Data from 3 trials were compiled to calculate microbial CP (MCP) production and MP requirements of growing calves on high forage diets. Individually fed steers (n = 335; 256 ± 15.6 kg midpoint BW) were utilized, each trial lasted 84. Diets consisted of 44% sorghum silage, 44% corn cobs, and 12% protein supplement. Source of protein within the supplement varied and included urea, meat and bone meal, soybean meal, feather meal, poultry by-product meal, or corn gluten meal. All trials included a urea only treatment. Dry matter intake of all calves within a trial was held constant, as a percent of BW, established by the urea supplemented group. The base diet was MP deficient, composition of the protein supplement varied with increasing amounts of test protein replacing urea. As protein in the diet increased, ADG plateaued. This methodology was used by Wilkerson et al. (1993) and is the basis of performance models used by the NRC (1996). Gain ranged from 0.19 to 0.56 kg/d, averaging 0.37 kg/d. Three microbial efficiencies were used to calculate MP. Maximum gain was then regressed against calculated MP to determine MP requirement for maintenance and gain. Method 1 (based on a constant 13% microbial efficiency suggested by the NRC, 1996) predicted an MP requirement of 3.4 g/kg BW^0.75 for maintenance and 461 g/kg gain (r² = 0.55). Method 2 (based on an equation developed by Patterson et al., 2006) predicted MP requirements of 2.9 g/kg BW^0.75 and 483 g/kg gain (r² = 0.56). Method 3 (based on an equation developed by Galyean et al., 2014) predicted MP requirements of 2.6 g/kg BW^0.75 and 449 g/kg gain (r² = 0.59). The factorial method of calculating MP maintenance requirements accounts for scurf, endogenous urinary, and metabolic fecal protein losses and averaged 4.2 ± 0.10 g/ kg BW^0.75, for the 3 trials summarized here. Factors affecting MCP production include TDN, RDP, and microbial efficiency. Dietary TDN and RDP have been measured on a variety of feedstuffs; microbial efficiency is not well defined and is a crucial component in calculating MCP production and MP requirements of growing cattle.