Hello Suomi,
you could spread it on the fields as is , once the snow has gone and the ground warmed up a touch.....but you may be better off stacking it in heaps and letting it go through it's composting process . Mixed with straw it should heat up and then start to break down . You can make the process even more effective by stacking it in a heap , letting it heat up for a few days then as it reaches it's hottest , turn it into another heap next to the first. You then let that heat up again and then move and mix again . This process is hard work , but it speeds up the composting process by weeks and you can then spread compost that resembles peat in both look and smell. Horse manure would really speed things up , but if you don't have any then the sheep and chicken will do fine. Spreading it direct and un composted on the fields will slow grass growth a bit ,so in your short growing season that may not be a good idea. Composting as described would allow you to spread direct without any burn or hold back to the grass. The time it would take to compost this way would be 4 weeks or so as long as you have the correct amount of straw to manure mix. The chicken manure would help speed things up greatly. Another alternative would be to build the manure heap and leave it till next year and spread it then . That way you end up with two heaps , one rotting down for the following year and one getting built with fresh muck .... Here in Wales and in most of the UK , it would take about 3 or 4 weeks for muck spread thinly to break down , but you would have to keep all stock off the field till all signs of manure had gone. If you could leave empty after an early spreading then you may even get a cut of hay or haylage or silage , depending on your climate/weather etc....
hope that makes some sense , and is of help....
cheers
Russ