Have a friend here in WC IA who has had quite a few cows die this winter, they came off grass looking bad and never really recovered. I finally think I have him convinced he is overgrazing, he has been running a pair every 2.5 acres, thats just too much. My honest opinion in driving around my area is that overgrazing had become a real problem. I think we forget we are a heck of a lot closer to Nebraska than Illinois, in my part of the state. He has gotten a new pasture that has been in CRP, hopefully that will take the pressure off of his other pastures a bit. In my area I have come to the realization that these cows on average just aren't getting what they need to be at there very best.
Not knowing the whole story and only from your post, I would suspect Johne's Disease. I would think that if they were overgrazed so bad to lead to death it would take very good fences and we would have been out of grass by the middle of the summer. I have seen pastures ate into the dirt and cows in poor condtion, but they didn't die because of it. Just a thought of something that may be looked into as another reason for all the death lose.
In north central Missouri, overgrazing is a common problem, even in good years. Then throw in a severe drought like last year and it's a given that most pastures and all of the severely overgrazed ones will have a tough go of it this spring, even with perfect weather. I'm certainly no expert, but I've grazed animals all my life, designed countless grazing systems in my county (I work for SWCD) and work with livestock producers all the time and I'm guessing that even with perfect spring weather, we are probably looking at forage production of maybe 2/3 normal. With the present winter that won't quit, a spring with "perfect" growing weather is already a thing of the past. The odds of the weather turning around and becoming more than perfect is next to impossible. Given the choice of winning the lottery or the weather changing enough to give us normal spring forage production, I'd opt for the lottery...and I never buy lottery tickets.
One doesn't want to forget that cows can die with a belly full of hay. Because of the severe drought, around here a lot of CRP was baled last fall. I have in front of me a forage analysis report for some CRP hay with 4.1% crude protein and 36.3% TDN. To give you an idea of just how inadequate that hay is, a dry cow needs a minimum of 8% CP and 53-54% TDN and a lactating cow 12% CP and 65-67% TDN. That's an example of how a cow can starve to death with a full belly.
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.