335 Metabolizable protein requirement of lightweight beef calves

The objective was to assess the metabolizable protein (MP) requirements of lightweight beef steers. The 2016 Beef Cattle Nutrient Requirements Model (BCNRM) prediction of MP requirements in lightweight beef steers (less than 250 kg) is limited by available performance data in beef calves. Fall born Angus × Simmental crossbred steers (n = 172; BW = 153 kg) were weaned at 70 d of age (± 26 d), backgrounded 73 d, implanted with Component TE-IS, blocked by BW as light (96–163 kg) or heavy (163–215 kg), and assigned to one of four treatments for 56 d. Treatment diets provided MP at: 0.59 (MP1), 0.69 (MP2), 0.85 (MP3), and 0.91 kg per d (MP4) based on observed DMI. Energy was similar across all treatments at 2.14 Mcal/kg ME. Shrunk body weights were collected on d 0 and 56 to estimate performance. Dry matter intake was affected (P < 0.01) by treatment and increased (linear; P < 0.01) with greater provision of MP. As a percent of shrunk body weight, DMI averaged 2.8% and tended (linear; P = 0.07) to increase with MP. Based on observed DMI, ADG for MP1, MP2, MP3, and MP4 was predicted using the BCNRM at 1.15, 1.55, 1.99, and 1.98 kg, respectively. Observed ADG increased quadratically (P = 0.01) with MP1, MP2, MP3, and MP4 gaining 1.86, 2.13, 2.3, and 2.3 kg, respectively. Final BW increased (quadratic; P = 0.02) with greater MP as MP1, MP2, MP3, and MP4 were 258, 273, 282, and 284 kg, respectively. Gain:feed increased quadratically (P = 0.04) with observed values for MP1, MP2, MP3, and MP4 being 0.323, 0.357, 0.359, and 0.360, respectively. Steer ADG response exceeded BCNRM predictions by 29%. Data suggest MP requirements of lightweight beef steers (BW = 213 kg) are 0.85 kg per d to achieve 2.3 kg ADG when fed a 2.14 Mcal/kg ME diet.