My experience with grunt calls is they work all season, if used correctly. There are more ways to use a simple grunt call than some people think. For starters, I've learned never to use a grunt call if a buck is very close, they will run away. Bleet calls have only worked around the rut (pre- and post-). I like to use short grunts early in season, I've called in bachelor groups every year with short grunts (leaves on trees) and other types (more aggressive) of grunts during other parts of the season.
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