Most winter pasture fields are planted in late August or early September to optimize the potential for getting grazing in the fall. Having a plan made beforehand can eliminate possible mistakes at planting time.
First, have a recent soil test for each field in front of you. This tells you two things: one is whether or not you have a suitable soil pH for winter pasture, and the other is how much of what type nutrients you need to add to optimize plant growth to meet livestock demands.
If your soil pH is over 5.5, that's good. You don't need to consider liming (unless you're also growing clovers, in which case your soil pH needs to be above 6.0). If your pH is less than 5.5, you have two options.
One is to use lime at the recommended rate. Lime will work best if applied before planting and incorporated, but don't think all is lost and not use it if you waited too long. If your soil pH is highly acidic, applying lime at any time and in any manner is better than not using it at all. Another possibility to consider if your soil pH is low and you can't get lime applied is to band phosphorus with the seed. Wade Thomason covered this, along with information on how to set up the drill with banding equipment. The phosphorus in the fertilizer will tie up some of the aluminum that damages plant roots at a low soil pH and reduce some of the damage caused by low soil pH. While this approach is not as good as liming on a long-term basis, it can get you by in the short term.
Next, look at your soil test phosphorus and potassium levels. The recommended amount of each of these nutrients should be applied at or near planting. If these nutrients are low in the soil and you don't apply them, you are limiting yield before you begin.
Now, think about nitrogen. How much per acre? When to apply? What source to use?
How much depends largely on your stocking rate, but if you're looking for the most forage for the money, Noble Research Institute research shows about 150 lbs. actual N per acre to be the best economic rate. If you're only running calves in the spring, probably 75 to 100 lbs. N per acre is more realistic.
When to apply depends on when you want the forage to grow. Noble Research Institute research shows that you'll grow the same amount of total forage regardless of whether you apply all the N at planting, all in February or split between planting and February. However, when you produce that forage does depend on when it is applied.
Noble Research Institute research shows that if you want good fall forage, apply a high percentage of your total amount of nitrogen near planting time. If you are grazing calves only in the spring, apply just enough N around planting time to let the plant get through the winter and then apply most of the N in February. If you're running a mixture of fall and spring calves, apply about two-thirds of the total N at or near planting and the remainder in February.
The source of nitrogen is not critical if you apply it correctly. If you use anhydrous ammonia, you may want to apply all the N in the fall to save a trip. For the February application, either urea or ammonium nitrate will be fine because urea is not lost in the cool temperatures at this time. If you use urea in August or September, it will work better if disked in within two to three days after application to reduce losses to volatilization. Price per pound of actual nitrogen should be a prime factor in determining nitrogen source.
Finally, keep the anhydrous ammonia and ammonium nitrate locked up and away from the meth makers and bomb makers and other card-carrying members of the "American Society of Lunatics." Work with local law enforcement to see what can be done to protect your fertilizer and your safety.