Dara,
Though I have never foaled out a mustang mare, I have raised Arabians and QH's for almost 2 decades. I apologize for not remembering if your mare was bred in captivity or if you adopted her from the wild and she was already pregnant. If the latter is true I might not be of much help in my advice here. Domestic vs. Wild as far as bone development will differ and the nutriton you give the foal will be measured differently I would assume. Others here who have foaled out mares who were bred before capture should be able to help you better than I. Now if your mare was bred in captivity I can be of some help.
I have had some foals who test out mom's grain right away and others who have no interest until they ar 1-2 months old. What I have found works best is offer a few handfuls while mom is eating to the foal. I keep it at that rate until they are well on their way to eating hay. The most important thing to remember is that good quality hay is your most important ingredient in your foals diet once they are weaned. If they are cleaning up the grain and leaving hay, then you need to back off the amount of grain.
As to the type of grain: I have fed numerous types of mare and foal grains over the years and the one I like best is Blue Seal's Vintage Mare and Foal. It is an extruded feed, which I have found the foals will eat right up. You might not be able to get this particular grain depending on where you live, but I went to the Blue Seal website and cut and pasted whats in this grain for your review.
VINTAGEĀ® MARE & FOAL
COARSE/EXTRUDED HORSE FEED
For Sucklings, Weanlings and Lactating Mares
GUARANTEED ANALYSIS
Crude Protein, Min. ......................................... 16.0 %
Lysine, Min. ..................................................... 0.85 %
Methionine, Min. ............................................. 0.35 %
Crude Fat, Min. ................................................. 5.0 %
Crude Fiber, Max. ............................................. 8.5 %
Calcium, Min. .............. 0.65 % Max. ......... 1.15 %
Phosphorus, Min. ............................................ 0.70 %
Copper, Min. .................................................. 50 ppm
Chelated Copper, Min. ................................... 12 ppm
Manganese, Min. ......................................... 125 ppm
Chelated Manganese, Min. ............................ 20 ppm
Selenium, Min. ............................................ 0.60 ppm
Zinc, Min. ..................................................... 155 ppm
Chelated Zinc, Min. ........................................ 40 ppm
Vitamin A, Min. .......................................... 5,500 IU/lb
Vitamin E, Min. ............................................... 75 IU/lb
INGREDIENTS
Steam Crimped Oats, Steam Flaked Corn, Steam
Flaked Barley, Wheat Middlings, Soybean Meal,
Ground Corn, Dehydrated Alfalfa Meal, Soybean
Hulls, Dried Beet Pulp, Rice Bran, Corn Distillers
Dried Grains, Yeast Culture, Cane Molasses,
Vegetable Oil, Salt, Calcium Carbonate,
Monocalcium Phosphate, L-Lysine
Monohydrochloride, DL-Methionine, Zinc Sulfate,
Zinc Proteinate, Copper Sulfate, Copper Proteinate,
Manganese Sulfate, Manganese Proteinate, Cobalt
Carbonate, Calcium Propionate, Calcium Iodate,
Ferrous Sulfate, Selenium Yeast, Sodium Selenite,
Vitamin A Supplement, Vitamin D3 Supplement,
Vitamin E Supplement, Choline Chloride, Riboflavin,
d-Calcium Pantothenate, Niacin, Folic Acid, Biotin,
Vitamin B12 Supplement, Thiamine Mononitrate,
Pyridoxine Hydrochloride, Menadione Sodium
Bisulfite Complex (Source of Vitamin K Activity),
L-Ascorbyl-2-Polyphosphate (Source of Vitamin C),
Oat Mill By-product, Brewers Dried Yeast, Dried
Enterococcus Faecium Fermentation Product, Dried
Lactobacillus Acidophilus Fermentation Product,
Dried Yeast Fermentation Solubles, Yucca Schidigera
Extract.
What you want to pay attention to the most in any grain is your Lysine and Calcium. This helps develop good bone and connective tissue. The level in this grain is appropriate. Also the protien level at 16% is a good percentage for a growing youngster. Some people will tell you that a high percent of protien will make a horse "hot", that is bull crap. The sugars in sweet feed and the corn content in some sweet feeds is what does that, not the protien. I have also been told the Omolene Mare and foal is a good grain with a similar analysis to the one listed above. Whatever grain you do settle on, you can use the one listed above as a guide to what to buy.
Anyway, I would start with a few handfuls like I said and build from there. Once the foal is weaned, slowy up your grain ration. There is no magic number here as to amount. You will have to see what works. Again, quality hay is the most important item in the diet and you "supplement" with the grain. On my QH weanlings, they are usually getting 2 to 2.5 lbs of grain 2x a day, along with their hay. The amount of hay should be between 12- 20 lbs of hay a day. Because mine have good quality pasture I include the amount of grass they getting in the lb number of hay. I always keep an eye on their hay intake though, if they are not cleaning up their hay, I back off on the grain.
Hope this helps. Sorry if it is more information than you wanted.
Kara