Q.Fertilizer
Is it good to dig in steer manure to your flower garden occasionally ?

Though most vegetables are heavy feeders you do need to make sure you do not feed too much nitrogen; as this will inhibit fruits and vegetables.
Manure must be properly composted and is high in nitrogen.
These articles will help you.
http://extension.colostate.edu/topic-areas/yard-garden/fertilizing-the-vegetable-garden-7-611/
https://www.gardeningknowhow.com/composting/manures/cow-manure-compost.htm
https://www.gardeningknowhow.com/composting/manures/the-benefits-of-manure-in-your-garden.htm