Feeding sunflower oil to partially defaunate the rumen increases nitrogen retention, urea-nitrogen recycling to the gastrointestinal tract and the anabolic use of recycled urea-nitrogen in growing lambs.

The objective of the present study was to delineate how interactions between feeding sunflower oil (SFO) to partially defaunate the rumen and altering dietary ruminally fermentable carbohydrate may alter urea-N kinetics and N metabolism in lambs. In a 4 × 4 Latin square with a 2 × 2 factorial arrangement of treatments, four Suffolk ram lambs (61·5 (se 4·0) kg) were used. Treatments were 0 ( - SFO) v. 6 % (+SFO) SFO and dry-rolled barley (DRB) v. pelleted barley (PB). N balance was measured over 4 d, with concurrent measurement of urea-N kinetics using continuous intra-jugular infusions of [(15)N(15)N]urea. Feeding SFO decreased (P = 0·001) ruminal protozoa and NH3-N concentrations. Urinary N excretion was lower (P = 0·003), and retained N was higher (P = 0·002) in +SFO lambs compared with - SFO lambs. Endogenous production of urea-N (urea-N entry rate; UER) was similar across treatments. Urea-N transfer to the gastrointestinal tract (GIT) (GIT entry rate; GER), expressed as absolute amounts (16·4 v. 13·1 g/d) or as a proportion of the UER (0·693 v. 0·570), its anabolic use (9·0 v. 6·0 g/d) and microbial N supply (14·6 v. 10·9 g/d) were higher (P ≤ 0·001) in +SFO lambs compared with -SFO lambs. As a proportion of the UER, GER was higher, whereas urinary urea-N loss was lower, in lambs fed PB compared with those fed DRB (P = 0·01). In summary, feeding SFO increased urea-N recycling to the GIT and microbial non-NH(3)-N supply, thus providing new evidence that the improved efficiency of N utilization in partially defaunated ruminants could be partly mediated by an increase in urea-N recycling.