Five agricultural by-products, maize stover and cobs, wheat straw, grape marc and sunflower husks, were treated with alkaline reagents under various conditions.
Maize stover and wheat straw reacted quickly with both sodium hydroxide applied at a level of 4 g NaOH/100 g dry residue and a mixture of calcium and sodium hydroxides (1:1), which led to changes in chemical composition and increases in digestibility. However, ensilage of the treated product at about 37% dry matter (D.M.) was not successful.
When dried grape marc was treated with sodium hydroxide (4·3 g NaOH/100 g dry residue) apparent lignin content increased, although other changes in composition and digestibility were similar to the above. Nevertheless, digestibility remained low. Moreover, when previously ground marc was treated in the same way lignin and cellulose fractions increased markedly while digestibility decreased.
Treatment of maize cobs with increasing amounts of sodium hydroxide (2·1–4·2 g/100 g dry residue) at 80% D.M. gave progressively more favourable results, although no improvement in digestibility was noted when half the sodium hydroxide was replaced by calcium hydroxide. Similar positive results were obtained under industrial conditions at application rates of 3·2 and 4·5 g sodium hydroxide per 100 g dry residue. Pelleting the product led to a further increase in digestibility.
Sunflower husks also showed a decrease in hemicellulose content and increases in digestibility and volatile acid content after treatment with 4·4 g NaOH/100 g dry residvie under industrial conditions. Lignin content was unchanged. The pelleted products (about 80% D.M.) were stable for more than 6 months when stored in paper sacks.
Except with grape marc loss of hemicellulose provided a general indication of the success of alkali treatment, although correlation with increased digestibility did not reach statistical significance (r = 0·34). Volatile acid production and alterations in lignin content were not suitable as general indicators.
In general, alkali treatment of the examined materials improved digestibility, although the products with a high water content were subject to undesirable fermentative changes during storage and therefore should be used within a few days.