Go here
www.elknut.com & click on "Elk Sounds" link. Click on "Battling Cows" this will give you a good idea as to the sound you are looking to imitate as far as your question goes. The "Lost Cow" sound is another good one for cows. The sounds are a bit sqealy sounding due to PC speakers, not the best quality! But you will get the idea, play the sound several times to really get the hang of the tone needed.
The reason "fighting Cow" sounds work is because cows fight for lead cow or a dominant position as bulls do, this is especially so early on in the rut but will carry on through out peak rut!! Good Luck!
ElkNut1