Supplemental cracked corn for steers fed fresh alfalfa: II. Protein and amino acid digestion

The effects of different levels of cracked corn on N intake, ruminal bacter ial CP synthesis, and duodenal flows and small intestinal digestion of amin o: acids (AA) in steers fed fresh alfalfa indoors were determined. Angus st eers (n = 6; average BW 338 +/- 19 kg) cannulated in the rumen, duodenum, a nd ileum were fed each of five diets over five periods in a Latin square de sign with an extra animal. Steers consumed 1) alfalfa (20.4% CP, 41.6% NDF) ad libitum (AALF); 2), 3), and 4) AALF supplemented (S) with three levels of corn (.4, .8, or 1.2% of BW, respectively), or 5) alfalfa restricted (RA LF) to the average forage intake of S steers. Average N intake and duodenal flow of nonammonia N (NAN) were greater(P < .01) in S than in RALF steers. Greater duodenal flows of NAN in S compared with RALF were due to a trend toward higher (P = .06) flows of both bacterial and dietary N. Levels of co rn decreased (P < .01) linearly N intake and increased (P < .01) linearly d uodenal flow of NAN owing to a numerical linear increase in nonbacterial N (P = .15) with no increase in bacterial N flow. Duodenal NAN flows as perce ntages of N intake increased(P < .01) linearly (69.3 to 91.0%) as corn incr eased. Ruminal NH3 N concentration, ruminal CP degradability, and the propo rtion of bacterial N in duodenal NAN were decreased (P < .01) linearly as c orn increased. Efficiency of net microbial CP synthesis was not affected (P > .05) by treatment (average 42.6 and 30.9 g N/kg of OM apparently or trul y digested in the rumen, respectively). Small intestinal disappearance of t otal N and individual AA, except for threonine and lysine, and small intest inal digestibility of N :md individual AA, except for methionine, histidine , and proline, increased (P < .01) linearly with level of corn and were gre ater (P < .01) in S than in RALF steers. Supplementing corn to steers fed f resh alfalfa reduced ruminal N losses and CP degradability and increased th e duodenal flow and the small intestinal disappearance and digestibility of total N and total, essential, and nonessential AA.