In the first two of four experiments, sheep were fed, ad libitum, sorghum stover supplemented with graded levels of foliage of the shrub leucaena (Leucaena leucocephald) or mulga (Acacia aneura), which provided between 0 and 0·34 or between 0 and 0·43 of the dietary dry matter (DM) respectively. A second treatment (with or without urea) was superimposed in a factorial design. The effect of treatments on liveweight (LW) was explainable by their effects on voluntary intake of apparently digestible organic matter (DOMI). D0M1 was increased by mulga, largely due to an increase in the total voluntary intake of organic matter (OM). Leucaena increased DOMI by increasing the ration OM digestibility and, at low levels of inclusion, intake of the basal diet. Roughage intake was greatest when leucaena provided 0·15–0·20 of the dietary DM. Leucaena increased rumen ammonia, and whenever roughage intake was increased by urea, leucaena also increased it.
In the third experiment, when diets were made isonitrogenous with urea, roughage intake was slightly greater when leucaena, rather than its ash or a mineral mixture, was supplemented. Total OM intake and DOMI were greatest when leucaena was fed.
In the final experiment, sheep were fed one of ten treatments: three basal diets (two of sorghum stover and the third of native pasture hay) each supplemented with legumes (leucaena to the hay and one stover diet and cowpea straw to the second stover diet), ash of the respective legume and formaldehyde-treated casein. The tenth treatment was sorghum stover plus urea. For sorghum stover diets with leucaena-based or cowpea straw-based supplements, DOMI responded linearly to the nonurea nitrogen concentration of the diet. On the other hand, for native hay with leucaena-based supplements, the response of DOMI to non-urea N was negligible. It was noted that the native hay (predominantly Flinders grass, Iseilema vaginiflorum), contained lower concentrations of polyphenols than sorghum stover.
It was concluded that browse foliage can increase the voluntary DOMI of sheep consuming low quality roughages by providing nitrogen and sometimes minerals and OM of greater digestibility. The slowly-degradable proteins in leucaena, cowpea straw or formaldehyde-treated casein are more effective with polyphenol-rich sorghum stover than with native hay of otherwise similar composition.