Winter crop for cows in greenhouse?

Hello. We are soon finishing a greenhouse on the farm. This will (if plans work out) be heated all winter, because we are using surplus heat from a walk in-freezer that is running all year anyway. We will be growing tomatoes and cucumbers, but I have been wondering about setting aside some square meters for growing something that could be cut daily to give the cows some variation from the conserved hay or silage. For example if we have 14 squares of equal size, each square could grow for two weeks before next cutting. The question is, what plants to use? Something that grows quickly, is nutritious and tasteful and doesn't need to much heat. And easy to harvest, with hand or scythe. We will have acces to the liquid manure (urine) from the cows, so we can keeps fertilizing the greenhouse crops all winter.
Any suggestions on plants to use? I have been thinking of rye grass (Lolium sp.), other grasses, Amaranth / quinoa (have no experience with this), comfrey??? (invasive, but may not be a problem in greenhouse). What temperature we can maintain in the house I don't know yet, but maybe between 0 - 10 centrigrades? I suppose we should have extra lights, it is quite dark here in winter.
Any other suggestions or comments?

Øystein S., Norway.

seems like a waste of space to me I'd want to tap every bit of that heated space for high value veggies for my table or the market

If I was going to be using up that kind of a resource for cows, barley, oats or triticale are the only crops that would be worth using up that space to me. Nothing beats annual grass in the boot stage and if it can be provided all winter that's a definite bonus.

However I think you overstate how much you are going to get out of it, things grow a lot more slowly in the winter time and it isn't just because of the temperature, it's because there isn't as much sunlight.

comefry or alfalfa might be decent choices, though nothing beats fresh grass!

adding some CO[sub]2[/sub] into your greenhouse could keep growth rates a bit higher during shorter and darker days.  so do your fermentation in the greenhouse, or house a few critters in there.

so we understand what's going on a bit better: what size will this greenhouse be and what temperature do you plan to keep it at?  how many cows are you wanting to feed?  will they be milking over the winter?

You might try a flooded bed with a biculture of rice and azolla.

Separately: since you have cow bedding material, have you looked into a compost-heated greenhouse? The basic idea is to build a large, hot compost pile, blow air through it, and then through the growing medium from below. From the compost pile, it imparts warmth, moisture, and ammonia (a culture of soil microbes develops in association with the roots, to convert this to nitrates), and then it flows up to the bottom of the leaves, still enriched in CO2. This might supplement the heat from the compressor, at any rate.