Introducing different species can certainly help improve the nutritional value of pastures, especially if there is a lack of desirable species. However, if you already have the ordinary species of grasses common to your area, then introduing another species won't necessiarily make any improvement. For 4 years I scooped up fresh livestock poop, froze it and sent it to Texas A&M for analysis. By analysising what was coming out the back of the animal, they could determine what was going in the front end, nutritionally that is. Cool season grasses, regardless of what species they were, in early spring would easily produce forage in the 20%+ CP and 80%+ TDN. Fescue, orchardgrass, bluegrass, timothy, smooth brome, etc. were all the same. When hot dry summer came along and the plant had gone to seed, those same grasses were down in the 8-10% CP and 58-65% TDN. And when in an overmature state, orchardgrass can be some of the poorest forage out there, but a bit better than wheat straw. Literally.
The secret is to keep the plant in a vegetative state by using rotational grazing or mowing to keep the plant from producing a seed head and setting seed. Do that and even in summer (and with a little rain) you will help keep the grass in the 12%CP and 65-68% TDN area. Having legumes in the mix will really help keep nutritional values up when hot dry weather comes along.
And don't overlook non-conventional forages. One year I had a carpet of cockleburrs with some waterhemp that flourished after I combined oats. I grazed dry hair sheep ewes on 12" burrs and the nutritional content on 8-17-2001 was 13.4% CP and 60.8% TDN. On 10/15/2005 I grazed sheep on clover field aftermath that was 20% red clover and 80% morning glory and waterhemp and it came back as 16.2% CP and 67.8% TDN. I will say that sheep and their nimble mouths are better able to select what they eat and my work shows that in the same setting, sheep will select a diet that is 2-3% higher in CP and 1-2% higher TDN.
The moral of the story is to manage your cool season grasses to keep them in a vegetative state and to not overlook the value of so called weeds. Young pigweed, waterhemp and lambsquarter can equal or exceed prime alfalfa in nutritional content. And incidently, on only a few drought type settings was the forage nutritional content the limiting factor on livestock gains, it was nearly always the insufficient forage quantity available that limited gains.
