If they are eating 25# of hay a day then they will be getting around 2-2.25# of protein/day. Which is enough for a last trimester non-lactating cow. If you are in a cold area, then you may need to be supplementing energy instead.
If they look to be losing weight, or aren't at the condition you want, feeding corn will help.
Or you can use a feed mix with supplement protein, and corn for energy.
My go to winter feed is made of corn and a protein source balanced out to make 22%-24% protein. Feed about 4-5#/head/day.
Last mix was 400#(66%) Corn, 200#(33%) SBM, and 10# (1.6%) salt. Cost me $.098 a pound, or $.49 per head/ per day (Just to give you an idea.)
You can decrease the protein more, if you desire more energy.
*This is my own opinion, use at your own risk*
