Polyethylene glycol is more effective than surfactants to enhance digestion and production in sheep fed mulga (Acacia aneura) under pen and paddock conditions

Abstract

Chemicals that interfere with the formation of tannin-protein complexes were evaluated as dietary additives for mulga-fed sheep in pens and under paddock conditions. Condensed tannins (CT) in mulga inhibit protein digestion, and the use of chemicals to precipitate CT or dissociate CT-protein complexes may improve production from sheep consuming a mulga diet. In a digestion study with mulga-fed sheep in pens, provision of polyethylene glycol (PEG) at a rate of 6 g/day significantly (P < 0·05) improved nitrogen (N), phosphorus (P), and sulfur (S) balance, and apparent N, P, dry matter (DM), and organic matter digestibility, and the rate of liveweight gain. Addition of the surfactants SDS or alkanate 3SL3 to the diet of mulga-fed sheep did not improve N balance or digestion; however, apparent digestibility of P, and P and S balance, were significantly improved by SDS. Teric PE64, a compound structurally similar to PEG, significantly improved S balance, but not DM intake or N balance. For sheep consuming a predominantly mulga diet under paddock conditions, provision of PEG at a rate of 12 g/day significantly improved clean wool growth and liveweight gain compared with unsupplemented sheep, by 9% and 100%, respectively (0·809 v. 0·745 mg/cm².day, and 44 v. 22 g/day). The studies demonstrated that although surfactants can affect mulga digestion, using PEG to precipitate CT is more effective to improve mulga digestion and animal production than the use of surfactants. However, the wool and liveweight production responses achieved with PEG were not sufficient to justify its wide-scale use for mulga-fed sheep. Consequently, alternative methods should be sought to reduce the negative effects of mulga CT on sheep production.