-R7) it may be a possibility.

Regardless of cause shatter can significantly impact yield especially if  it begins weeks prior to harvest ripe (Image 3). As a general rule of thumb a loss of 4 seeds per square foot equals one bushel per acre yield loss. I have yet to reach an average of 4 seeds per square foot in my research plots however if you are concerned and are questioning harvest timing to minimize shatter in your early maturity group soybeans please see Check Combine Settings to Minimize Soybean Harvest Loss.

 Image 1. Open pod with exposed seed.

 Image 2. Curled pods post shatter. 

 Image 3. Shattered seed on the ground.

Monday, September 3, 2012

Spider Mites: My Lessons Learned and Varietal Differences Noted

Like many growers and crop consultants if I don't see spider mites for another 24 years it will be too soon. As we transition into physiological maturity and put this season behind us it is always a good idea to reflect back on what we have experienced and learned so a documented record is present for the next time these infrequent events occur. So here is what I learned battling spider mites in 2012.

  1. There are product efficacy differences. I sprayed our Janesville, East Troy, Fond du Lac, and Arlington (twice) sites in 2012 for spider mites. All of the locations were sprayed with Dimethoate except Fond du Lac which received bifenthrin. For the most part I was pleased with control at all locations. 
  2. I waited too long to spray. I am a strong proponent of IPM guidelines and follow them religiously with excellent success. My mistake was that I did not effectively scout the large border areas surrounding our Arlington research trials. This did not lead to any significant plot loss or impact my research but it did lead to a continuous re-supply of spider mites due to egg numbers that I had to manage.
  3. Rain did not help. Rain did knock down populations for a couple days but they quickly rebounded and required chemical control.
  4. Varietal differences are evident. Plot to plot variability was noted at our Janesville and Fond du Lac sites so we took plot notes to quantify (0-10, where 0 = no plot damage and 10 = 100 of plot injured. this variability. Analysis of the data indicated significant varietal differences (0 to 8 at Janesville and 0 to 3 at Fond du Lac). It is unclear whether these differences where due to initial selection, preferential feeding, or increased fecundity (more eggs) but they were real. Given the infrequency of spider mites for Midwest growers I would not select varieties based on this criteria, but it may help explain infestation or control variability for 2012.