Growth performance and carcass characteristics of Dorset lambs fed different concentrates: Forage ratios or fresh grass

Forty male Dorset lambs were divided at weaning into four dietary treatment groups: ad libitum concentrates (C), restricted concentrates (RC), zero grazing (ZGR) and grazing (GR). All the lambs were weaned and slaughtered at similar weights, 24 kg for weaning and 47 kg for slaughter. The average daily gain (ADG) of the RC-fed lambs (347 g/d) was lower than that of the C-fed lambs (449 g/d) but higher than that of the lambs in the ZGR (267 g/d) and GR (295 g/d) treatments (P < 0.0001). There was no significant difference between the ZGR and GR lambs for ADG. To reach slaughter weight, the RC and ZGR-GR lambs required 20 and 40 additional days, respectively (P < 0.0001), compared to the C-fed lambs. The lambs fed C had better feed efficiency than the lambs on mixed (RC) or forage-based (ZGR, GR) diets (P < 0.0001). Values for body score, in vivo (P < 0.05) back fat thickness (P < 0.0001), and back fat thickness after slaughter (P < 0.05) were higher in the carcasses of the C-fed lambs compared to the values obtained with the other dietary treatments. No difference was observed among the treatments for leg and shoulder muscle classification (P > 0.05). However, the loins of the C-fed lambs obtained a higher classification score than those of the lambs raised under ZGR or GR (P < 0.05). Carcass yield was greater (P < 0.0001) for the C-fed lambs compared to the RC and ZGR lambs, mostly because of a lighter full digestive tract (P = 0.0007). The carcasses of the grazing lambs obtained a lower global rating classification (P < 0.05), mainly because of a lack of back fat thickness. Feeding system had a significant effect on subcutaneous fat lightness (L*) (P = 0.004) and yellowness (b*) (P < 0.0001) but did not affect redness (a*). Overall, forage-based diets may prevent excessive carcass fat in heavy lambs while producing similar muscle development, resulting in a leaner product for consumers.