Abstract:The effects of various purified carbohydrates, given as supplements to a basal diet of grass silage, on ruminal fermentation and on the urinary excretion of purine derivatives (PD) were investigated using six sheep in a 6 x 6 Latin square experiment with period lengths of 14 days. The six experimental treatments were (i) the basal diet of 4 kg day-' of grass silage, supplemented with (ii) sucrose, (iii) lactose, (iv) xylose, (v) wheat starch, and (vi) fructose. All supplements were given at 200 g day-'. The basal diet and the supplements were given in two equal meals each day. Relative to the basal diet, xylose, starch and fructose all reduced (P < 0.05) the mean ruminal pH; for sucrose, the depression was non-significant (P > 0.05) whilst lactose produced virtually no depression of pH. All supplements reduced (at least P < 0.05) the ruminal concentrations of ammonia compared with the basal diet but, excepting xylose, all the sugar supplements produced a greater (P < 0.05) reduction than did the starch treatment; for xylose, this effect just failed to reach statistical significance (0.10 > P > 0.05). Starch did not alter the volatile fatty acid (VFA) pattern from the basal diet but the sugars produced very marked changes in the molar proportions of VFA. All sugars reduced the proportions of acetic acid (for xylose, 0.10 > P > 0.05, for the others, at least P < 0.05) but compensation was in terms of increased proportions either of propionic acid (xylose and fructose) or of butyric acid (sucrose and lactose). All four sugars increased (P < 0.05) the urinary output of PD over that seen with the basal treatment but the much smaller increase with the starch supplement was statistically non-significant (P > 0.05). The calculated supply of microbial nitrogen to the small intestine was 10.2, 14.8, 14.3, 13.1, 11.9 and 13.7 g day-' for the basal, sucrose, lactose, xylose, starch and fructose treatments respectively; the value for sucrose was significantly (P < 0.05) greater than that for starch but differences between sugars were statistically non-significant (P > 0.05). It is concluded that sugars, particularly sucrose, are clearly superior to starch as an energy source for the microbial fixation of nitrogen in the rumen.

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The object of the experiment was to test the hypothesis that altering the degree of synchrony in the ruminal release of available energy and nitrogen would affect microbial protein synthesis (MPS) when the diet contained a high proportion of readily fermentable carbohydrate. Four lactating dairy cows were given a basal diet of (kg DM day−1) 8.0 grass silage, 4.2 barley and 1.8 groundnut meal containing 31.4 g N kg−1 DM. The experiment was designed as a 4 × 4 Latin square with periods lasting 14 days. The treatments were (1) the basal diet given in two equal meals at 10:00 and 22:00 h (BASAL), supplemented with (2) 2.0 kg maltodextrin given as a continous intraruminal infusion (CONT), (3) 2.0 kg maltodextrin as two 6‐h infusions starting at 10:00 and 22:00 h (SYNC) and (4) 2.0 kg maltodextrin given as two 6‐h infusions starting at 16:00 and 04:00 h (ASYNC). All three infusion treatments reduced (P < 0.05) the concentration of ruminal ammonia relative to BASAL but only the CONT and SYNC treatments increased (P < 0.05) MPS over the level with BASAL; the value for ASYNC was the same as that for BASAL. Lactic acid was a minor product of the ruminal fermentation with all treatments. All three infusions reduced (P < 0.05) the plasma concentration of urea and the urinary output of nitrogen but there were no differences among the infusion treatments. It is concluded that with this diet, containing about 30% of DM as fermentable carbohydrate, altering the degree of synchrony in the rates of ruminal release of energy and nitrogen had a marked effect on MPS. © 1999 Society of Chemical Industry

The ef®ciency of transfer of L-histidine into milk protein was measured in two experiments in which L-histidine was infused intravenously into dairy cows eating a basal diet of grass silage and a cereal-based supplement containing feather meal. Both experiments used Latin square designs, and infusion periods lasted 10 days. In Experiment 1, histidine was infused alone at doses of 3, 6 and 9 gday À1. The output of milk protein increased up to the 6 g day À1 dose but fell back to the basal level when 9 g day À1 was infused. The ef®ciency of transfer was highest for the 6 g day À1 dose, for which the value was 0.38. In Experiment 2 the same three histidine doses as in Experiment 1 were used, but this time the histidine was accompanied by 8 g L-methionine, 28 g L-lysine and 2.5 g L-tryptophan, to ensure that histidine remained ®rst-limiting over the whole dose range. The output of histidine in milk protein (Y) increased linearly with histidine dose (X) such that Y = 0.431 X 0.070 r = 0.998; n = 4, indicating an ef®ciency of transfer of 0.43.

The effects of abomasal infusion of casein or soya-bean-protein isolate (SPI) on milk production were investigated in four Friesian cows in mid-lactation receiving a basal diet of grass silage and barley which supplied energy and protein considerably in excess of requirements for milk production by conventional rationing standards. Three levels of infusion were used for each protein source, the corresponding doses being isonitrogenous for each of the proteins: 100,220 and 330 g/d for casein and 115,230 and 345 g/d for SPI. Casein produced much greater effects on the yield of milk and milk constituents than did SPI. On the highest dose of casein, milk yield was increased by 3.5 kg/d, fat output by 15% and protein output by 36%; corresponding values for the highest dose of SPI were 1.6 kg/d, 12% and 13% respectively. Increases in the yield of milk-protein were linear for casein but for SPI there was no increase beyond the first level of infusion. It was calculated that casein infusion had a marked effect on the utilization of energy: the increases in milk production could be explained either by a channelling of an extra 12 MJ metabolizable energy (ME)/d away from body tissue synthesis and into milk synthesis or by an increase in the efficiency of utilization of ME for lactation from 0.50 on the basal diet to 0.58; the measurements made did not allow the two mechanisms to be clearly differentiated. There are a number of reports of increased milk production of dairy cows in response to the infusion of casein into the abomasum (see Clark, 1975; 0rskov et al. 1977;Rulquin, 1982;Konig et al. 1984;Whitelaw et al. 1986). Responses were greatest when energy intake in early lactation was restricted to ensure some dependence on body energy stores (0rskov et al. 1977;Whitelaw et al. 1986). Abomasal protein infusionsTwo questions are raised : (1) If casein infusion can stimulate mobilization of body fat when energy intake is restricted in early lactation, can it influence the pattern of energy use in cows in mid-lactation consuming energy and protein in excess of their requirements for milk production? (2) how do the responses elicited by casein relate to those obtained with a protein more representative of those used in practice?The experiment reported here compared the effects of abomasal infusions of casein and of soya-bean protein. Both proteins were infused at three dose levels in cows in midlactation receiving a diet containing a fixed amount of barley and high-quality grass silage offered ad lib.