Feeding goats naturally seems to be a complicated subject, which I'm just starting to wrap my head around (so readers should feel free to jump in if you think I'm off base).  From what I understand, the ancestors of our cultivated goats would have been eating browse --- rough plant matter like tree leaves, blackberry brambles, and so forth.  They needed to consume lots of this roughage to get enough energy to live and grow, and their digestive system has evolved to require that kind of bulk fiber.

Grain is a recent introduction to the goat's diet, and while it can help a doe keep on weight when she's producing lots of milk, grain can also be hard on a goat's gut since she isn't really adapted to eat it.  That makes the supplement more of a tradeoff than you'd find with your dogs --- fancier rations will probably just make your canines happy, but more grain can make a goat sick.

So why give her any grain at all?  Humans generally want the most that we can get out of our livestock, so we've bred dairy goats to produce much more milk than their ancestors would have in the wild.  Producing that milk requires extra energy and protein, and most goats will get really skinny if milked hard on pasture alone.  Think of this as a bit like Appalachian Trail thru-hikers --- they physically can't carry enough food to keep them well nourished while on the trail, so they often gorge on ice cream and other concentrates when they hit towns.  Grain is the dairy goat's ice cream --- it helps a doe put back on the pounds, but it's not necessarily good for her in the long run.

Now, there are alternatives to grain that still provide concentrated nutrition.  For example, Nita provided some excellent guest information in my ebook $10 Root Cellar about how she grows and prepares carrots, beets, parsnips, rutabagas, and (sometimes) mangels to keep her milk cow in fine flesh while milking over the winter.  And there's also something to be said for choosing individual milking animals that have well-developed rumens from a childhood on pasture and that don't produce quite as much milk, since animals like this probably won't need as much nutritional supplementation.  But, in the end, most people end up giving at least a little bit of grain to their milking animals at certain times.

Returning to your point, there is some controversy about when a doe most needs extra nutrition --- while milking heavily, or when pregnant.  Most sources will only mention the latter, but others tell you that it's imperative to have your doe in relatively good condition at the start of her pregnancy if you want her to easily get and stay pregnant.  The trouble is that if you feed much grain while she's actually pregnant, the kid(s) will grow extra large, which can result in complications during birth.  So, your best bet is to get your doe in tip-top shape before she gets pregnant, to cool it on the grain while she's actually pregnant, then to pick back up with grain (or other supplements) in proportion to how much milk she's producing after she gives birth.

Of course, all of this is still book learning at the moment.  In a year or two, I'll probably laugh at the lack of nuance in my understanding of the subject but, for now, I'll keep Abigail's treats to a minimum but will provide her with lots of excellent browse to keep her healthy.

(As a side note, during day one with two goats, it rained like crazy, so our girls have been enjoying room service --- masses of Japanese honeysuckle torn off the side of the barn, a handful of oat leaves, several sweet-corn stalks, two sunflower plants, some comfrey leaves, and a sorghum stalk.  Abigail thinks the oats are the best, followed by honeysuckle and comfrey, while our little doeling still looks a bit befuddled by her new home but chowed down on quite a bit of honeysuckle.  Hopefully it'll dry off enough to get them out on pasture for a better quality photo shoot soon!)