Two in vitro experiments were conducted in order to determine if microbial fermentation of Eragrostis curvula hay is depressed by high levels of added tallow. Two levels of tallow, to reach 0.06 (T6) and 0.12 (T12) of the ether extract (organic matter basis) in food were compared with a control (T0, 25 g ether extract per kg). The first experiment studied the pattern of gas production. From 24 h onwards, gas volume for T0 was higher (P < 0.05) than for T6 and T12. However, lag time was shorter with tallow, probably because of utilization of the released glycerol. Fermentation of a similar amount of fat included in T6 and T12 as the only substrate (fat-6 and fat-12) depressed gas production compared with the blank, irrespective of fat level. In a second experiment, characteristics of microbial fermentation were studied, including volatile fatty acid (VFA) production, bacterial adhesion to fibrous particles (measured according to purine bases concentration) and polysaccharidase and glycosidase activities, at 6, 12, 24 and 48 h incubation. Total VFA was higher (P < 0.05) in T0 compared with T6 but not with T12. Acetate: propionate ratio diminished with tallow in the food. Higher total xylosidase (P > 0⋅05) and glycosidase (P < 0⋅001) activities were observed for T0 than for T6 and T12 and similar responses were observed regarding specific activities. Bacterial adhesion was not different between T0 and T6 but it was smaller in T12. The inhibition of microbial fermentation by tallow addition is more related to specific polysaccharidase and glycosidase activities, rather than to a depressed bacterial adhesion.

A series of three replicated 3 × 3 Latin-square design experiments was carried out with lactating dairy cows (15 in experiments 1 and 3, and 18 in experiment 2). The objectives of these experiments were to investigate if the feeding of high concentrations of molasses would be detrimental to the performance or health of cows, and if the strategic supplementation of nitrogen or unprotected fat could enhance the efficiency of molasses feeding. In experiment 1, complete diets were based on grass silage with three inclusion rates of a liquid molasses-based supplement (Molaferm 20) to supply molasses dry matter (DM) of 125, 250 and 375 g/kg DM respectively. In experiment 2, the control diet contained (g/kg DM) 460 grass silage, 310 'molaferm 20' and 192 barley and 22 soya-bean meal, and was then supplemented with urea (12 g/kg DM) or soya-bean meal (177 g/kg DM). In experiment 3, the control diet was based on (g/kg DM) grass silage (460), molaferm 20 (310), barley (130), soya-bean meal (65) and oatfeed (20). Two concentrations, low and high, of unprotected tallow were added to the control diet (12·5 and 25·8 g/kg DM, respectively).

In experiment 1, cows suffered from some scouring on the highest molasses treatment, but recovered when changed to either the medium or low molasses treatment. However, there were no clinical symptoms of ill health observed during experiments 2 and 3. In experiment 1, animal intake and performance on the diets containing molasses DM of 125, 250 and 375 g/kg DM respectively were: total DM intake, 12·8, 16·2 and 18·6 kg/day (s.e.d. 1·03, P < 0·001); milk yield, 15·5,17·4 and 17·6 kg/day (s.e.d. 0·53, low v. medium or high, P < 0·01); fat concentration, 39·6, 39·7 and 40·0g/kg (s.e.d. 0·70, P>0·05); protein concentration, 31·6, 32·7 and 33·6g/kg (s.e.d. 0·22, P < 0·05). In experiment 2, supplementation of the control diet both with urea and soya-bean meal significantly increased DM intake (P < 0·01); milk yield (P < 0·01) and protein concentration in milk (P < 0·05), but had no significant effects on fat concentration in milk (P > 0·05). In experiment 3, there were no significant differences in DM intake between the three treatments. However, supplementation with both low and high concentrations of unprotected tallow significantly increased milk yield (P < 0·01) and significantly decreased fat (P < 0·01) and protein (P < 0·001) concentrations in milk.

It is concluded that lactating dairy cows could be given molasses up to 250 g/kg DM in grass silage-based complete diets without adverse effects on milk production or health. When a diet contained a high concentration of molasses (248 g/kg DM), supplementation with urea and soya-bean meal increased intake and milk production, and supplementation with unprotected tallow also increased milk yield, but fat and protein concentrations in milk were decreased.

Two fats, soya-bean oil and tallow, were blended in the ratios 70: 30, 50: 50 and 30: 70. The ratios of unsaturated to saturated fatty acids (U/S) of the blends were 3·18, 2·08 and 1·49 respectively; values for soya-bean oil and tallow were 6·69 and 0·93. These five fat sources were each included into a basal diet at levels of 40, 80 and 120 g/kg. The digestible (DE) and metabolizable (ME) energy together with the apparent fat digestibility (AFD) of the experimental diets were measured in a metabolism trial based upon four 4 × 4 Latin squares using 16 gilts of 30 kg initial live weight. Linear equations relating dietary DE and ME values to level of inclusion of fat were extrapolated to 1000 g/kg to obtain values for the fats. DE values for soya-bean oil and tallow were 38·06 and 34·16 MJ/kg respectively. Corresponding data for ME were 35·63 and 32·53 respectively. These figures were used to calculate values of the mixtures. Data thus derived did not differ significantly from those determined directly, indicating that there was no synergism between the two fats. DE values of fats determined as the product of AFD of fats and their gross energy followed similar trends. The majority of the increase in values of fats was associated with a change in the U/S ratio from 0·93 to 2·08.