You need to get with your veterinarian and get feed samples - and a 'fresh' aborted fetus & placenta, and serum sample from the dam (and any other cows that have aborted recently) to the diagnostic lab.
Excessive nitrates would be a concern - as would Neospora, Leptospira, IBR, BVD, and any number of other potential causes. All folks on an internet discussion board can do is conjecture about possible causes.
Get the appropriate testing done.