Abstract Ryegrass, harvested before ear emergence, was ensiled in triplicate in laboratory silos with and without pre-ensiling treatments. These were: 4.7 M sulphuric acid (39 and 79 g kg −1 DM); formalin (28 and 58 g kg −1 DM); ‘Add-F’ (22.5 M formic acid; 36 and 71 g kg −1 DM); formalin (28 and 58 g kg −1 DM); ‘Farmline’, a commercial additive containing acids and formalin (31 g kg −1 DM); heat (30 and 60°C for 1 h); and Lactobacillus plantarum inoculum + glucose (4.6 g kg −1 DM). The ensilage period was 120 days. Samples of each silage were incubated, in artificial fibre bags, in the rumens of four Hereford-cross steers fitted with permanent rumen cannulae. Total nitrogen (TN) disappearance was measured after incubation periods of 1, 2, 4, 7, 12 and 24 h. Hydrochloric acid and the heating pre-treatments had no significant effect ( P > 0.05) on the pH, water-soluble carbohydrates and fermentation acid values compared with those of the control. Fermentation acid production was suppressed to an increasing extent by formalin (28 g kg −1 DM), sulphuric acid (79 g kg −1 DM), formic acid (36 g kg −1 DM), formalin (58 g kg −1 DM) and formic acid (71 g kg −1 DM). The application of L. plantarum + glucose increased fermentation acids from 90.3 g kg −1 DM in the control to 227 g kg −1 DM, owing to a 10-fold increase in lactic acid content from 21.7 to 202 g kg −1 DM. All treatments, with the exception of 30°C heating, produced silages with significantly ( P < 0.001) more total nitrogen as protein than the control. This ranged from 334 g kg −1 TN (60°C heating) to 748 g kg −1 TN (formalin; 58 g kg −1 DM). All treatments significantly ( P < 0.05) reduced TN loss, compared with the control silage values, after a 7-h incubation period in the rumen. After 24 h, however, TN disappearance from the treated silages was not significantly different from that of the control with the exception of the two silages treated with formalin alone. With these silages, the proportion of TN disappearing was 55.0 and 40.0% for the low and high application rates, respectively, compared with a value of 74.0% for the control.