Agronomic life as we in north central Oklahoma have known it has shifted… changed, this summer. Glyphosate resistant Palmer Amaranth pig weeds have popped up all over Garfield, Grant, and Kay counties.
In the summer of 2013 we had verified them in two fields within close proximity to one another in Grant County. I knew of none in Garfield. This year they are spread throughout both counties as well as some in Kay County. How did they do that, this quickly?
The members of the pigweed family that have a significant presence in our part of the world are: Water Hemp, Prostrate, Red Root, and (probably the worst and most common) Palmer Amaranth. Water Hemp has been glyphosate resistant for a number of years now and although we do have them they have not had as numerous or prolific numbers in most of our fields as has the amaranth. We already had atrazine and SU resistant amaranths, now the amaranth is glyphosate resistant. Let that sink in a moment.
Distinguishing between the different pigweed species can be difficult especially when they are seedlings. Some of the characteristics of amaranths are a rough raspy seed head often over a foot long, a long petiole (the structure between stem and leaf) usually longer than the leaf itself, a single barb like hair at the tip of the leaf, and often a chevron shaped water mark on the leaf itself. However not every amaranth has all these features all the time. Sometimes they may have one, two or all, sometimes none can be found.
Palmer Amaranths produce hundreds of thousands of seeds per plant, can emerge from at least May to September, and are able to grow at times in their life cycle up to 3 to 4 inches per day. As they gain in height they can have over a hundred growing points from which they can start growing again if not completely destroyed.
The advent of glyphosate resistant amaranths probably has the most significant impact in our area in soybeans. It has been found that amaranths can reduce soybean yields up to 79%. I have seen them make the crop unharvest able, thus leading to 100% yield loss. Until this summer you could basically control Palmer amaranths with glyphosate, so most producers with RR Ready soybeans just applied some form of glyphosate on their soybeans and that was the extent of their chemical program. Not anymore. Mother Nature wins.
What do we do now?
- Plan ahead with your rotations. Perhaps you shouldn’t plant soybeans in a field that has a history of high pigweed populations. Go to winter crops such as wheat or canola, or a summer crop of corn if you have acres that will produce corn. There are more chemical options for control of amaranths in corn.
- Start clean. Eliminate as many weeds as possible while you have the opportunity to use growth hormone chemistries such as 24Ds and or dicamba.
- Give yourself time. You often have to allow time depending on the chemicals used, between applications and planting to allow the chemicals to breakdown so they will not interfere with seed germination. Spur of the moment decisions to plant beans into wheat stubble can often times lead to weedy messes.
- Use pre-emergents. There are no chemicals to apply on post emerged soybeans that have good efficacy on large pigweeds. IF the pre-emergent chemical gets activated by rainfall it seems to significantly reduce the pigweed population. I have seen them come through the chemical barrier but usually at much reduced numbers.
- Use all adjuvants and additives called for at the correct rate. If the chemical label ask for ammonium sulfate at 17 pounds/hundred gallon solution, with a methylated seed oil, then put them in!
THE MOST IMPORTANT POINT! Not just on pig weeds but on all weed control.
- Get them while they are SMALL. Be timely with your chemical applications. All the chemicals you are using out there list chemical rates based on weeds that are 2 to 4 INCHES tall. You are off label every time you spray the chemical on weeds larger than 4 inches. If the weed is 48 inches (4 foot) tall do you put on 12 times the label rate? There are post emerge chemicals other than glyphosate, for soybeans, that have good efficacy on 3 to 4 inch pigweeds.
In my entire career in this industry I have never seen weeds so out of control over basically all of our farmland as has been the case this summer. Our extremely heavy reliance on chemicals without the knowledge of how to use them properly and timely might have led to this. Yes, we have had unusual weather and a strange harvest, but as an industry (producers and suppliers) we can and must do better. We are going to have to or Mares tails, Water Hemps, Palmer Amaranths, Rag weeds, Kochias etc. are going to be our crops in the future.
We can do this, but we will need to tighten our management.
P.S. Palmer Amaranths are edible, maybe we can eat them.
Palmer amaranths in Garfield County soybean field after two separate applications of 1 pound active ingredient of glyphosate.