Dietary cis-9, trans-11-conjugated linoleic acid (CLA) is generally thought to be beneficial for human health. Fish oil added to ruminant diets increases the CLA concentration of milk and meat, an increase thought to arise from alterations in ruminal biohydrogenation of unsaturated fatty acids. To investigate the mechanism for this effect, in vitro incubations were carried out with ruminal digesta and the main biohydrogenating ruminal bacterium, Butyrivibrio fibrisolvens. Linoleic acid (LA) or a-linolenic acid (LNA) was incubated (1·67 g/l) with strained ruminal digesta from sheep receiving a 50:50 grass hay -concentrate ration. Adding fish oil (up to 4·17 g/l) tended to decrease the initial rate of LA (P¼ 0·025) and LNA (P¼ 0·137) disappearance, decreased (P, 0·05) the transient accumulation of conjugated isomers of both fatty acids, and increased (P, 0·05) the accumulation of trans-11-18:1. Concentrations of EPA (20:5n-3) or DHA (22:6n-3), the major fatty acids in fish oil, were low (100 mg/l or less) after incubation of fish oil with ruminal digesta. Addition of EPA or DHA (50 mg/l) to pure cultures inhibited the growth and isomerase activity of B. fibrisolvens, while fish oil had no effect. In contrast, similar concentrations of EPA and DHA had no effect on biohydrogenation of LA by mixed digesta, while the addition of LA prevented metabolism of EPA and DHA. Neither EPA nor DHA was metabolised by B. fibrisolvens in pure culture. Thus, fish oil inhibits ruminal biohydrogenation by a mechanism which can be interpreted partly, but not entirely, in terms of its effects on B. fibrisolvens.

Ten hays harvested at three stages (early bloom MB, mid bloom MB or in seed) made from lucerne (Medicago sativaj, sweet clover (Melilotus segetalis), Persian clover (Trifolium resupinatum) and pre-bloom (PB) Italian ryegrass (Lolium multiflorum var.), were offered ad libitum to four Merino male sheep and daily intake (g dry matter (DM) per kg M0·75) and DM apparent digestibility (DMD) were measured. In sacco DM degradation (g per 100 g DM), gas production (ml per 200 mg DM), in vitro digestibility and fibre composition (g/kg DM) of the hays were also studied. Gas production or DM degradation were calculated at 6, 12, 24, 48, 72 or 96 h and their kinetics were described using the equation p = a + b(1 - e-ct). Intake and in vivo DMD of the hays were variable (P < 0·01). Lucerne EB and Persian clover (all stages) had the highest nutritive value, whereas sweet clover (all stages) had the lowest. Apart from neutral-detergent fibre, which was only related to intake (r = -0·68; P < 0·05), chemical components and in vitro digestibility were poorly (P > 0·05) related to animal performance. Between 12 and 96 h incubation, intake and in vivo DMD were better related to DM degradation (r = 0·79 to 0·83; r = 0·61 to 0·77) than to gas production (r = 0·73 to 0·80; r = 0·58 to 0·78). Prediction of intake and in vivo DMD from the (a + b) values did not provide a great advantage over using some of the static values of gas production or DM degradation.Multiple regression using separated kinetics of degradation resulted in highest accuracy for predicting intake and apparent digestibility from gas production (R2 = 0·63; R2 = 0·78) and nylon bag degradability (R2 = 0·77; R2 = 0·89). It was concluded that the gas test has good potentiality as it was capable of predicting not only apparent digestibility, but also intake to a level close to that of the nylon bag technique.

The study compared using chemical components (i.e. crude protein (CP), neutral- and acid-detergent fibre or lignin (NDF, ADF and ADL) g/kg dry matter (DM), the in vitro digestibility (Tilley and Terry, 1963), the in situ (nylon bag) DM degradation (g/100 g DM) and gas production (ml/200 mg DM) techniques to predict voluntary daily intake (g DM per kg M0.75) and in vivo apparent DM digestibility (DMD) of 10 graminaceous hays individually offered ad libitum to four Merino male sheep. Gas production or DM degradation were determined after 6, 12, 24, 48, 72 or 96 h incubation and their characteristics described using the equation p = a + b (1 – e-ct). Intake and in vivo DMD of the hays were variable and poorly related (r = 0·52; P > 0·05). The in situ DM degradation was significantly (Y < 0·05) related to in vivo apparent DMD at 48 to 96 h incubation (i = 0·76 to 0·75) and to intake at 24 to 96 h (r = 0·71 to 0·75) incubation. However, fibre components, the in vitro digestibility or gas production were either related to daily intake or in vivo apparent DMD, but not to both on the same occasion. Accurate prediction of intake (r = 0·90; P < 0·05) and in vivo apparent DMD (r = 0·88; P < 0·069) were achieved using NDF, ADF, ADL and CP in a multiple regression. Using the (a + b) and the rate (c) of in situ DM degradation, both in vivo apparent DMD (r = 0·77; P < 0·05) and intake (r = 0·83; P < 0·05) were predicted with accuracy. However, using the (a + b) and (c) of gas production, only intake was predicted accurately (r = 0·87; P < 0·01). The lower performance of the gas test was attributed to the small contribution to gas production and higher buffering capacity resulting from protein fermentation. When data of the graminaceous and other data from leguminous hays were combined, the most accurate prediction of both intake and apparent digestibility was by using characteristics of in situ DM degradation followed by those of gas production. The latter was more accurate than using chemical components or the in vitro digestibility. Addition of CP in the multiple regression improved the prediction of intake and in vivo apparent DMD from characteristics of gas production. It was concluded that despite the need to overcome the problem of protein fermentation in the gas test, accurate prediction of both intake and apparent digestibility can be achieved simply from the degradation characteristics of foods.

By using separated values of kinetics of in situ dry matter (DM) degradation or in vitro gas production (Menke and Steingass, 1988) of leguminosae hays, Khazaal et al, (1993) reported high correlation with intake (r= 0.88; r= 0.79) and in vivo DM digestibility (DMD) (r= 0.94; r= 0.88). The aim of the present study was to extend the range of samples used and compare the ability of the 2 stages in vitro digestibility (Tilley and Terry, 1963), the in situ DM degradation or the gas production techniques to predict daily intake (g DM/ kgW0.75) and in vivo DM digestibility (DMD) of 19 leguminous and graminaceous hays fed to sheep.Three harvesting stages (early bloom EB, mid bloom MB or in seed IS) made from lucerne (Medicago sativa), sweet clover (Melilotus segetalis), Persian clover (Trifolium resupinatum), Rye (Secale cereale), Triticale (Triticale hexaploid), oat (Avena stativa) and a pre-bloom (PB) Italian ryegrass (Lolium multiflorum ). Each hay was fed ad libitum to 4 Merino male sheep and their intake and in vivo DMD recorded. Gas production (ml/ 200 mg DM) or in situ DM degradation (g/ 100 g DM) were determined as described by Khazaal et al, (1993) after 6, 12, 24, 48, 72 or 96 h incubation. Measured gas production or DM degradation values were fitted to the equation p=a+b(l-e-ct)(McDonald, 1981) where p is gas production or DM degradation at time t and a, b and c are constants. For nylon bag the washing loss (soluble fraction) was defined as A, the insoluble but fermentable matter was defined as B=(a+b)-A, and c is the rate of fermentation or degradation (Ørskov and Ryle, 1990).

Without chemical analysis, Blummel and Ørskov (1992) adapted the in vitro gas test (Menke and Steingass, 1988) and used the exponential equation p=a+b (l-e-ct) to describe the kinetics of fermentation of 10 straws. They reported high correlation with animal performance. The aim of the present study was to adopt a similar approach and assess the gas test in comparison with nylon bag for the prediction of intake or DM digestibility of 10 different quality hays.Ten hays consisting of 3 harvesting stages of lucerne (Medicago sativa), sweet clover (Melilotus segetalis), persian clover (Trifolium resupinatum) and Italian rye-grass (Lolium multiflorum). Each hay was fed ad libitum to 4 Merino male sheep and their intake (g dry matter (DM)/day/KgW0.75) and in vivo DMD were recorded.