Deactivation of condensed tannins in Acacia cyanophylla Lindl. foliage by polyethylene glycol in feed blocks : Effect on feed intake, diet digestibility, nitrogen balance, microbial synthesis and growth by sheep

Abstract

Unmolassed feed blocks were used as supplement of Acacia cyanophylla Lindl. (acacia)-based diets. They were made, on the basis of crude weight, with olive cake (42.2%), wheat bran (26.7%), wheat flour (10.7%), quicklime (10.7%), urea (4.4%), salt (4.4%) and a mineral and vitamin supplement (0.9%). Polyethylene glycol (PEG, mol. wt. 4000) was introduced in these feed blocks at five increasing levels (0 (control), 6, 12, 18 and 24% on the basis of ingredient mixture before moulding) to deactivate acacia condensed tannins. The five dietary treatments (levels of PEG) were evaluated simultaneously and each diet was tested on 6 yearling male Barbarine sheep (average initial weight 19±1.5 kg) and five Barbarine rams (average initial weight 47±3.0 kg). Animals were held in individual pens and metabolism cages, respectively. They were adapted to experimental conditions for 10 and 21 days, respectively before the commencement of the measurement periods. Freshly cut foliage of acacia was distributed ad libitum and the feed block was permanently available in the trough. Feed intake and daily gain were measured on yearlings during a 60-day period. Feed intake, in vivo diet digestibilities, nitrogen balance and microbial nitrogen synthesis using urinary excretion of allantoin were measured on rams during 10 consecutive days. Dry matter intake (DMI) of feed blocks ranged from 13.7 and 16.5 g/kg W 0.75 for yearling sheep and from 15.5 and 24.3 g/kg W 0.75 for rams. PEG had no effect on feed block intake. Acacia DMI by sheep given feed blocks without PEG was low (37.7 and 39.9 g/kg W 0.75, respectively for yearlings and rams). Supplementation with feed blocks containing either 6, 12, 18 or 24% of PEG increased acacia DMI by 25, 50, 56 and 45%, respectively for yearling sheep and 22, 25, 56 and 71%, respectively for rams. Providing PEG in feed blocks at a rate of 6, 12, 18 or 24% increased digestible organic matter intake (DOMi) by 3.4, 4.3, 9.4 and 18.2 g/kg W 0.75 and digestible crude protein intake by 17, 12, 23 and 36 g/day, respectively. The higher the rate of PEG introduced in feed blocks the greater N retention and urinary excretion of allantoin. Estimated microbial N (g/kg DOMi) was increased by 37, 94, 135 and 153% with feed blocks containing 6, 12, 18 and 24% of PEG, respectively. The clear-cut improvement of the nutritive value of acacia-based diets following PEG supply resulted in a linear increase of the growth rate of sheep. The optimum responses of acacia intake, nitrogen retention, microbial N yield and daily gain were obtained in sheep given feed blocks with 18% of PEG which corresponded to a PEG consumption of about 23 g/day. It was concluded that feed blocks may be used as carrier of PEG for improving the nutritive value of tannin-rich diets.