Ensilage as a means of reducing the concentration of cyanogenic glycosides in the pods of 'Acacia sieberiana' and the effect of additives on silage quality

Abstract

Cyanogenic glycosides are anti-nutritional components capable of liberating hydrogen cyanide, a respiratory poison that has been reported to be responsible for the death of livestock that browse some Acacia species containing these compounds. The objective of the study was to improve the nutritive value of the pods of 'Acacia sieberiana' by reducing their contents of cyanogenic glycosides through ensiling, and to examine the influence of various levels of added molasses and urea on the aerobic stability of the silage. In experiment 1, laboratory silages, prepared by mixing ground pods with water in two ratios (60:40 and 40:60 w/w pod material:water), were put into 250 ml polythene bottles, ensiled for 4, 7, 14, 21, 28 or 35 days and analyzed for cyanide. In experiment 2, silages prepared as described above but using only one of the ratios (40:60), and with the addition of molasses (0, 30 or 60 g kg⁻¹) and urea (0, 2.5 or 50 g kg⁻¹) were included in a 3 x 3 factorial design, to study the effects of these additives on the stability of the silage. The period of ensilage was 14 days. In experiment 3, the concentrations of molasses and urea were modified to 0, 45 or 90 g kg⁻¹ and 0, 7.5 or 15 g kg⁻¹, respectively, and the period of ensilage was extended to 45 days. The parent material contained 130.6 mg CN kg⁻¹ DM and ensiling the material for 35 days reduced the concentration to 18.1 mg CN kg⁻¹ DM. Moisture content at ensiling significantly (P < 0.001) affected the concentration of cyanide in the stored silages. Silages opened after 14 days were unstable irrespective of the use of additives. However those with combined urea and molasses exhibited a slow rate of deterioration and low fungal counts. Extending the period of ensiling to 45 days improved the aerobic stability of all the silages. The addition of urea significantly (P < 0.001) increased the pH, crude protein content, gross energy and fermentation acids while molasses treatment increased (P < 0.001) the lactic acid and ash content of the silages. Ensiling ground pods for 45 days was enough to reduce the cyanide content to non-toxic levels and produced a silage which was aerobically stable, while the inclusion of additives further improved the quality of the silages.