Calculating winter forage is an exercise in sliding scales. While the number of animals may be static, their size or stage of gestation is not. Calves are getting bigger, both on the ground and in utero, so their forage requirements are going up. If the weather is very cold, everyone has to burn more calories to stay warm, so paddocks have to be larger; conversely, if it’s warm and wet the food value of the standing grass is going down, meaning larger paddocks still. Standing grass quality is going to go down in February anyway, whatever the temperatures, so there will have to be some supplementation — hay — if we want the lactating cows to keep their production up; but as the same cows reach the last two months before their calving dates they are dried off, moving them back down in caloric needs, as well as moving them off the better-quality pastures out front and putting them out back with the dry cows on the rougher pastures.
Keeping up with these variations isn’t just difficult — it’s impossible. Does that make grazing calculations hopeless? Not at all. On a day-to-day basis we can gauge how much pasture to allow by experience seasoned with a good dose of intuition. On a whole-season basis we haven’t got it figured out yet, and maybe never will, meaning we can’t know how much hay we’ll need in a given winter until the winter is over. So we fill the barns in summer and scope out sources we can tap for late winter hay in case we reach March/April with more cows than forage.
In any case, we’ll be glad of every mouth we have out there when the grass comes on in May.