Retention time of macerated alfalfa hay and silage in sheep

Fresh alfalfa was mowed and conditioned mechanically at four levels: a control (rubber rolls), macerated once (a single passage through three finely corrugated rolls set at 1-mm clearance), macerated twice (two passages), and macerated thrice (three passages). Alfalfa was then field-wilted either for 45 h and conserved as chopped silage at 30% dry matter (DM) or for 94 h and stored as baled hay at 85% DM. The eight forage treatments (four mechanical conditioning levels x two conservation systems) were fed to 24 sheep (three replications per treatment) during 5 wk. At the beginning of wk 5, a 15-g sample of chromium-mordanted forage (3.5% Cr) was fed to each sheep, and feces samples were collected at 30 different times over 7 d, between 10 h and 168 h after Cr ingestion. Four models were used to estimate the passage rates, the time delay, and the mean retention time (MRT). A two-compartment time-dependent model and a multicompartment model produced the best fit (average r2 of 0.96) to represent the Cr concentration in the feces over time. When compared with alfalfa hay, alfalfa silage had a higher (P < 0.01) time-dependent turnover rate (0.0949 vs 0.0733/h), a lower (P = 0.03) time delay (9.1 vs 11 h), and a lower (P = 0.04) MRT (57.8 vs 64.4 h). Maceration did not affect significantly (P > 0.10) the time delay or the MRT. However, the MRT of macerated alfalfa hay tended to be higher than the MRT of control hay. Experimental data based on marker concentration in the feces can be used satisfactorily to assess differences in MRT between treatments, but they should be used with caution to estimate the partition of retention time within the gastrointestinal tract.