Influence of Rumen Protein Degradability and Supplementation Frequency on Ruminants Consuming Low-Quality Forage
Seven rumen and duodenal cannulated steers (264 ± 8 kg BW) consuming low-quality forage (5% crude protein) were used to determine the influence of crude protein degradability and supplementation frequency (SF) on ruminal fermentation characteristics using an incomplete 7 × 4 Latin square design with four 24-day periods. Treatments included an unsupplemented control and degradable intake protein (DIP) or undegradable intake protein (UIP) provided daily, every 3 days, or every 6 days. The DIP treatments (18% UIP) were calculated to provide 100% of the DIP requirement while the UIP treatments (60% UIP) were provided on an isonitrogenous basis compared with DIP. The DIP supplement was composed of soybean meal and the UIP supplement was composed of expeller processed soybean meal and blood meal. Ruminal ammonia nitrogen was increased on the day of supplementation with supplemental crude protein (P = 0.04) and for DIP compared with UIP (P < 0.01). Also, because ruminal ammonia nitrogen increased at a greater rate with DIP compared with UIP as SF decreased, a linear effect of SF ´ crude protein degradability interaction (P = 0.02) was observed. In addition, ammonia nitrogen was greater on the day before supplementation for supplemented treatments (P = 0.04) and decreased linearly (P < 0.01) as SF decreased. Concentration of total volatile fatty acids increased linearly (P = 0.02) as SF decreased on the day of supplementation while, on the day before supplementation, total volatile fatty acids were greater for UIP compared with DIP (P = 0.01) and decreased linearly (P < 0.01) as SF decreased. An interaction concerning the linear effect of SF and crude protein degradability (P = 0.02) was observed for ruminal liquid volume on the day of supplementation. This was the result of an increase in liquid volume with DIP as SF decreased compared with a minimal effect with UIP. In contrast, there was no influence of supplementation on liquid volume the day before supplementation. Ruminal liquid dilution rate was greater (P = 0.02) with crude protein supplementation on the d of supplementation. We did observe a quadratic effect of SF ´ crude protein degradability interaction (P = 0.01) for dilution rate because of a quadratic response with DIP (greatest value with the every 3 day treatment) compared with a decrease as SF decreased for UIP. On the day before supplementation, ruminal liquid dilution rate decreased linearly (P = 0.02) as SF decreased. These results suggest that DIP and UIP elicit different effects on ruminal fermentation when supplemented infrequently to ruminants consuming low-quality forage while not adversely affecting nutrient intake and digestibility.