Abstract 275

Moore,-JA; Swingle,-RS; Hale,-WH

Dep. Animal Science, Univ. Arizona, Tucson, AZ 85721, USA.

Journal-of-Animal-Science. 1986, 63: 4, 1267-1273; 23 ref.

Two random digestion trials were made, each with 12 beef steers (325 kg initial weight), to estimate changes in digestibilities of fat and of forage components when fat was added to diets containing 62 to 76% wheat straw. Trial 1 diets contained no added fat or 6.3% added fat from whole cottonseed (30% of the diet), cottonseed oil or animal fat; diets were formulated to contain equal amounts of cottonseed husks and cottonseed meal. Trial 2 diets contained 0, 2, 4 or 8% added animal fat. In all forms and at all levels, added fat increased apparent digestibility of dietary lipid (P <0.05). Estimated true digestibility of lipid decreased (from 94 to 71%) as added fat was increased from 0 to 8% (P <0.05). Up to 6.3% added fat increased digestible energy (DE) content of the diet. Fat additions of 2 and 4% increased daily DE intake (P <0.05) and did not depress digestibility of diet components (P <0.05). Fat additions of 6.3% or greater, as free fats or as whole cottonseed, reduced (P <0.05) mean acid detergent fibre digestibility from 40 to 28%. In addition to depressing fibre digestibility, 8% added fat reduced (P <0.05) digestibilities of DM (from 54 to 47%), organic matter (60 to 52%) and gross energy (60 to 51%). Oil fed as whole cottonseed caused digestibility depressions similar to free fat addition at the same level. The results are interpreted to show that up to 4% fat may be added to low quality forage diets without depressing digestion of diet components. Addition of 2 to 4% fat may be expected to stimulate feed intake and increase DE intake by steers. Attempts to increase energy density of low quality forage diets by fat additions greater than 4% may be counterproductive.