A word about Marestails

In recent years marestails (horseweeds) have become increasingly hard to kill. Many of these plants we see growing in the fields are already glyphosate (Round Up) resistant. It is not unusual to see a clump of three marestails INCHES apart and two will be crispy dead and one will be untouched.

Compounding the problem in a non inclusive list are the following:

1. Marestails are annuals and can emerge as early as February and well into the summer months. This means you might have 3 inch marestail in the same field as 4 foot marestail requiring different chemical rates (the 4 foot marestail would take fairly massive rates of chemical). Or you might burn down the marestail early and many more might be in the ground waiting to come to the memorial.

2. Marestail stand erect with small lance like leaves and do not offer a large leaf area to apply a chemical to, making it difficult to get enough chemical into the plant to kill it.

3. When you do get a chemical in them and burn them down to the ground with a 2 inch stump sticking up, their large tap root has the ability to remain alive and start sending branches off of the stump.

4. When they are stressed they are very fast and very good at going into survivor mode. When it is 105 degrees out, or dry, or both together, these plants shut down fast. CHOP it’s head off with a combine and then try to kill it and you are fighting stress, heat, dry, and even less leaf area to hit and you have a problem. The same can be said about chemical stress. Put a too light of a chemical load into the plant and give it a stomach ache only, then they become almost impossible to kill.

4. Most if not all are now glyphosate resistant. This weed is just getting scary! When we tank mix other chemical modes of action in the mixture it is starting to show resistance to them as well. We have recently began seeing mature hardened off marestail not die with chemical treatments and rates that always got them before including the chemical family of phenoxies (24d, dicamba, tordon, etc.) and that folks is scary.

Are Marestails just having a couple of good years and will mother nature take them down or slow them down next year……………or are they becoming the monsters we fear they are? Only time will tell but right now they are very hard to kill.


I wrote this last year about July.  Now in June of 2010 I only think this weed is worse, a true "monster".  Killing this weed in growing soybeans is very difficult if not impossible (depending on size) and fairly expensive, so do everything you can to control it prior to planting.  All of us are trying many different chemicals and are still waiting on the results.  It has been my experience not to trust the first look after trying a chemical.  Marestails can look like you had great efficacy for a week or two and then start growing again. In some instances tillage with sweeps (cut their legs out from under them) may be the best (only) answer.


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