The effect of diet supplements and gamma irradiation on dissimilation of low-quality roughages by ruminants: III. Effects of feeding gamma-irradiated base diets of wheaten straw and rice straw to sheep
Authors: McManus W. R. , Manta L. , McFarlane J. D. , Gray A. C. 
Published: March 2009

Results of two experiments in which gamma-irradiated low-quality cereal straws were fed ad libitum to sheep, with and without supplements, are reported.

In Exp. I 12 adult Merino wethers were fed chopped wheaten straw irradiated at 50 Mrad or non-irradiated wheat straw, each roughage was fed with and without a supplement of urea, minerals and vitamins. In Exp. II 16 adult Merino wethers were fed diets of chopped rice straw or chopped rice straw irradiated at 25, 50 or 75 Mrad; each of these roughages was fed with and without a supplement of casein, minerals and vitamins.

Gamma-irradiated diets were invariably associated with reduced mean retention times of undigested feed particles in the digestive tract, and a depressed apparent digestibility of dietary dry matter. Voluntary feed intake of sheep given unsupplemented wheat straw irradiated at 50 Mrad was not significantly affected; nor was the voluntary feed intake of sheep fed unsupplemented irradiated rice straw. Irradiation depressed apparent dry-matter digestibility of unsupplemented wheat straw by 5·2 percentage units; irradiated rice straw diets were depressed by 20·2 (25 Mrad); 9·0 (50 Mrad) and 27·2 (75 Mrad) percentage units compared to non-irradiated diets.

Supplementation of irradiated wheat-straw diets tended to restore them to a nutrient worth approximating their original levels in terms of digestibility and intake. Supplementation of rice-straw diets increased voluntary feed intake, because mean retention time was reduced, but diet dry-matter digestibility remained depressed. Supplementation always increased the animals’ state of nitrogen balance; irradiation per se had little effect.

Distribution of gastro-intestinal-tract dry-matter content showed a strong tendency for a greater proportion of the ingesta to be located in the hind gut of animals fed irradiated rice-straw diets, thus confirming the enhanced rate of passage of these diets through the tract as determined by the use of dual stained feed-particles. For sheep fed wheat straw the average mean retention time for non-irradiated particles was 50·9 % greater than for irradiated particles. For rice straw diets non-irradiated feedparticles had mean retention time (pooled across supplementation and irradiation treatments) 30·8% greater than for irradiated particles.

Distinct changes occurred in the ratio of acetic to propionic acid in the rumen liquor of sheep fed irradiated diets which are suggestive of an alteration in species composition of the foregut microbial population.

Evidence is presented for the presence in irradiated diets of a factor or factors toxic to the ruminal microbiota.

Irradiation at the levels used decreased feed particle size, but any advantage so obtained was offset by an enhanced rate of passage of feedstuff through the tract with consequent depression in digestibility. It is concluded that further work is required to elucidate the effects reported and to further establish optimum conditions for use of gamma irradiation as a technique in the treatment of low-quality roughages for use by ruminants.