Influence of rumen protein degradability and supplementation frequency on performance and nitrogen use in ruminants consuming low-quality forage: cow performance and efficiency of nitrogen use in wethers.

Two studies were conducted to determine the influence of CP degradability and supplementation frequency (SF) on ruminant performance and N efficiency. Treatments included an unsupplemented control (CON) and degradable intake protein (DIP; 82% of CP) or undegradable intake protein (UIP; 60% of CP) provided daily, every 3 d, or every 6 d. Seven wethers (36+/-1 kg BW) were used in the digestibility study with DIP and UIP treatments formulated to meet CP requirements. Eighty-four Angus x Hereford cows (512+/-42 kg BW) in the last third of gestation were used for the performance study. The DIP treatments were calculated to provide 100% of the DIP requirement and UIP treatments were provided on an isonitrogenous basis compared with DIP. Basal diets consisted of low-quality (5% CP) meadow hay. Forage DMI and N intake by lambs decreased (P < 0.05) linearly as SF decreased. Additionally, DMI, OM intake, N retention, N digestibility, and digested N retained were greater (P < 0.01) for supplemented wethers than for controls with no difference due to crude protein degradability. Nitrogen balance, DMI, and OM intake decreased linearly (P < 0.05) as SF decreased. Plasma urea (PU; mM) was measured over a 6-d period and supplemented lambs had increased (P < 0.01) PU compared with CON. Plasma urea linearly decreased (P < 0.01) as SF decreased. Pre- and postcalving (within 14 d and 24 h of calving, respectively) cow weight and body condition score change were more positive (P < 0.05) for supplemented groups than for controls. Results suggest CP supplements consisting of 20 to 60% UIP can be effectively used by ruminants consuming low-quality forage without adversely affecting N efficiency and animal performance, even when provided as infrequently as once every 6 d.