In situ digestion of detergent fiber nitrogen in alfalfa stems

Abstract

The detergent fiber (van Soest) system for partitioning carbohydrates may be used to partition nitrogen (N) in feedstuffs. We studied the ruminal disappearance of N in neutral detergent fiber (NDFN), acid detergent fiber (ADFN) and available fiber N (AFN). Alfalfa ( Medicago sativa L.) stems were harvested on 21 May, 31 May and 1 June in spring, and 10 July, 20 July and 30 July in summer of 1984. Ground stems were placed in dacron bags and incubated ruminally in steers for 0, 24 and 48 h. Bag residues and original samples were analyzed for dry matter (DM), N, NDF, ADF, NDFN, ADFN and AFN. N concentrations in bag residues were corrected for bacterial N by assuming contamination of 180 and 360 g bacterial N kg −1 residue N at 24 and 48 h, respectively. Disappearance of NDFN and ADFN at 48 h ranged from 563 to 387 g kg −1 of NDFN and 346 to 243 g kg −1 ADFN. Disappearance of AFN averaged 653 g kg −1 AFN, whereas ADFN disappearance averaged 275 g kg −1 ADFN. N undegraded after 48 h in the rumen ranged from 70 g residual N kg −1 original N from the 21 May harvest to 150 g residual N kg −1 from the 30 July harvest. Undegraded N was closely related ( r 2 = 0.93, root mean square error (RMSE) = 8.9 g kg −1) to concentration of ADFN (as a proportion of total N), whereas AFN was not ( r 2 = 0.38, RMSE = 26.6 g kg −1).