Thirty-two Friesian cows in early lactation were given one of tour completely mixed diets based on equal proportions of barley straw and concentrate. Two varieties of barley straws had been identified as varying in degradation characteristics (Corgi and Gerbel) and the straws were either given untreated or treated with anhydrous ammonia. The ad libitum intake for 10 weeks closely reflected degradation characteristics. For Corgi, treated and untreated and Gerbel, treated and untreated the 48·h degradabilities were 71·9, 66·8, 61·9 and 52·2 (g/100 g) respectively. The voluntary intakes of dry matter were 17·1, 14·8, 15·0 and 13·1 (s.e. 0·51) kg/day and average yields of fat-corrected milk for 10 weeks were 26·7, 21·5, 22·6 and 22·4 (s.e. 1·0) kg/day respectively.
It was calculated that the differences between actual digestibility and potential degradability were greatest for ammonia-treated straws. Also it was observed that the degradability of faecal dry matter was greater from animals given ammonia-treated straws.