Utilization of alkali-treated straw by sheep

Abstract

Sheep were fed on a semi-purified diet based on alkali-treated straw, to determine the nutritive value of such forage and to study various aspects of microbial synthesis in the rumen. Finely ground wheaten straw was treated with sodium hydroxide, which was subsequently neutralized with acetic and mineral acids; this forage was supplemented with minerals, branched chain fatty acids, and urea, the latter increasing the crude protein content of the diet from 4 to 13%. The feed was offered to mature Merino wethers at 1000, 700, and 400 g/day.

Of the organic matter consumed, 45–55% was apparently removed from the stomach. However, when the synthesis of bacterial organic matter was taken into account, the loss of dietary organic matter in the stomach was actually 64–70% of intake. The digestibility of cell wall constituents in the whole tract was 80–85% of intake, and 83–92% of this digestion occurred in the stomach. By contrast the cell contents fraction derived from the straw component of the diet was less readily digested, and only 34–44% of the amount consumed was digested in the stomach. Nitrogen from dietary urea was substantially incorporated into bacterial cells, the calculated rate of synthesis being equivalent to 2.6 to 3.7 g nitrogen per 100 g straw organic matter fermented in the stomach. Most of thenitrogen that passed into the intestines was present in bacteria, the amino acid and long chain fatty acid composition of which were similar to published figures. True digestibility of protein in the intestines was about 72%.

The main parameter affected by level of food intake was the rate of flow of digesta, which ranged from 5 to 10 l/day from the rumen and from 6 to 13 l/day from the abomasum. The residence time of a soluble marker rose from 10 hr at the highest food intake to 15 hr at the lowest. The efficiency of protein synthesis was not consistently affected by changes in flow parameters.

The organic matter intakes recorded, and the nutrients released from this diet were similar to those previously observed with forage diets containing 8–11% protein.