I liked the way that the player's field of view is obscured - This and the art style that you went with reminds me of Teleglitch, and Monaco. 

Some of the instructions were not fully visible, so I ended up wasting my reload early on whilst figuring out the controls. I would then realise there isn't infinite reloads. A possible improvement would be to replenish spent ammo from a pool instead of using the full box. Shooting and reloading felt satisfying - the little gun twirl on reloading is a nice touch.

The inclusion of the theme felt a little tacked on, as there wasn't much in-game to suggest that it's taking place on a farm. If it were zombie animals, it might have fitted the theme better.

A retro-style zombie shooter on a farm.

As a big fan of retro games, the SFX caught my attention. I also like how u integrated the tuto into the level environment.
But greeting the player with tons of text is not a good idea. and in terms of balancing, u might need to increase the magazine count or add a shot cooldown bcs it runs out super fast.
ALso, i dont see how this has anything to do with the theme. Maybe if u add some art for the farm it would be better. Imagine the walls being hay stacks, and the zombies being infected farm animals, and the player being the farmer. that would be reaally good !! 
Other than that good job for the effort mate !