Rumen capacity can constrain the voluntary intake of roughages. Few published experiments have combined manipulation of rumen digesta volumes with measurements of intake, eating behaviour, and rumen volume and outflow. All these measurements were made in the experiment reported here.

Three Holstein steers (680-750kg) with large rumen canulae were used. They were given hay ad libitum (degradation characteristics a = 35.2, b = 45.6, c = 0.0397 when degradation (P) in ‘t’ hours is given by P = a + b(l-e-ct). The hay was ‘chopped’ to lengths of about 40-60mm and supplemented with urea, sodium sulphate, minerals & vitamins. There were seven periods of five days. Periods 1, 3, 5 & 7 were controls, and periods 2, 4 & 6 treatment (3x3 latin square). As treatments the reticulo-rumen was totally emptied once daily, and the digesta weighed, sampled, and A) 1.15 (add), B) 0.75 (remove), or C) 1.00 (replace) returned to the rumen (digesta from B was used to add to A). Daily DM intake and excretion, feeding behaviour, and digesta flow (pulse dose of PEG on the last day of all seven periods) measured. Digesta was manipulated at 11.30h, the process taking less than an hour. Treatment effects were analysed as differences from the means of the controls proceeding and succeeding the appropriate treatment. These differences (‘change’ or ‘Δ’ values) were analysed as 3 x 3 latin squares, with period and animal effects included in the residual variance.