1. Two experiments were conducted to determine the feeding value of high moisture wheat straw treated with ammonia. The straw was chopped, water added to attain 350 g dry matter per kg treated straw, then treated with 84 or 90 g anhydrous ammonia per kg wheat straw dry matter and stored in a large plastic bag.
2. In Experiment 1, the treated straw was provided ad libitum to lambs in individual pens with either a control supplement (molasses and minerals), or 200 or 400 g of a soya bean meal- or maize gluten meal-based supplement.
3. Lambs fed the control supplement consumed 24·8 g of treated straw dry matter per kg live weight and lost 32 g live weight per day during the 28-day study. Source of supplemental protein did not affect live-weight gain or treated straw consumption. Level of supplement increased live-weight gains (P < 0·01) but did not affect treated straw consumption.
4. In Experiment 2, the lambs were fed one of three levels of supplement that each supplied 58 or 116 g of crude protein per day. Supplemental protein level increased cellulose digestibility (P < 0·01) with no effect on hemicellulose digestibility. Protein level did not affect treated straw consumption or live-weight gain.
5. Supplement level decreased cellulose digestibility (P < 0·01), hemicellulose digestibility (P < 0·01), and treated straw intake (P < 0·01) but increased live-weight gain.
6. The results are discussed in relation to the effects of amount and type of supplemental protein and level of supplementation on utilization of the treated straw.