GROWTH AND TISSUE ACCRETION OF LAMBS FED CONCENTRATE IN DRYLOT, GRAZED ON ALFALFA OR RYEGRASS AT WEANING, OR AFTER BACKGROUNDING ON RYEGRASS

Weaned Targhee x Hampshire lambs (average BW 27.6 kg) were used to det ermine the effects of concentrate feeding, forage grazing, or finishin g on concentrate after grazing upon growth and carcass tissue accretio n. Lambs were assigned randomly and balanced by weight and sex to five replicated treatments (12 lambs/treatment): all-concentrate in drylot (DL); rotational grazing alfalfa (ALF); rotational grazing ryegrass ( RG); RG for 62 d, then DL (RGDL); RG for 62 d, then ALF (RGALF). Lambs were slaughtered when fat thickness over the ribeye was estimated at 3.8 to 5.6 mm. Lamb growth and carcass measurements included ADG, accr etion of bone, lean, and fat in the carcass, and final BW. Lambs on DL had the highest (P = .001) ADG, whereas lambs on RG treatments tended to have the lowest ADG and heaviest final BW. Compared with DL lambs, ALF lambs had lower(P <.O5) ADG but comparable final BW. Lambs that g razed RG had more (P =.001) carcass lean weight than lambs fed in DL, but carcass lean weight of lambs grazed on ALF did not differ (P >.05) from that of lambs on the DL or RG treatment. Carcass fat was less (P =.001) for ALF lambs than for the DL, RGDL, or RGALF treatment group. Daily accretion of bone, lean, and fat was highest(P =.001) for DL. D aily accretion of lean and fat for the RG group was less (P =.001) tha n for the ALF group but did not differ(P >.05) from that of RGALF lamb s. Lean:fat ratio in weight gain for DL lambs was less (P <.01) than r atios for the ALF and RG groups, which were similar to those for RGDL and RGALF lambs. When slaughtered at the same level of fat over the ri beye, DL-fed lambs had higher ADG and fewer days on test than grazed l ambs. However, lambs finished or backgrounded on forage had high lean: fat tissue gain and a higher percentage of lean in their carcasses tha n DL-fed lambs.