Thanks for that question. I've laid out an overview of winter grazing below as a starting point for this big topic, with links for where you can get more information on my website. So here goes...
Winter grazing is not something you start at the flick of a switch, but rather a process/skill that you grow into by learning how to extend your grazing season longer and longer each year. There are a lot of dominoes that need to line up before you attempt it, but once when you reach that point, it is a lot of fun! It can also significantly reduce your feed costs - for obvious reasons when cattle instead of tractors and harvesters and feed wagons are allowed to do the harvesting for you, manure continues to be spread by the cows instead of piling up in feed pens, and your time is freed up for more productive things.
Here are some of the dominos that need to line up for winter grazing:
1. Although any herd can extend its grazing season for part of the winter with either winter pastures or swath grazing, if you have a cow/calf herd, year-round grazing or winter grazing through the majority of the winter is ONLY a safe option if calving during the growing season (likely early to mid-June in your region, same as when the deer and moose calve). If you have a cow/calf herd, you need to adjust your calving date before even attempting to winter graze, both for the health and safety of your pregnant cattle and to protect their fertility in the next breeding season. You can read more about this in my article about picking the perfect calving date. If you buy stockers and don't have your own cow/calf herd, then calving date is irrelevant for whether or not you can winter graze through the whole winter.
2. Mediocre pastures, short grass, or weak grass varieties don't work for winter grazing. Start learning how to create high-quality winter pastures - by this I mean pastures that are tall pastures of prime quality at the very end of the growing season (so the pasture is at it's maximum quality, height, and nutrient content going into the dormant season). A lot of time and effort and planning during the growing season needs to go into preparing a high quality winter pastures. You have to give the cattle something worth digging for. And you have to create a winter pasture that is taller than your snowpack so there is a trail of cookie crumbs for the cows to follow to find the grass. The grass also has to stick out through the top of the snowpack in order to keep the snowpack from crusting over (through freeze-thaw cycles or wind crusting) to the point where the cows cannot get through. You can learn more about how to create high quality pastures for winter grazing at this link.
3. Winter grazing MUST be accompanied by a monthly forage analysis program. You have to work with a livestock nutritionist who can compare the nutrient profile of your winter pastures with the cattle's nutritional needs and design a mineral and nutrient (i.e. protein) supplement program for your winter pasture rotation. That supplement program will change from month to month as the grass quality changes through leaching and frost damage, and the nutritional needs of your cattle change based on age, growth stage, pregnancy, lactation. Those calculations will also tell you the maximum length of your winter grazing season before the pasture quality deteriorate too far, or your supplements get to expensive, or the fat reserves of your cattle get too low. However, you will still need to monitor your cattle visually to ensure that no animal is allowed to slim down below a body-condition-score of 5. Cattle should only ever need to draw upon their fat reserves, NEVER on muscle. And growing cattle must continue to gain weight even as they reduce their fat reserves so that healthy growth is not compromised. It is not something you can gauge by eye - it must be calculated specifically for your herd by a livestock nutritionist based on your monthly forage analyses. When you get it right, with minimal supplementation you can let your cattle supplement themselves off of the fat on their backs during the winter, and then rebuild those fat reserves essentially for free during the growing season when grass is cheap and abundant.
The Planning for Winter Grazing and the Herd Nutrition Plan chapters of my book go into detail on how to set up your monthly forage analysis program and how to work with a livestock nutritionist to support your winter grazing program.
Your winter grazing strategy also MUST use DAILY pasture moves to ration out your winter pastures, slice by slice. The DAILY pasture moves are the only way that your cattle continue to have a consistent feed quality from day to day during the winter grazing season. And it is the only way to make sure that the pastures under the snowpack remain accessible - if the cattle disturb the snow and don't eat what is under it that same day, the snow will recrystallize and set like concrete, locking away what's underneath.
5. Always have a emergency hay reserve ready, even if you never use it, in case you miscalculate, or in case the ice-storm of the century comes through and buries everything.
Once you've got all your dominoes lined up to start your first winter grazing season, in your first attempt start with all your usual hay reserves standing by, but create enough winter pastures to extend your grazing season by a month or two. Assess what worked and what needs adjusting, and then extend again the following year, and so on. Having your usual hay reserve ready means there is no panic if something doesn't go according to plan.
As you gain experience, you'll get a sense of how long you can continue to winter graze each year - once your cattle overcome their ingrained habit of waiting for the feed wagon, they generally get trained to winter grazing far quicker than you do. Learning this strategy is one of the keys to low-cost beef production, regardless of whether you plan to market it as grass fed beef or whether you sell cattle through the commodity markets.