Abstract:
The aim of this study was to investigate the effect of different carbohydrate sources, with rapid and medium fermentabilities, on the digestibility and rumen fermentability of Atriplex nummularia cv. De Kock fed to sheep. Maize was used as the medium fermentable carbohydrate source and barley as the rapid fermentable carbohydrate source. The trial ran in four sequential experimental periods using two groups of animals. The two groups stayed constant during the entire trial, one group receiving the maize treatment and the other the barley treatment. During each experimental period the groups of animals received different levels of the two carbohydrate sources. Each experimental period consisted of a digestibility trial and a rumen fermentation trial. Four different levels of supplementation were used, namely 0, 15%, 30% and 45%. Measurements included dry matter intake, water intake, percentage dry matter digestibility, percentage neutral detergent fibre digestibility, rumen pool sizes, rumen pH, rumen ammonia nitrogen and rumen volatile fatty acid production. Supplementation of A. nummularia cv. De Kock with an energy source tended to increase feed and water intake. The tendency of energy sources to increase dry matter and neutral detergent fibre digestibilities diminished when the level of supplementation was raised from 15% to 30% and from 30% to 45%. The results suggest that maize and barley supplementation at a level of 15% resulted in the highest incremental increase in dry matter and neutral detergent fibre digestibilities of A. nummularia cv. De Kock. The results also indicated that energy supplementation tended to increase dry matter intake and rumen fill. Rumen pools of dry matter, neutral detergent fibre and total nitrogen all increased with an increase in the level of supplementation. The tendency of energy supplementation to increase the different rumen pools diminished as the level of supplementation increased from 30% to 45% for both the energy sources. The results also indicate that the rumen environment was more favourable for fibre degradation when maize was used as an energy source. In the final phase it is reported that ruminal ammonia nitrogen increased as the rate of supplementation was increased from the 0% to the 15% level. The drop in rumen ammonia nitrogen as the rate of supplementation was increased to 30% indicates an improvement in microbial protein synthesis. Ruminal pH decreased as the rate of supplementation increased with barley supplemented animals having lower rumen pH values. The decrease in rumen pH supports the increase in volatile fatty acid production as the level of energy supplementation was increased. It is concluded that an energy supplement of maize or barley will increase the intake and digestibility of A. nummularia cv. De Kock and that a level of 30% should be optimal for microbial protein synthesis without significantly affecting fibre degradation.
