Hello,
Thanks for your question.  I have a few in response before I send recommendations:

• How will you be irrigating?
  
• How will you be mowing this area (if at all)?
  
• Do you want a single species manicured look, or a mix of grasses that will naturalize?
  

I look forward to hearing from you,

We have ditch rights and I have built a holding pond with irrigation pump.
I have a riding lawn mower.
The soil is sandy, rocky and mowed weeds right now I don't want it to be a continuation of the Kentucky blue grass yard. So mixed grass pasture like.

Hello,
Thanks for the additional information!  For your needs, I recommend some kind of native grass seed mixture. From the Colorado Forage Guide (http://sam.extension.colostate.edu/wp-content/uploads/sites/2/2016/07/forage-guide.pdf):

Small acreage pastures can be reseeded to a single species (solo planting or monoculture) or a mix of species. Mixtures are more ecologically desirable because of stand diversity. They survive disease and insect problems better, and extremes in climatic conditions. Mixtures are more efficient users of available moisture, light, and nutrients through the growing season. An established stand of multiple species can be expected to provide better competition against weeds.

The Forage Guide has a lot of great information about characteristics of various grasses and which ones combine well.  If you like to do your own research, you can choose what you'd like to mix and purchase separately (seed source reference below); as stated in the guide, don't mix native and introduced grasses; don't mix warm and cool season grasses.       OR, in checking out some sources from said reference, I found two cool mixes that you can buy online (more options out there, I'm sure):

1. Short Grass Prairie Grass Seed Mix from Western Native Seed:  http://www.westernnativeseed.com/SGPgrassmix.html.
   
2. Native Grass Seed Mix from the Jefferson (County) Conservation District:  http://www.jeffersonconservationdistrict.org/noxious-weeds/native-seed-sales/
   

If you decide you really want a single species of grass (monoculture) rather than a mix, two good warm season options for our area (both of which can either be mowed or left native) are:

• Buffalo grass http://extension.colostate.edu/docs/pubs/garden/07224.pdf
  
• Dog Tuff Grass http://plantselect.org/plant/cynodon-pwin04s/
  

Here are some additional resources that I found in researching your question that I'll list in case they are of interest/use to you.

• Grass Seeding Plan for Colorado Small Acreages:  http://sam.extension.colostate.edu/wp-content/uploads/sites/2/2016/07/seeding-plan.pdf
  
• Native Grasses For Use In Colorado Landscapes:    http://www.ext.colostate.edu/mg/Gardennotes/581.pdf 
  
• Colorado Forage Guide (http://sam.extension.colostate.edu/wp-content/uploads/sites/2/2016/07/forage-guide.pdf) same as above
  
• Native Plant Revegetation Guide for Colorado https://cpw.state.co.us/Documents/CNAP/RevegetationGuide.pdf
  
• Native American Grasses for Dryer Sites:  http://extension.colostate.edu/docs/pubs/garden/07232.pdf
  
• Sources of Native Colorado Plants http://www.ext.colostate.edu/mg/Gardennotes/583.pdf
  

Happy reading and planning!  Best regards,