Thirty steers were used in two pen experiments (Expts 1 and 2), and 27 of these in a third (Expt 3), to quantify their responses of hay intake, rumen ammonia nitrogen (RAN) concentrations, and liveweight to inputs of rumen soluble nitrogen (urea) and rumen undegradable protein (formaldehyde-treated casein; F-casein) when added to a basal diet of low quality hays. The hays were made from unimproved native pastures typical of those grazed by cattle in the subtropics of Australia and contained 7·8 g N/kg dry matter (DM) with coefficient of organic matter digestibility of 0·503 in Expts 1 and 2, and 5·2 g N/kg DM with a digestibility range from 0·385 to 0·448 in Expt 3. The steers (15 months old) were either Brahman (B), Hereford (H) or the F1 Brahman×Hereford (BH) cross. Steers were offered supplementary minerals with the hays in each experiment. In Expt 1 (35 days) urea was sprayed on part of the hay, allowing for daily urea intakes (g/steer) of either 0, 5, 11, 16 or 26. In Expt 2 (42 days), F-casein was offered daily (g/steer) at either 0, 75, 150, 225 or 300 and in Expt 3 (56 days) discrete offerings were made of soluble casein (225 g/day), of urea (18 g/day)+F-casein (225 g/day) or of nil.

There were significant linear effects of urea intake upon hay intake and liveweight change of steers. However, B steers had smaller increases in intake and liveweight change than did H steers, and B steers did not have a linear increase in RAN concentrations with increasing urea intake as did H and BH steers. In Expt 2 there were significant linear effects of F-casein supplements on hay intake and liveweight change of steers and a significant improvement in their feed conversion ratio (i.e. DM intake: liveweight change). The B steers did not differ from H and BH steers in liveweight change but had significantly lower hay intakes and non-significantly smaller increases in RAN with increasing F-casein intake. In Expt 3, hay intake of the steers increased with soluble casein (by 16·8%) and with urea+F-casein (24·5%). Only steers given urea+F-casein had a high RAN concentration (94 mg/l) and a high liveweight gain. The B steers had a liveweight loss and a lower hay intake than H or BH steers in Expt 3 but a higher RAN concentration.

These studies have indicated the importance of the form and quantity of additional N required by cattle of differing breed types to optimize their feed intake and liveweight gain when offered low-N, low-digestible hays.