Factors affecting the nylon-bag technique, for examination of digestion in the rumen of extrusa and hay samples of low nutritional value, were studied. Effects of: the method of placement of bags in the rumen; saliva in extrusa; drying and milling techniques; a second-stage digestion with acid-pepsin solution; sample size; and losses of particulate matter through bag cloth were studied. Digestion of dry matter (D.M.) and cell walls after 12, 24, 48, 72 and 168 h in the rumen were used to indicate treatment effects.Attaching bags with nylon cord to a weight was a better method than enclosing them in perforated jars in the rumen which restricted digestion. Drying technique had little effect on digestion, although frozen extrusa was digested more than oven-dried feed, especially in samples of Heteropogon contortus. Chewing of forage was sufficient to obtain maximum rates of digestion. Milling extrusa through 1 or 2 mm screens did not increase digestion. An extra period of digestion in acid-pepsin solution increased digestion of feed samples more than that of extrusa, but the effects were small. It reduced between-bag variation slightly, but did not alter the significance of differences between treatments. The effect of sample size on digestion was small. Sample weight (3, 6 and 9 g) was kept proportional (42 mg/cm 2 ) to bag cloth area.Digestion of D.M. after 24 h reflected digestion of cell walls. Digestion of cell walls was faster in grasses than in legumes. The higher digestibility of legumes was due to solubilization within 24 h of the greater amounts of cell contents found in legumes. Cell contents were only partially solubilized, and up to 10 g per 100 g original D.M. were not removed. Losses of particulate matter averaged 4-6% (range 0-5-10-6%). Losses were high when samples were finely milled (1 mm screen) but varied between species. Pore aperture of cloth was 25 fan. Methods for determining loss and correction of digestion data for such losses are given.of forages in the reticulo-rumen. Rate of digestion can be measured using the nylon-bag technique In the drier tropics, pasture production is very (Lowrey, 1970). This technique provides a simple seasonal and grazing pressures are lenient for most and cheap way of measuring forage quality, and of the year. As a result, cattle show strong selec-allows the use of large frozen, un-milled samples, tivity for particular plant parts and plant species This avoids the oven-drying of wet extrusa samples when grazing most tropical pastures. Thus, it is which may cause non-enzymic browning reactions, necessary to develop methods for prediction of and thus depress digestion and cause changes in nutritional value, which use extrusa samples chemical composition (Van Soest, 1965; Noller et obtained through an oesophageal fistula (OF). One al. 1966). Extrusa samples contain saliva which measure of nutritional value is the rate of digestion adds soluble dry matter (D.M.) to the plant material, and which, if not corrected, would give falsely high * Present address: CSIRO Di...

The rate of release of N, S, P, Ca, Mg, K and Na from four milled tropical hays in nylon bags in the rumen of cattle over a 168 h period were measured. The hays were: Medicago sativa, Stylosanthes humilis, Chloris barbata and Heteropogon contortus.Three ways were used to express results: (a)'element as % of element initially present; (b) element as % of dry matter initially present, and (c) element as its concentration in residual dry matter. The proportions of elements removed during digestion were positively related to initial concentration of the elements in the hays, with the exception of P in C . barbata hay. High proportions of Mg and K were released within 48 h in all hays. Only about 60 % of N, P and Ca initially present was removed even after 168 h in the rumen. Amounts of elements remaining became constant for each element after 48 h and were for N, 6.0; Ca, 3.0; K, 1.0; P, 0.7; Na, 0.7; S, 0.5 and Mg, 0.3 g element kg-1 DM initially present. Concentrations of elements in the residual dry matter after decreasing in the first 24 h, increased in the latter stages of digestion, even though cell wall digestion continued. The four hays released the seven elements differently. Generally, the elements in Medicago were most soluble and those in Heteropogon the least. However, the extent of solubilisation in Stylosanthes and in Chloris varied with the element concerned. In Heteropogon hay, which had low element concentrations initially, less than half of the N, S, P, Ca and Na were released even after 168 h digestion.

Thirty crossbred wethers (60 kg avg initial wt) were used to study the time-dose response to dietary Se as sodium selenite (Na2SeO3). Sheep were fed a basal diet (.20 mg/kg Se, M basis) for 10 d; three wethers were killed and tissues were collected for controls. The remaining 27 sheep were assigned randomly to diets supplemented with either 3, 6 or 9 mg/kg Se (as-fed basis) from reagent grade Na2SeO3 and fed for 10, 20 or 30 d. Feed offered was restricted to 1,200 g daily and tap water was available ad libitum. Sheep were stunned and killed by exsanguination and liver, kidney, muscle, heart and spleen were removed and frozen for Se analysis. No toxic effects were noted as expressed by feed intake or hemoglobin concentration. Added dietary Se increased Se linearly (P less than .01) in liver, kidney, and serum. Selenium in liver, kidney and serum also increased (P less than .01) as time advanced. Serum, liver and kidney were more sensitive to dietary Se than were muscle, heart and spleen. Ten days appeared to be an adequate length of time for further Se bioassay studies of this nature. Reagent grade Na2SeO3 was nontoxic when fed to sheep for 30 d at levels up to 90 times the Se requirement.

An experiment was conducted with 192 day-old male Cobb chicks to study tissue uptake of Se as an estimate of the bioavailability of supplemental inorganic Se sources fed at high dietary concentrations. A basal corn-soybean meal diet (.18 mg Se/kg diet, dry matter basis) was supplemented with 0, 3, 6, or 9 mg Se/kg diet (as-fed basis) as either reagent grade Na2SeO3, CaSeO3, or Na2SeO3 plus fumed amorphous carrier or 6 mg Se/kg diet as either Na2SeO4 or Se metal, and fed for 1 wk. No toxic effects were noted as expressed by mortality; however, there was a reduction (P less than .01) in feed intake and daily gain when 9 mg Se/kg diet was fed, suggesting onset of toxicosis. Selenium concentration in liver, kidney, muscle, and plasma increased linearly (P less than .01) as dietary Se increased from all sources. Selenium metal produced lower (P less than .01) Se concentrations in kidney and muscle than other supplemental sources. Multiple regression slope ratios were used to estimate relative bioavailability values of 100, 103, 99, 112, and 83 for Na2SeO3, CaSeO3, Na2SeO3 + carrier, Na2SeO4, and Se metal, respectively. When these ratios were corrected for the analyzed dietary Se concentration, relative values were 100, 96, 94, 109, and 81 for the above sources, respectively.