Also depends on what breed they are, different breeds mature at different weights, for breeding. What the commercial pen of three heifers is, it's raising replacements, that are confirmed bred before show date. They aren't your normal show calf, they are raised to be producers. Preferrably pulled from your own pasture..or purchased from a local ranch. 10# of grain is not alot when you compare it to the amount of feed you'd be feeding if you bought the show mixes from the feed store. Those rations get up to 20 lbs a day and no where near the gain we got from this mix. The heifers were by no means fat. I'm lookin forward to hearing from the man that bought them, since they should be calving this month.
Here's something that might be helpful
Table 1. Estimates for heifers reaching puberty at various weights
Heifers Reaching Puberty
Breed 70% in heat 90% in heat
lb
Angus 600 650
Brahman 725 750
Brangus 650 700
Charolais 750 775
Hereford 650 700
Santa Gertrudis 725 750
Shorthorn 600 650
Simmental 750 775