Protein levels in beef cattle finishing diets: industry application, university research, and systems results.

Consulting nutritionists were surveyed to determine current formulation and management practices for finishing beef cattle. Among the six consultants surveyed, percentage of CP in finishing diets ranged from 12.5 to 14.4%, with urea levels ranging from .5 to 1.5% of DM. Finishing diets were based primarily on highly processed, rapidly fermented grains (steam-flaked and high-moisture grain), with roughage levels ranging from 3 to 11% of DM. All six consultants considered feed bunk management to be a critical factor affecting feed intake and performance; five of the six consultants used aggressive implant programs based on estrogen + trenbolone acetate. Recent university research was reviewed with respect to CP level and source in finishing diets. Finishing cattle managed on aggressive implant programs seem to respond to higher levels of CP better than would be expected from the factorial calculation approach. Moreover, improvements in performance noted in recent research seemed to be more consistent when supplemental CP was derived from ruminally degraded vs undegraded sources. Calculation of protein requirements with a metabolizable protein (MP) system yielded estimates of protein needs by finishing cattle that agreed more closely with current industry practices than did calculation based on the factorial method. The difference between the MP system and the factorial method was primarily a result of accounting for microbial N needs in the MP system. Reasons for production responses to CP levels that are greater than those calculated by the factorial method include increased accretion of protein by rapidly growing, implanted cattle, particularly during the initial phase of the finishing period, alleviation of a microbial N deficiency, and ruminal and systemic effects of ruminally degraded N on acid-base balance of beef cattle fed rapidly fermented, high-grain diets. Reasons for production responses to supplemental CP need further research.