Do NOT put a pregnant mare on a diet. Do NOT.
Timothy is good. Brome is good. Orchard grass is good.
Out of the 3 choices, timothy has the most balance when it comes to calcium : phosphorous ratio. Timothy must be baled at prime when the flower head is 2 inches or less. Timothy tends to start tasting a little bit like straw if the flower head gets too long. Timothy also has decent protein.
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Brome is fine too. I feed brome, timothy, and alfalfa to my mares. Sometimes some crested wheatgrass goes in too.
For your mare, you need a balancer ration that will fit whatever hay you get her so that it can add the necessary calcium and whatnot.
The nice thing about adding some alfalfa is there is lysine in the alfalfa and protein. It’s not so much the calcium because you can get calcium from other sources. What baby needs in the last 3 months is calcium, phosphorous, copper, and protein and if the protein doesn’t come from her diet, then baby pulls it out of her muscles. The other 3 elements plus other vitamin/mineral necessities can be had in a good pregger ration balancer.
Momma is fluffy. Better fluffy than too skinny by a long shot. Still, you can afford to feed her judicious amounts of a variety of hays to maintain her current level of fluffiness until she is lactating. Once she is lactating you can just keep status quo to allow lactation pull down her fluffiness, but still maintain her on a vitamin/mineral balancer so baby doesn’t end up pulling calcium out of mommy’s skeleton. Lactation requires protein, so inevitably Madame Fluffy is going to need some alfalfa too.
Really fluffy mares do much better feeding them small meals very frequently. 3-4 times a day, if you can swing it, plus a bedtime snack. Their blood sugars level out and they become less voracious.