Rumen escape nitrogen from forages in sheep: comparison of in situ and in vitro techniques using in vivo data

Abstract

The objective of this study was to relate in vivo data of rumen escape N (REN) of forages with REN estimated from models and with determinations of rumen undegradable N. For these determinations and models measurements from in situ and in vitro techniques were used. Eleven forages were investigated in vivo using sheep with cannula in the rumen, duodenum and ileum. These forages were fresh, silage and hay from lucerne and orchard grass, and fresh, silage and haylage from red clover, and silage and hay from perennial ryegrass. Digesta flows were measured with the double marker technique using 51 Cr -EDTA and 103 Ru -phenanthroline. To measure the duodenal flow of microbial nitrogen (N), 15 N was infused as well as purine derivatives were measured in urine excretion. In vivo REN, expressed as g N kg −1 of N intake or as g N kg −1 of duodenal flow of non-ammonia N (NAN), was calculated from duodenal flows of NAN and microbial N and with assumptions for the duodenal flow of endogenous N. REN was also estimated from the models estimating effective undegradable N, using measurements from the in situ nylon bag technique or using Cornell net carbohydrate and protein system with data from CPM (Cornell, Penn, Minor Institute) Dairy Beta program (CPM-REN). With the in situ technique REN was calculated from N residues of forages incubated in the rumen, with and without corrections for microbial contamination. These in situ measurements were applied in cows fed a standard diet and in sheep fed the same forage as incubated in the nylon bag. CPM-REN was calculated from five N fractions determined with in vitro techniques. Undegradable N of the 11 forages was measured as N residue after 72 h incubation in nylon bags in the rumen of cows (in situ residual N), after 24 h incubation with protease and as acid detergent insoluble N (ADIN). REN from different in situ measurements and in situ residual N had no relationships with in vivo data. CPM-REN and the in vitro technique using protease had also no relationship with in vivo data. ADIN had a moderate relationship with different in vivo REN determinations and these relations improved when fresh and conserved (silage, hay and haylage) forages were separated ( R 2 = 0.83–0.87; coefficient of variation = 0.08–0.16). It was concluded, that ADIN has potency to predict in vivo REN of forages.