The best thing you could do is contact Bob Scriven, Kearney NE, 308-440-7385. He is an irrigated pasture consultant that can help walk you through some of the economics, planning and decision making, and would likely save you time, money and frustration. He is available for on site and continuing consultation. He has worked for many years with irrigated and other pasture/forage consulting across the US.
Two other excellent resources are:
Bruce Anderson, UNL Extension Forage Specialist 402-472-6237,
banderson1@unl.edu
Jerry Volesky, UNL Range and Forage Specialist, 308-696-6710,
jerry.volesky@unl.edu
As you suspected irrigating warm season native grasses is usually not economical,but don't completely rule it out. You are probably looking at a cool season grass/legume mix, as they will make more efficient use of the added water and fertility. Grass species may include Meadow Brome, Orchardgrass, Tall Fescue, Meadow Fescue, Smooth Brome, Intermediate Wheatgrass, and Festulolium. Legumes may include, Alfalfa, Birdsfoot Trefoil, Red Clover, White Clover, and Sainfoin. Species selection will depend on the existing soil fertility and your plans on how much additional water and fertility you can economically add. Generally in northern Nebraska, simulating around a 30" rainfall enviroment with 125-150 units of N spread throughout the season will be in the optimal ballpark.
We have worked with a lot of irrigated pasture establishment in Nebraska and the surrounding states, and irrigated pasture seed mixtures were our main business until corn prices skyrocketed a few years ago. Thankfully about that time cover cropping interest exploded, and we had been dealing and promoting that for years before it became popular. In one year our gross sales increased 30%, but from 80%+ pasture mixes to 95%+ cover crop seed. As we buy our seed 6-12 months in advance, that rapid and drastic change created an inventory and cash flow nightmare!! Now with high cattle prices, a shortage of pasture due to converting to crop land, and low corn prices, irrigated pasture is making up a larger percentage of my inquirys. I expect our pasture seed sales to be much higher in the next few years.