The objective of this work was to investigate the effects of changing proportions of readily fermentable carbohydrate and fibre in the diet on the rates of lipolysis and fatty acid hydrogenation by sheep rumen digesta. Three experiments were carried out using rumen-fistulated Romney wethers. In the first experiment rumen digesta from one sheep on a high-fibre diet was incubated in vitro with [l-14 C]linoleic acid and 0, 0-1, 0-2 and 0-5% sucrose. It was found that sucrose increased the rate of hydrogenation of linoleic acid by up to 40%.In the second experiment five rumen-fistulated Romney wethers were fed five diets according to a latin-square experimental design. These diets had decreasing proportions of fibre (42-8-19-5%) and increasing proportions of starch (12-2-35-7%). Lipolysis and hydrogenation rates decreased significantly with decreasing fibre content which was reflected in the amounts of stearic and octadecenoic acids in the rumen digesta.In the third experiment sheep were fed the high-fibre diet used in the first experiment and the rumen digesta incubated in the presence of 0,0-15, 0-45, 0-75 and 1-50% starch. Lipolysis rates increased but no effect of added starch on hydrogenation was observed.It was concluded that the short-term addition of energy to the rumen digesta increases lipolysis and hydrogenation rates. However, long-term replacement of fibre by starch results in a reduction of these rates.

Particulate fractions prepared from meadow hay, ranging in size from (M to 2 mm, were incubated with rumen digesta from four cannulated Romney sheep fed the same hay. The rates of gas production, lipolysis of corn oil and hydrogenation of linoleic acid were measured.The rate of gas production per g fermentable particles (FP) was approximately 30 % lower with 1-2 mm than with the 0-1-0-4 mm particles. However, per m 2 surface area the rate for the larger particles was found to be approximately 600% greater.The rates of lipolysis of triacylglycerols and hydrogenation of linoleic acid were respectively 25 and 60% higher per g FP and 1100 and 1200% higher per m 2 FP surface area with the 1-2 mm particulate fraction.The same hay particulate fractions were incubated with pure cultures of Ruminococcus flavefaciens, since this organism is active in both lipid metabolism and cellulose fermentation. The rate of gas production and the number of organisms adhering to the particles were determined.The effects of particle size on gas production were similar to those found when incubations were carried out with rumen digesta. Per g FP the rate was 40 % lower with 1-2 mm than with Q-1-0-4 mm particles. However, per m 2 surface area the rate was found to be approximately 450% greater with the former.It was further found that although the density of the bacterial population on 1-2 mm particles was 600% higher than on the 0-1-0-4 mm particles, the rate of gas production per 10" bacteria remained unchanged.We conclude that per m 2 surface area fermentation, lipolysis and hydrogenation were more rapid with particles ranging from 1 to 2 than from 0-1 to 0-4 mm in size. This was due, at least in part, to microbial population density.

Two experiments were carried out to test the effects of ryegrass maturity on rumen lipid metabolism. In the first experiment the effect of stage of maturity of perennial ryegrass on lipid metabolism in the rumen was studied with grazing sheep fitted with rumen cannulae. The pasture was either immature (13-8% crude protein), mature (8-1% crude protein) or senescent (5-5% crude protein).The rates in vitro of triacyl glycerol lipolysis and linoleic acid (18: 2w6) hydrogenation were found to decrease with increasing age of the ryegrass.In the second experiment the sheep were dosed with emulsified linseed oil (30 g) via rumen cannulae while grazing immature or senescent ryegrass and the rumen digesta and blood plasma sampled at 0, 4 and 8 h after dosing.The proportions of linseed oil retained in the rumen were greater and blood plasma linoleic (18:2w6) and linolenic (18:3w3) acid concentrations higher when senescent ryegrass was fed.It was concluded that the rates of rumen lipolysis and hydrogenation decreased with the age of pasture and that after dosing with linseed oil the polyunsaturated fatty acid concentrations in blood plasma increased. INTRODUCTIONacids in the rumen and their appearance in blood plasma. Previous work has established that diet affects lipid metabolism in the rumen. Thus concentrate MATERIALS AND METHODS diets reduced the rates of lipolysis and hydrogenation by rumen digesta (Gerson, John & King, Animals 1985) and enhanced the unsaturation of tissue Romney wethers aged 2 years and weighing lipids (Leat, 1977), plasma lipids (Moore, Noble & between 45 and 52 kg, fitted with 85 mm (i.d.) Steele, 1968) and milk fat (Latham, Storry & rubber rumen cannulae were used. For Expt 2 the Sharpe, 1972). Diets low in N resulted in low rates cannulae were fitted with probes for dosing and of lipolysis and hydrogenation of unsaturated fatty sampling. These consisted of 2 cm diameter, 10 cm acids (Gerson, John & Sinclair, 1983) and increased long perforated metal cylinders, covered with 1 mm unsaturation of fatty acids in rumen digesta, plasma nylon mesh placed at the end of 20 cm polythene and perirenal fat (Gerson et al. 1982).tubing. In the present work the objective of the first experiment (Expt 1) was to study the effect of rye-Diets grass of varying maturity on the rates in vitro of The ryegrass pasture {Lolium perenne L. var. lipolysis and hydrogenation by sheep rumen di-Grasslands Nui) was strip grazed at a high stocking gesta.density (about 300 sheep/ha) in order to minimize The purpose of the second experiment (Expt 2) pasture selection. Dry matter (D.M.) density of was to examine the effects of dosing sheep fed pastures cut to ground level were: immature (preimmature or senescent ryegrass with linseed oil and bloom) 2-2 t/ha, mature (after seed head formation) to compare the retention of polyunsaturated fatty 2-6 t/ha and senescent (mostly dead) 4'2 t/ha.