Corn Silage Rumen Undegradable Protein Levels and Extent of Digestion

The amount of RUP in corn silage and the extent to which it is digested in the small intestine are uncertain. Moisture content and length of ensiling period both affect the digestibility of grain in corn silage. Three studies were conducted to assess the amount and extent of digestion of RUP in corn silage. In Exp. 1, four feeds (35% and 42% DM corn silage, soybean meal (SBM), and Soypass (enzymatically browned SBM; 50% CP; 75% RUP)) were utilized in an in vitro setting with 100-mL bottles incubated for 16 or 24 hours (bottle was experimental unit). Purine analysis was done to correct for microbial N when calculating RUP content. Exp. 2 utilized in situ methods with two ruminally cannulated steers and one duodenally cannulated steer to compare RUP content and RUP digestibility of two corn silages (35% and 42% DM) along with Empyreal (wet corn milling byproduct; 75% CP; 65% RUP) and Soypass. Samples were ruminally incubated for 20 or 30 h and ½ of the samples were then duodenally incubated to determine RUP digestibility (each experimental unit consisted of 4 bags). In Exp. 3, dry rolled corn (14% DM) was reconstituted to 25, 30, 35, and 50% moisture and ensiled in mini silos (2265 cm³) for 30 days (silo was experimental unit). After ensiling, samples were ruminally incubated for 20 or 30 hours to determine RUP content. In Exp. 1, RUP as a % of CP was greatest for Soypass (P = 0.02) with no differences between the two corn silages, averaging 23.2%, 7.4 SEM, similar to SBM. As a % of DM, RUP was also greatest for Soypass (P < 0.01), intermediate for SBM, and least for the corn silages, averaging 1.7%, 1.6 SEM. In Exp. 2, RUP as a % of DM was greatest for Soypass (P < 0.01) intermediate for Empyreal, and least for the corn silages, averaging 1.2%, 0.28 SEM. As a % of CP, RUP was greatest for Soypass (P < 0.01), intermediate for both corn silages averaging 14.7%, 1.1 SEM, and least for Empyreal due to washout of bags. In Exp. 3, as moisture content of the corn grain increased, RUP as a % of DM decreased linearly (P < 0.01). The RUP content and RUP digestibility of corn silage is low. Increased moisture content makes the grain portion of corn silage more digestible in the rumen, further decreasing total RUP levels.