We use fly papers, a bug lamp, available from most DIY stores, and a fly swatter.
Keeping all food covered even if it only stands out for a minute while you are preparing a meal helps.
Otherwise sugar dissolved in water in a jam jar or shallow bowl traps flies.
With a field of cows nearby, I would consider mosquito or fly nets on all windows - they can be bought in standard sizes that fit most modern windows.
If you have an old house where the screens don't fit, you should be able to buy nylon net by the metre from any dress-making shop that sells material. I did for our boat and we attached the nets by sewing velcro to the nets and the other side of the velcro we glued to the window frames.
I guarantee no flies, wasps, bees or midges will get past that system, it kept two cats on board too!