After our last post on winter grazing, an interested farmer posed this question: 

How can you tell there is enough forage under the snow?

Great question — long answer.

First of all, we know how much forage was there before the snow came, because we’ve been watching it since we started to stockpile in July/August — so we have a starting point from which to estimate paddock size. But that’s only part of the answer.

Over the course of the winter there will be a number of factors affecting how large a paddock (and consequently, how much forage) we give the animals, snow being one of them. Wet snow or dry? Dry snow is easy for cows and sheeep to nose or paw through, so they can find the forage easily; wet snow and slush are hard to move, and they lay the snow down, pack underfoot (mashing down the forage so it is hard for the animals to wrap their tongues around), and, over time, forage quality is diminished.
Since, under snow, I can’t know for sure exactly how much the animals will harvest, I have to do some of my calculations after the fact, each day when I move the animals onto a new paddock. At that point, I’m looking for full rumens (the hollow under the left hip bone) and eager but not anxious animals moving onto the fresh forage — heads down, but little or no shoving. I want to see fluffy, shiny coats, and there should be plenty of manure on the previous paddock.

Large patches of undisturbed snow probably mean the paddock was too big. On the other hand, in icy conditions, it probably means that the ice crust on the snow is too thick for the cows to get through comfortably, in which case I need to feed broken bales in the paddock for the time being. (More northerly graziers report that their cows learn to paw through ice crust (rather than nose through), and consequently can graze under more stringent conditions. My cows don’t usually see more than a week or two of snow cover at a time, and haven’t learned that lesson.)
As the winter progresses, the forage will slowly decrease in both quality and quantity, depending mostly on the weather. For us, a cold, dry winter preserves forage value better than a warm, wet one. I don’t believe that snow reduces forage quality unless the snow is wet and heavy, pressing the snow to the ground where soil microbes can get to work on it.
Forage type matters. After the weather turns cold, clover breaks down quickly, and it doesn’t seem to play a big role in stockpile after December. Bunch grasses hold well, and some, like fescue, that my cows don’t favor most of the year, are more palatable when they are frozen.
Another variable, also largely dependent on the weather, is the amount of forage the animals need to stay warm and comfortable. Early in the winter the forage quality is high (note: our standing forage will always be at least as good as hay in the barn, right up into April) and the animals are in high condition, so a relatively small area of pasture is adequate. As the cold intensifies, and the forage begins to break down, we have to increase the paddock size to keep everyone warm and well fed. Again, rumen fill and contented behavior are good indicators.
Planning the winter grazing ahead of time reminds me of my (lamentable) attempts to undertand calculus and probability in college math classes. There are multiple sliding variables that will affect paddock size — not only will the weather be an issue, but the animals themselves are not static, moving through stages of maturity, fertility, gestation, and lactation — and some of these can only be guessed at before the fact.

We generally seem to have found that our end-of-winter paddock size is going to be close to double the size it was in early winter; that is, if we were allowing fifty square paces per cow equivalent in November/December, we may have to allow close to 100 square paces in March. For the sake of planning, we take the average and consider each cow equivalent as 75 square paces per day. (You understand, I am sure, that the base number here is specific to our pastures and not necessarily transferable to another farm.)
Best of luck with your grass!