A finishing dietary protein concentration of 12.5% may be optimal for both steers and heifers, University of Arizona researchers found.

Researchers assigned feedlot steers and heifers to one of three dietary protein concentrations — 11%, 12.5% or 14% — to determine the effect on finishing performance and carcass characteristics.

Heifers consumed significantly less dry matter and gained less than steers, and were significantly lower than steers in carcass weight, marbling score, backfat and yield grade, but feed conversion, dressing percentage, and ribeye area did not differ between genders.

DMI and ADG increased from 11% to 12.5% protein and these tended to decline from 12.5% to 14%. Carcass weight, backfat and yield grade increased linearly as dietary protein increased from 11% to 14%. Dressing percentage, ribeye area and marbling score weren't affected by dietary protein concentration.

Regarding feed conversion, steers were most efficient at 12.5% protein, while heifers tended to be most efficient at 14% protein, which could be a reflection of the heifer's overall advantage in leanness. Nevertheless, the authors concluded that the peaks in DMI and ADG at 12.5% for both sexes suggest 12.5% protein concentration is optimal for either steers or heifers

(Bailey et al. 2006. Midwest Section ASAS. Abstract 318).
— Michigan State University Beef Cattle Research Update, Summer 2006