SUMMARYThe yield and composition of milk from Friesian cows grazing either perennial ryegrass (Lolium perenne) (G, ten cows) or white clover (Trifolium repens) (C, nine cows) were evaluated between d 21 and 129 post partum. The two forages, of similar digestible energy content, were the sole source of nutrients and were offeredad lib. (exp. 1). Digestion and flow at the duodenum were measured on 13 occasions in early lactation with comparable cows fitted with rumen and duodenal cannulae and grazing similar forages (exp. 2). The gross milk yield (22·2, G; 25·0, C, kg/dP< 0·05) and the yield of protein (0·66, G; 0·77, C, kg/d,P< 0·01) were higher, but the protein content was similar and the fat content lower for cows fed C compared with G. Cows fed G were heavier at the beginning of the experiment and lost weight more rapidly than cows fed C. Milk energy output, adjusted for tissue energy gain or loss, was 83·9 for cows fed C compared with 71·8 MJ/d for cows fed G (P< 0·001), during the period of tissue energy repletion (weeks 11–18). From week 18 to the end of lactation all cows from exp. 1 were fed silagead lib. and 502 kg dry matter of concentrates. The total (305 d) difference in lactation response to grazing C compared with G was 931 1 (5657 1, C; 4726 1, G); this was a direct response during the experiment of 301 1 and a residual response of 630 1. In exp. 2, more organic matter (6·47, G; 8·01, C, kg/ d,P< 0·001), and more non-ammonia N (433, G; 575, C, g/d,P< 0·001) entered the duodenum of cows grazing C compared with G.

I. Pure swards of perennial ryegrass (Loliwn perenne cv. Melle) as a primary growth (May), trimmed primary growth (early June) and regrowth (late June), and white clover (Trifolium repens cv. Blanca) as a mature primary growth (July) and vegetative regrowth (August), were grazed by twelve Friesian steers (mean body-weights throughout experiment 152-231 kg) at daily allocation rates of forage which provided dry matter (DM) intakes ranging from 28 to 36 g/kg body-weight).2. Total nitrogen contents of the three ryegrasses declined with season (37-20 g/kg DM), but in vitro organic matter (OM) digestibilities were relatively constant (0.7U.79). The clovers had higher N (average 366 g/kg DM) and lower fibre and water-soluble carbohydrate contents than the grasses, and in vitro OM digestibilities of 0.70 and 076 respectively.3. Duodenal digesta samples were obtained, using a portable sampling apparatus from the animals grazing the pasture, and estimates of the flow of nutrients into the small intestine were derived using two indigestible markers which were continuously infused into the rumen using a portable infusion apparatus. Forage intakes were calculated from estimates of faecal output of indigestible OM and the predicted in vivo OM digestibilities of the forages consumed. Coefficients of variation for OM flow to the small intestine and OM intake were 11.8 and 10.9 O h respectively.4. The apparent digestion of OM in the rumen ranged from 722 to 741 g/kg digestible OM intake and from 681 to 711 g/kg digestible OM intake for the grass and clover diets respectively. Substantial losses of ingested N before the small intestine were measured on all diets except the regrowth ryegrass. Losses were 030 and 040 g/g N intake on the primary growth ryegrass and the regrowth clover respectively; N contents were 37 and 39 g/kg DM respectively. 5.Comparison of the values obtained at pasture with that obtained when similar diets were offered to housed cattle (Beever et al. 1985) indicated that combined relations relating duodenal OM and non-ammonia-N (NAN) flows to OM and N intakes respectively could be established for each diet. For two diets (primary-growth grass and regrowth clover) the relations were curvilinear; for the remaining diets, the derived relations were linear.6. The effects of forage species, stage of maturity and variations in the stem:leaf ratio in the grasses, on nutrient supply in grazing animals are discussed, and a combined relation for all grazed and harvested forages examined in this and in a previous study reported by Beever et al. (1985) is established. The relation Y = 1.430-0.0169X (r072, residual SD 0.140). where Y is NAN flow/N intake (g/g) and X is N content in the crop (gfkg OM), indicates that net losses of N across the rumen will occur on diets which contain more than 25.5 g N/kg OM.7. Possible limitations in the techniques available to measure nutrient digestion and supply in grazing ruminants are discussed.

1. A total of twenty Friesian steers were grazed on pure swards of either perennial ryegrass (Lolium perenne cv. Melle) or white clover (Trifolium repens cv. Blanca) from May to late August to examine the effect of forage species and season on nutrient digestion and supply. Within each forage species, two daily allowances of forage (i.e. 30 and 60 g dry matter (DM)/kg live weight) were given, and nutrient flow into the small intestine was measured on thirteen separate occasions (viz. seven grasses and six clovers).2. Total nitrogen content of the grasses varied between 28 (early season), 19 (mid-) and 33 (late) g/kg DM, whilst the clovers showed a much narrower range and all values were higher (39-45 g/kg DM). In vitro organic matter (OM) digestibilities of plucked samples ranged from 742 to 809 g/kg OM (grass) and 712 to 790 g/kg OM (clover), the lowest values being noted in late June (grass) and midJuly (clover).3. OM intakes calculated from estimated faecal OM output (using unlabelled ruthenium) and predicted OM digestibility of the consumed forage indicated mean values of 20.9 (grass) and 26.0 (clover) g/kg live weight ( P <0.001), whilst the higher forage allowances increased OM intake by approximately 8% on both grass and clover (P < 0.01) compared with the low allowance. OM intake was significantly ( P < 0.01) lower on the two primary growths of grass examined in May (mean 16.8g/kg live weight) than the other five grass diets.4. Rumen molar propionate levels declined with season but were at all times higher on the grass than on the clover diets, whilst acetate levels were unaffected by forage species. Apart from the late-season grass, mean rumen ammonia concentration was less than 100 mg NH,-N/I on all grass diets, whilst values on the clover diets ranged from 200-350 mg/l. 5.Daily duodenal OM flows in relation to animal live weight were approximately 20% higher on the clover than on the grass diets (grass 9.2, clover 11.2 g/kg live weight, P < 0.001), but estimates of the proportion of digestible OM apparently digested in the rumen were not significantly influenced by forage species (grass 0.69, clover 6. Non-NH,-N (NAN) flow to the small intestine varied from 0.41 to 0.76 g/kg live weight on the grass diets in response to season and forage allowance compared with values of 0.6W.94 g/kg on the clover diets, with the overall forage species effect being statistically (P < 0.001) significant (grass 0.60, clover 0.76 gfkg live weight).In relation to estimated N intakes, however, these findings revealed a considerable loss of N between mouth and duodenum on the clover diets equivalent to approximately 35% of N intake. 7. A significant regression of NAN flow/unit N intake (NI) (g/g) on N content in the forage OM (g/kg) was obtained for all forages examined:indicating that efficiency of utilization of the N in fresh forages in the rumen was more closely related to forage N content than forage species per se. 0.71).NAN/NI = 1.507-0.0185 m/OM], residual SD 0.007, r 0.929, Forages contribute over 70% of the metabo...

1.A primary growth crop of peren;iial ryegrass (Lolium perenne L., cv. S23) was partially wilted and ensiled after the application of either formic acid-water (1:3, w/v; 7.1 I/t fresh herbage; control diet C) or formic acid-formaldehyde (1 : I, w/w; 8.8 l/t fresh herbage; formaldehyde treated diet F) which supplied 50 g HCHO/kg crude protein (nitrogen (N) x 6.25). Thc two silages were fed separately and a third diet comprising formaldehydetreated silage, supplemented with urea (20g/kg dry matter DM) at the time of feeding was also examined (diet FU).2. The three diets were fed at a level of 16 g DM/kg live weight to six 3-to six-month-old cattle fitted with rumen and re-entrant duodenal cannuks in two 3 x 3 Latin Square experiments, and measurements were made of the digestion of energy, carbohydratz and N.3. The formaldehyde-treated silage had a lower content of fermentation acids and ammonia-N, and a higher content of water-soluble carbohydrate and total amino acids. The apparent digestibility of organic matter, energy and N were depressed (P < 0.05. P < 0.05 and P < 0.01 respectively) by treatment with formaldehyde, but cellulose and neutral-detergent fibre digestibility were unaffected.4. Within the rumen the digestior. of organic matter, cellulose and neutral-detergent fibre were unaffected by formaldehyde treatment or supplemmmtation with urea. Microbial protein synthesis in the rumen was similar for the three diets (average 131 g/kg apparently digested organic matter in the rumen). 5.The application of formic acid-formaldehyde increased (P i 0.05) the amount of food protein escaping degradation in the rumen (4.76 diet C, 6.89 diet F; 7-07 diet FU g/kg protein intake). The contribution of amino acids of dietary origin at the duodenum increased (P < 0.05) from 50 (diet C) to 80 (diet F) and 82 (diet FU) g/kg DM intake, and the flow of total amino acids at the duodenum was 33% higher (P < O W I ) in cattle fed formic acid-formaldehyde silage ditts compared with the control silage due to the reduction in degradation of protein at ensiling and in the rumen.During silage fermentation, extensive proteolysis occurs with the final silage having a protein of high solubility whilst the non-protein-nitrogen can be as much as 60% of the total N. Formaldehyde has been used to protect protein from degradation in the rumen Siddons et al. 1979) of silage fed to sheep has previously been studied. In the present paper quantitative measurements were made of the digestion by cattle of the protein and energy in grass silage conserved with formic acid and with formic acid plus formaldehyde. In addition, a third diet comprising the formaldehyde-treated silage supplemented with urea at the time of feeding was studied, in order to examine what effect extra non-protein-N may have on microbial and total protein flow to the small intestine, should N supply to the microbes in the rumen be limited when formaldehyde-treated silage is fed (Beever et al. 1977).

1. The effect of harvesting date of perennial ryegrass (Lolium perenne) on the nutritive value of the resultant silage and the effect of substitution of late-cut silage with barley was examined in growing cattle. The diets comprised early-cut (H) and late-cut (L) silage offered alone or with 280 (LCI) or 560 (LC2) g rolled barley/kg total dry matter (DM) substituted for late-cut silage.2. Both silages were prepared with the addition of formic acid (850 g/l; 2.4 litres/t fresh weight) to a partially wilted crop, and were judged to be well fermented (pH 3.9,3.8) with lactic acid contents of 108 and 73 g/kg DM, total nitrogen contents of 24.6 and 18.4 g/kg DM and ammonia-N contents of 121 and 124 g/kg total N (values for early-and late-cut silages respectively).3. Two experiments were conducted to measure duodenal non-NH,-N (NAN) supply in relation to N intake on the four diets (feeding level 18 g DM/kg live weight (LW)) and to examine the partition of the metabolizable energy (ME) supply from the four diets using open-circuit indirect calorimetry (three feeding levels, 14, 17 and 20 g DM/kg LW). The experiments were undertaken with eight and nine Friesian male castrates respectively with a mean starting weight of 300 kg and age 12 months. The animals used in Expt 1 had been previously fitted with cannulas into the dorsal rumen and the proximal duodenum.4. NAN supply was significantly higher on diet H than all other diets which were similar irrespective of the level of barley inclusion. Mean ME contents (MJ/kg DM) of the two silages differed markedly (H 11.9, L 9.7) and barley addition (LCI and LC2) restored values to 10.7 and 11.1 MJ/kg DM respectively. Estimated NAN absorption in relation to energy supply was significantly higher for diet H (1.47 g/MJ ME) than for all other diets (mean 1.25 g/MJ ME).5. Partition of ME supply using conventional linear analysis indicated dietary differences with respect to estimated ME for maintenance (L H, LCI and LC2) and efficiency of utilization of ME supplied above maintenance (L > H, LCI and LC2), but difficulties in biological interpretation of these findings led to the use of exponential curve analysis. This provided an improved description of the findings, and whilst dietary differences were apparent, none were statistically significant. It was concluded that a single exponential equation could be used satisfactorily to describe all values.6. The consequence of these findings in relation to the carcass retentions of energy. fat and protein reported by Thomas ef al. (1988) is discussed and possible reasons for the discrepancies in energy retention measured by comparative slaughter balance and open-circuit indirect calorimetry are considered.In the previous paper (Thomas et al. 1988) the consequence of delaying the harvesting of grass for ensiling on the nutritive value of the resultant silage when offered to beef cattle was examined, and the response to barley substitution of the later-cut material was quantified. That study was concerned with the examination of body and ...