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

Unmolassed feed blocks were used as supplement of Acacia cyanophylla Lindl. (acacia)-based diets. They were made, on the basis of crude weight, with o live cake (42.2%), wheat bran (26.7%), wheat hour (10.7%), quicklime (10.7% ), urea (4.4%), salt (4.4%) and a mineral and vitamin supplement (0.9%). Po lyethylene glycol (PEG, mel, wt. 4000) was introduced in these feed blocks at five increasing levels (0 (control), 6, 12, 18 and 24% on the basis of i ngredient mixture before moulding) to deactivate acacia condensed tannins. The five dietary treatments (levels of PEG) were evaluated simultaneously a nd 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, respecti vely. They were adapted to experimental conditions for 10 and 21 days, resp ectively before the commencement of the measurement periods. Freshly cut fo liage of acacia was distributed ad libitum and the feed block was permanent ly available in the trough. Feed intake and daily gain were measured on yea rlings during a 60-day period. Feed intake, in vivo diet digestibilities, n itrogen balance and microbial nitrogen synthesis using urinary excretion of allantoin were measured on rams during 10 consecutive days. Dry matter int ake (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 fe ed 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). Supplemen tation 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 ra te 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 introduce d 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% w ith feed blocks containing 6, 12, 18 and 24% of PEG, respectively. The clea r-cut improvement of the nutritive value of acacia-based diets following PE G supply resulted in a linear increase of the growth rate of sheep. The opt imum responses of acacia intake, nitrogen retention, microbial N yield and daily gain were obtained in sheep given feed blocks with 18% of PEG which c orresponded to a PEG consumption of about 23 g/day. It was concluded that f eed blocks may be used as carrier of PEG for improving the nutritive value of tannin-rich diets. (C) 2000 Elsevier Science B.V. All rights reserved.