It's almost certainly some kind of senna, but I don't have any photos of the mature plant. I collected the seeds here on the property where it grows in the riverbed (only in the riverbed - not in the bottomland) in relatively dry years. It behaves as an annual reaching up to about 8'. Blooms are yellow. Sort of an interesting plant, so I collected some of the beans and here's a couple sprouting now. They are really GREEN! The dry bean was sort of a smutty brown, but these sprouts may be the greenest seedlings I've ever had come out of a seed. They are almost flourescent. I hope next year to get real photos of the mature plant and find it's true identity. I think it's probably a bad plant for the cattle should they ever decide to eat any. It's here naturally and so far as I know they have never bothered with it.

Not much to go by without a photo of the mature plant. But I am guessing what it may be. Once the true leaves emerge it will be easier to identify. If you can remember what the mature plants looks like, check out images of 'tall bagpod' aka Sesbania and/or Sesbania vesicaria, or drummondii. It's an invasive and yes, it will harm your cows if they eat this.

Edited to add:
Sesbania drummondii
http://en.wikipedia.org/wiki/S...

Thanks Greene! This will be a big step toward identifying it.

I think this is getting really close here. The pods are wrong for the photos of Sesbania drummondii in the database, but the photo for Sesbania herbacea looks really accurate for the plant. Googling for photos of S. herbacea led me to another similar appearing plant - Glottidium vesicarium - where the plant also looks right and the a photo of the seedpods really, really look accurate. The pods ranged from only 1 - 3 beans in a pod with most of them having two beans. The last photo at this site http://www.sbs.utexas.edu/bio4... is perfect for the dried pods I shelled. They all had that inner membrane around the beans. I'm planning on making an effort to get photos of the plant with pods and blooms to try and get a good identification next year if I'm given the chance. It doesn't seem to be very invasive here. It only grows sparsely down in the dry river bed. On years with enough rainfall that the river runs more often, there is just the occasional plant. The last few years have been leaving the riverbed mostly dry, so there hasn't been any running stream to wash them up and away. I have never seen one growing even half way up the river banks. But in the meantime, I'm going to tag the photo with S. herbacea and G. vesicarium as possibilities. I will note also that along with another Sesbania, S. vesicaria is listed as a synonym for G. vesicarium in the ATP database. So there's something of a taxanomical mix-up somewhere. One drawback to Glottidium is the red in the bloom description. I'm only remembering yellow, so I'm going to have to wait for the blooms. It's not a new plant here. It's been growing in the river bed for years and years.

Great! Keep working on the identification. The one I have has yellow flowers that are just tinged with red.
I should say that I plant this (or something similar) on purpose because I save the tall stalks to use as tomato stakes. Mine grow over 8 feet tall, sometimes up to 12 feet tall. Amazing what a weed can do if it gets pampered and fertilized.

I've been looking around on the net. I found another site that says G. vesicarium is a synonym for S. vesicaria. The exact opposite of what the other one said. I think this is going to prove to be the correct plant and all I need to do is verify it when I can see it blooming and making pods. The info on this site http://essmextension.tamu.edu/... really makes me think it's what I have. It confirms a very toxic nature for cattle should they eat the bean. It also lists my specific area in Texas as an area of natural distribution. Good photos that match closely what I've seen as well as good descriptions. I expect the proof will be in the details.