Around here the grunt call works well pre rut, during rut, but after the rut they wont usually come to it they will just stop and look!
I've watched a number of bucks that were out of range or going away from me that stopped , turned and came in to the call.
Be careful not to blow with too much force or you wont get good results because it changes the pitch. It should be a soft guttural sound that varies in length and lasts only about 10-15 seconds.
I use my voice to make a low grrrr sound while blowing the call and it gives a better tone.
Works good for me anyway.
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