Yes, she is very light for her age based on your estimate. 10-12 month old Hereford heifers (twins or not) would weigh that much. This is management, nutrition, and/or genetics. Have you dewormed her? I would start there, with a good dewormer. If she has not been dewormed with a grub control this year, you need to check with your local extension or a vet as to the latest date you can use a degrub wormer. Michigan should be about the same as it is here in Central NY. Nov 5 is our last date to use a grubicide dewormer.
Not to crticize, but as info for your next livestock project. At 3 months of age, she needed a high protein diet to replace the super high protein milk she was not getting from her dam. There are special grain rations designed for young calves. As they grow (size and weight) their protein requirements reduce. Most beef calves stay on their dam until around the age of 6-8 months old, Herefords would weigh 450 - 650(?) at that age. Their protein requirement would be less than a 3 month old, but it would still be high (about 15%). A "normal" Hereford heifer (twin or not) at 19 months of age should/could weigh about 900-1100# and normally would have been bred at the age of 14-16 months old. Being a freemartin (female twin to a bull), she is/would be treated as a steer & fed out for harvest. If you want to only feed grass/legume/hay, that is find, and an animal can be finished out on that sort of diet, but it requires really GOOD QUALITY feed and free choice minerals, and a good health program which inludes deworming.
In all reality, a 19 month heifer should be ready to harvest ( or, with a roughage diet, should be getting close). With a good grain diet from weaning on, cattle are able to be harvested at 1100-1300# at 14-16 months of age.
Being a twin, generally only affects their growth when both calves are left on their single mom to raise without suppliment. In that case, the dam only has so much milk to raise both calves, and a lot of cows don't have enought to do a good job on two calves. But, after weaning, they should gain the same daily rate as their contemporaries.
