Specialized feeding of lambs for optimized performance during the finishing phase

[摘要] ENGLISH ABSTRACT: The aim of this study was to determine whether a lamb finishing ration that isbalanced for essential amino acids (EAA), at a specific level of non-structuralcarbohydrates (NSC) would yield better feedlot performance when compared to astandard commercial finishing ration.Metionine (Met) and Lysine (Lys) were identified as the first limiting amino acids forgrowing lambs. These two amino acids, as well as Threonine (Thr), Arginine (Arg),Leucine (Leu), Isoleucine (Ile) and Phenylalanine (Phe) were included in anoptimized protein (OP) feedlot ration at optimal levels. This ration was formulated tocontain 157 g/kg crude protein (CP) and 477 g/kg NSC, with Met, Lys, Arg and Thr at2.48, 7.45, 8.51 and 8.12 g/kg on dry matter (DM) basis respectively. The secondtreatment, FIN, was a standard commercial lamb finishing feed without optimizedamino acids and contained similar total protein and NSC to OP of 152 g/kg CP and468 g/kg NSC but with Met, Lys, Arg and Thr at 2.08, 5.49, 7.47 and 4.80 g/kg onDM basis respectively. A third treatment, a low protein (LP) diet served as a positivecontrol and was formulated on lower specifications (139 g/kg CP, 455 g/kg NSC) andwas also not optimized for amino acids. This treatment contained Met, Lys, Arg andThr at 1.93, 4.99, 6.66 and 4.73 g/kg on DM basis respectively. Lambs grazingkikuyu pasture served as the negative control (CON) group. These lambs alsoreceived additional supplementary feed at 500 g/day as a production lick to becomparable to a scenario where lambs are finished on grazing.Forty cross-bred Merino x Döhne-Merino lambs with an average weaning weight of24.35 ± 0.648 kg were finished in a feedlot for 57 days where after they wereslaughtered at an average weight of 41.41 ± 1.259 kg. During the feedlot trial lambperformance was measured by monitoring daily growth rates and feed intake. Also,an in vivo digestibility study was carried out on the OP and FIN lambs. During theslaughter process the rumen was removed which was done to collect a sample ofthe rumen wall from next to the rumino-reticular fold. These rumen samples weremounted onto slides so that the development of the rumen could be examined. The M. longissimus dorsi from both sides of the carcass between the 2nd and 3rd lastthoracic vertebra and the 4th and 5th lumbar vertebra were removed.There was no significant difference (P > 0.05) between any of the concentrate feedtreatments with regards to the average daily gain (ADG), feed conversion ratio (FCR)or dressing percentage (DP). The CON lambs, as expected, had lower growth rates(P < 0.05) than the concentrate fed lamb and thus showed significant differences (P< 0.05) in terms of ADG. The DP of 45.96 % ± 0.711 for the CON lambs differedsignificantly (P < 0.05) from the OP (51.44 % ± 0.358), FIN (52.72 % ± 0.653) and LP(51.74 % ± 0.611) treatments. As expected the concentrate feeds were much moreeffective in maintaining higher growth rates when compared to the CON lambs whilethe optimizing of EAA in the OP diet did not lead to improved feedlot performance asthe FIN and LP treatments were able to achieve similar (P > 0.05) growth rates.Within the feedlot treatments there was no significant difference (P > 0.05) withregard to the papillae length and rumen wall thickness. There was however anumerical increase in the papillae length as the NSC levels in the feed increased.The CON lambs differed from the OP lambs (P < 0.05) in terms of papillae length.This illustrated the importance of having increased levels of NSC in a feedlot diet asit is this fraction that is responsible for the initialisation and maintenance of rumenmorphological development.The in vivo digestibility study therefore confirmed that the commercial finishing feedwas just as effective as the optimized feed in terms of nitrogen retention as well as inmaintaining suitable energy balance. Although the in vivo digestibility for Met and Lysin the OP feed was higher (P < 0.05) than the FIN feed, this did not lead to improvedfeedlot performance of the OP lambs. The increased digestibility of these aminoacids is due to the fact that the OP diet was higher in levels of bypass amino acidsthan the FIN feed. The in vitro true digestibility (IVTD) of the OP feed was higher (P < 0.05) than that of the FIN feed.

[发布机构] Stellenbosch University