Urea supplementation compared with pretreatment. 1. Effects on intake, digestion and live-weight change by sheep fed a rice straw

Abstract

The intake, digestion and utilization by sheep of diets comprising milled untreated (RS) or urea-pretreated (URS) rice straw (920 g kg −1), molasses (50 g kg −1) and minerals (30 g kg −1) with and without supplements of urea and sulphate infused into the rumen were compared. The intake, excretion and apparent retention of nitrogen and sulphur were also measured. Urea pretreatment of the straw decreased the proportion of hemicelluloses in neutral detergent fibre (NDF) from 0.30 to 0.24 and increased the potential digestibility of NDF from 57 to 69%. Urea pretreatment or supplementation with urea and sulphate increased organic matter (OM) intake from RS 580 g per day to URS 780 g per day, supplemented RS 800 g per day and supplemented URS 930 g per day. Associated with these increases were increases in digestibility of OM and cell-wall constituents. NDF digestibilities were: RS, 41%; URS, 60%; supplemented RS, 49%; supplemented URS, 63%. Although rumen fluid pH was not affected by diet, ammonia nitrogen concentrations were increased from 12 mg Nl −1 on RS to 57 mg Nl −1 on URS, 104 mg Nl −1 on supplemented RS and 203 mg Nl −1 on supplemented URS. Both urea pretreatment and supplementation increased apparent nitrogen and sulphur balances in the sheep. As a consequence of the increases in digestible OM intakes resulting from urea pretreatment or urea and sulphate supplementation, live-weight losses were reduced from 140 g per day on RS to about 15 g per day on URS and supplemented RS, while sheep given supplemented URS gained 20 g per day. These results are discussed in relation to previous reports on urea pretreatment and supplementation, and the likely effects of straw quality on the responses obtained to either of these strategies.