1. Four experiments have been carried out to determine the effect of treatment of spring-sown barley straw (var. Deba Abed) with sodium hydroxide (NaOH) solution and subsequent neutralization with hydrochloric acid (HCl) on its digestibility and intake by sheep.
2. In Experiment 1, chopped straw was mixed with 4·5 or 9·0 g NaOH in 200 or 800 ml water/100 g, straw, and after 24 hr neutralized with HCl and left for a further 24 hr before being offered to castrated male sheep (wethers) in a maintenance diet containing 35% concentrates. Organic-matter digestibility of straw significantly increased, by 8 and 11 percentage units respectively, after treatment with 200 ml solution containing 4·5 and 9·0 g NaOH. Volume of solution did not affect digestibility.
3. In Experiment 2, straw treated as in Experiment 1 was given ad libitum. The highest intake was for straw treated with 200 ml solution containing 4·5 g NaOH/100 g straw, treatment with 9·0 g NaOH giving a significantly lower intake but higher than that of untreated straw.
4. In Experiment 3, the in vitro digestibility of milled straw, treated as in Experiments 1 and 4, increased with increasing volumes of solution up to 120 ml/100 g straw, but the response to successive increments of NaOH declined progressively.
5. In Experiment 4 chopped straw was mixed with 4·5, 6·75 or 9·0 g NaOH in 30, 60 or 120 ml water/100 g straw and offered as in Experiment 1. Treatments significantly increased straw digestibility, by 8 to 16 percentage units. Increasing the volume of water from 30 to 60 ml significantly improved digestibility, by 5 percentage units at the two lower levels of NaOH. The response to an increase in the level of NaOH was less, and inconsistent. In vitro and in vivo digestibilities were significantly correlated, but it is concluded that the in vitro technique used overestimates the digestibility of treated straw.