Alternative approaches to predicting methane emissions from dairy cows.

Previous attempts to apply statistical models, which correlate nutrient intake with methane production, have been of limited value where predictions are obtained for nutrient intakes and diet types outside those used in model construction. Dynamic mechanistic models have proved more suitable for extrapolation, but they remain computationally expensive and are not applied easily in practical situations. The first objective of this research focused on employing conventional techniques to generate statistical models of methane production appropriate to United Kingdom dairy systems. The second objective was to evaluate these models and a model published previously using both United Kingdom and North American data sets. Thirdly, nonlinear models were considered as alternatives to the conventional linear regressions. The United Kingdom calorimetry data used to construct the linear models also were used to develop the three nonlinear alternatives that were all of modified Mitscherlich (monomolecular) form. Of the linear models tested, an equation from the literature proved most reliable across the full range of evaluation data (root mean square prediction error = 21.3%). However, the Mitscherlich models demonstrated the greatest degree of adaptability across diet types and intake level. The most successful model for simulating the independent data was a modified Mitscherlich equation with the steepness parameter set to represent dietary starch-to-ADF ratio (root mean square prediction error = 20.6%). However, when such data were unavailable, simpler Mitscherlich forms relating dry matter or metabolizable energy intake to methane production remained better alternatives relative to their linear counterparts.

Effects of postrumen starch infusion on milk production and energy metabolism in dairy cows.

Two experiments were conducted to determine effects of postrumen starch infusion on milk production and energy and nitrogen utilization in lactating dairy cows. In experiment 1, four cows in early lactation fed grass silage and concentrates were continuously infused into the duodenum with water or 700, 1400, or 2100 g of purified maize starch daily for 10 to 12 d in a 4 x 4 Latin square design with 2-wk periods. Starch infusion increased milk yield linearly and decreased milk fat concentration in a quadratic manner such that increases in fat-corrected milk and calculated milk energy yield were minimal except at the highest rate of infusion. Changes in milk energy output suggest that even at the highest infusion rate metabolizable energy supplied by infused starch was used for tissue energy or oxidized. In experiment 2 energy and nitrogen balance were measured in four cows in late lactation fed a mixture of dehydrated lucerne, grass silage, and concentrates during the last 6 d of 2-wk abomasal infusions of 1200 g of purified wheat starch daily or water in a balanced switchback design with 5-wk periods. Measurements of fecal starch concentration indicated nearly all the starch infused was digested, but decreased fecal pH and apparent nitrogen digestion suggested an increase in hindgut starch fermentation. Starch infusion decreased urine nitrogen output in part because of increased tissue nitrogen retention but had no effect on milk nitrogen output. In absolute terms, numerical decreases in feed energy intake and energy digestion reduced the recovery of starch energy infused as digestible and metabolizable energy, but in terms of changes in total energy supply with starch infusion, 79% was recovered as metabolizable energy. Starch infusion had no effects on heat or milk energy but increased net energy for lactation due to a numerical increase in tissue energy, implying that in late-lactation cows, starch digested postruminally was used with high efficiency for tissue energy retention as protein and fat.

A mechanistic model of whole-tract digestion and methanogenesis in the lactating dairy cow: model development, evaluation, and application.

Dietary intervention to reduce methane emissions from lactating dairy cattle is both environmentally and nutritionally desirable due to the importance of methane as a causative agent in global warming and as a significant loss of feed energy. Reliable prediction systems for methane production over a range of dietary inputs could be used to develop novel dietary regimes for the limitation of feed energy loss to methane. This investigation builds on previous attempts at modeling methanogenesis and involves the development of a dynamic mechanistic model of wholerumen function. The model incorporates modifications to certain ruminal fermentation parameters and the addition of a postruminal digestive element. Regression analysis showed good agreement between observed and predicted results for experimental data taken from the literature (r2 = 0.76, root mean square prediction error = 15.4%). Evaluation of model predictions for experimental observations from five calorimetry studies (67 observations) with lactating dairy cows at the Centre for Dairy Research, in Reading, U.K., shows an underprediction (2.1 MJ/d) of methane production (r2 = 0.46, root mean square prediction error = 12.4%). Application of the model to develop diets for minimizing methanogenesis indicated a need to limit the ratio of lipogenic to glucogenic VFA in the rumen and hindgut. This may be achieved by replacing soluble sugars in the concentrate with starch or substituting corn silage for grass silage. On a herd basis, the model predicted that increasing dietary energy intake per cow can minimize the annual loss of feed energy through methane production. The mechanistic model is a valuable tool for predicting methane emissions from dairy cows.

Examination of energy utilization in cattle offered a forage diet at near- and sub-maintenance levels of feeding.

Eight Friesian calves were reared from birth to the start of the experiment at a predetermined growth rate of 1.0 kg/d. Four calves (group 1) commenced the experiment at 22 weeks of age and the remainder (group 2) at 34 weeks, and feed intake was calculated to provide sufficient metabolizable energy to sustain zero energy balance (Em). Between experimental days 30 and 56 at maintenance levels of feeding, respiratory exchange measurements were made using open-circuit calorimetry to provide indirect estimates of heat production (H) simultaneous with measurements of faecal and urinary excretion of energy and N. A datum point for Em and H was established for each animal giving mean values for Em of 491 and 537 and for H of 476 and 511 kJ/kg live weight (LW)0.75 per d for groups 1 and 2 respectively. Treatment levels calculated as 0.25, 0.5, 0.75 and 1.25 of dry matter intake required to sustain the measured Em, were imposed on individual animals after day 56 and further measurements of H and faecal/urinary balance were made between days 72 and 84 followed by measurements of fasting heat production (FHP) during days 86-90 from the two animals in each group which had received the 0.75 and 1.25 levels of intake. Regression analysis of the treatment levels indicated separate linear models which predicted Em at 419 and 473 kJ/kg LW0.75 per d for groups 1 and 2 respectively. The incorporation of FHP with partitioning of faecal and urinary energy losses measured during fasting altered the relationship but not the predicted Em. Overall predicted Em (days 72-84) from all models were 406 and 478 kJ/kg LW0.75 per d for groups 1 and 2 respectively which were significantly lower (P < 0.05) than Em measured during days 30-56.

The effect of fishmeal supplementation of a straw-based diet on growth and calorimetric efficiency of growth in heifers.

Thirty-two 160 kg dairy heifers were used to measure the effects of increasing dietary protein content on growth and heat production. A basal diet containing (g/kg) 550 sodium hydroxide-treated straw, 220 barley, 220 sugarbeet pulp and 10 urea was offered with 0, 76 and 152 g fishmeal/kg dry matter of the basal diet (F0, F1 and F2 levels respectively). The three diets were each given at two levels of feeding (low, L; high, H): 57.6 g/d per kg metabolic body-weight (W0.75) for the LFO diet and 74.7 g/d per kg W0.75 for the HFO diet. Apparent digestibility of the diets increased in response to the addition of fishmeal. Mean dry matter digestibility values were 0.67, 0.67, 0.69, 0.66, 0.68 and 0.69 and those for acid-detergent fibre digestibility were 0.60, 0.63, 0.66, 0.58, 0.60 and 0.65 for diets LF0, LF1, LF2, HF0, HF1 and HF2 respectively. Nitrogen retention increased in response to both fishmeal and feeding level. Live-weight gains were 170, 296, 434 g/d for the LF0, LF1 and LF2 diets and 468, 651 and 710 g/d for the HF0, HF1 and HF2 diets respectively. There were significant effects of increasing the plane of feeding and the level of fishmeal in the diet on live-weight gain. Dietary effects on live-weight gains were accompanied by increases in mean energy retention of 23, 45, 82, 94, 160 and 152 kJ/d per kg W0.75 for diets LF0, LF1, LF2, HF0, HF1 and HF2 respectively, but no definite evidence was obtained that dietary supplementation with fishmeal modified the efficiency of utilization of metabolizable energy for growth.

The effect of date of cut and barley substitution on gain and on the efficiency of utilization of grass silage by growing cattle. 2. Nutrient supply and energy partition.

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 (LC1) 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 milligram; 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-NH3-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 (LC1 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 greater than H, LC1 and LC2) and efficiency of utilization of ME supplied above maintenance (L greater than H, LC1 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.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of forage species and stage of harvest on the processes of digestion occurring in the rumen of cattle.

1. Pure swards of perennial ryegrass (Lolium perenne L. cv. Melle) or white clover (Trifolium repens L. cv. Blanca) were harvested daily as either primary growth (May-June) or mid- (July) and late- (August-September) season 4-week regrowths and offered to Friesian steers at two levels of feed allowance (18 and 24 g dry matter (DM)/kg live weight), to examine the effect of forage species and stage of harvest on nutrient digestion and supply. 2. The early- and mid-season grasses had low nitrogen (23 g/kg DM) and high water-soluble carbohydrate (169 g/kg) contents whilst the late-season grass had a higher N content (28 g/kg). All clover diets had high N (average 45 g/kg) and low water-soluble carbohydrate (89 g/kg) contents, and DM digestibility on all diets ranged from 0.77 to 0.83 (mean of two feeding levels). 3. Mean total rumen volatile fatty acid concentrations were significantly (P less than 0.001) higher on the clover diets, whilst on the grass diets molar proportions of propionate showed a slight but not significant decline with advancing season and tended to be higher than those on the clover diets. Mean rumen ammonia concentrations were significantly (P less than 0.001) lower on the early- and mid-season grasses (59 mg NH3-N/l) than the late-season grass (242 mg/l) and early-season clover (283 mg/l) which were all significantly (P less than 0.01) lower than the mid- and late-season clovers (372 and 590 mg/l) respectively. 4. Rates of organic matter (OM) and N digestion in the rumen were estimated using established nylon-bag techniques and found to be high on all diets, but significant effects due to forage species (clover greater than grass; P less than 0.001) were detected, whilst overall potential degradability in the rumen exceeded 0.89 for both OM and N on all diets. 5. Significantly (P less than 0.001) more OM entered the small intestine of calves fed on white clover (10.2 g/kg live weight) than those fed on ryegrass (8.33 g/kg) and similar effects due to level of feeding (g/kg; low 7.9, high 10.6; P less than 0.05) and stage of harvest (g/kg; early 8.3 v. mid 10.0, late 9.50; P less than 0.05) were also detected.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of forage species and season on nutrient digestion and supply in grazing cattle.

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 mid-July (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 less than 0.001), whilst the higher forage allowances increased OM intake by approximately 8% on both grass and clover (P less than 0.01) compared with the low allowance. OM intake was significantly (P less than 0.01) lower on the two primary growths of grass examined in May (mean 16.8 g/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 NH3-N/l 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 less than 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 0.71).(ABSTRACT TRUNCATED AT 250 WORDS)

The efficiency of energy utilization in growing cattle consuming fresh perennial ryegrass (Lolium perenne cv. Melle) or white clover (Trifolium repens cv. Blanca).

1. Twenty Friesian steers (225 kg live weight) were fed on mid- (M) (June-July) and late- (L) (August-September) season crops of either fresh perennial ryegrass (Lolium perenne cv. Melle) (G) or white clover (Trifolium repens cv. Blanca) (C). Each of the forage diets was offered at three restricted planes of nutrition above maintenance to compare the effect of forage species on the efficiency of energy utilization. All diets were harvested daily from swards of regrowth forage of intended equivalent digestibility. 2. Faecal and urine excretions were measured for 7 and 5 d respectively, followed by two consecutive 24 h measurements of methane, carbon dioxide and oxygen exchange in open-circuit respiration chambers. 3. The apparent digestibility of the energy in perennial ryegrass (0.759) was marginally higher (P less than 0.01) than that in white clover (0.748); the mid- and late-season forages were of similar (P greater than 0.05) digestibilities. Metabolizable energy (ME): digestible energy (DE) in diet G (0.837) was significantly (P less than 0.001) different from that in diet C (0.812). The partition of energy losses when expressed as MJ/GJ gross energy intake (GEI) indicated that energy lost as methane was not significantly different (P greater than 0.05) either between forages (G 62.8, C 63.4) or between seasons (M 63.2, L 63.1). Energy excretion in urine was higher for cattle fed on diet C (77.5) compared with diet G (60.5) (P less than 0.001). Heat production was similar (P greater than 0.05) between forages (G 480, C 478), but lower (P less than 0.01) for L (471) compared with M(486). Energy retention (by difference) was lower (P less than 0.001) for diet C (132) than G (156) and for M (138) than L (149) (P less than 0.05). 4. Parallel-line analysis of unscaled ME intake (MEI) in relation to retained energy (RE; MJ/d) indicated that the efficiency of utilization (kt) was similar (P greater than 0.05) between perennial ryegrass (0.42) and white clover (0.46). Linear extrapolation of the values to zero energy retention indicated that maintenance requirements of ME (Em; MJ/d) were 23.3 for diet G and 28.8 for diet C. The extrapolated Em when expressed in relation to a measured fasting heat production (FHP) of 22.8 (MJ/d) resulted in a derived efficiency of utilization of ME for maintenance (km) of 0.97 (G) and 0.79 (C), suggesting an underestimate of Em for diet G.(ABSTRACT TRUNCATED AT 400 WORDS)

The digestion of fresh perennial ryegrass (Lolium perenne L. cv. Melle) and white clover (Trifolium repens L. cv. Blanca) by growing cattle fed indoors.

1. Pure swards of perennial ryegrass (Lolium perenne L. cv. Melle) or white clover (Trifolium repens L. cv. Blanca) were harvested daily at three and two stages of growth respectively, and offered to housed cattle. The grass diets comprised primary growth (May) and two later regrowths of contrasting morphology (i.e. leaf: stem values of 1.54 and 2.84 respectively), and were characterized by high contents of water-soluble carbohydrate and neutral-detergent fibre and comparable in vitro dry matter (DM) digestibilities (mean 0.80). Total nitrogen content was higher on primary growth grass (34 g/kg DM) than on regrowths (23 g/kg DM) but lower than values obtained for the two clover diets (38 and 43 g/kg DM, respectively). The clover diets had lower water-soluble carbohydrate contents than the grasses, comparable cellulose, but lower neutral-detergent fibre contents and in vitro DM digestibilities of 0.70 and 0.77 respectively. 2. The experiment lasted from May until August, during which time a total of twenty-one young Friesian steers (initial average live weight 130 kg) were used to determine both nutrient supply to the small intestine (twelve animals) and apparent digestibility (nine animals). Each diet was offered at three levels of DM intake (i.e. 18, 22 and 26 g/kg live weight). A further six steers, all fed at the rate of 22 g DM/kg live weight, were used to determine the metabolizable energy contents of the five diets by means of open-circuit calorimetry. 3. The three grass diets and the later-cut clover had, as intended, quite similar in vivo organic matter digestibilities, but that of the earlier-cut clover was lower, and this was associated with a large number of flower heads in this crop at the time of feeding. 4. On the clover diets, proportionately less of the ingested organic matter appeared to be digested in the rumen (0.40) compared with the grass diets (0.58) (P less than 0.001). On the high-N primary grass and the clover diets, substantial rumen losses of N were detected (P less than 0.01) compared with regrowth grasses. 5. The metabolizable energy content of the primary growth of grass was 12.2 MJ/kg DM, whilst the values for the other two grass diets were lower (11.6 MJ/kg DM), despite no marked decline in overall energy digestibility. Values for the two clover diets (mean 10.5 MJ/kg DM) were considerably lower than all values noted for the grasses.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of grinding and pelleting on the digestion of Italian ryegrass and timothy by sheep.

1. Primary growths of Italian ryegrass and timothy were harvested in late May, high-temperature dried and either retained in the chopped form or ground through a 2 mm sieve and pelleted. All diets were fed to four sheep fitted with re-entrant cannulas into the proximal duodenum and measurements of the sites of energy and protein digestion and the synthesis of volatile fatty acids (VFA) and microbial protein were made. 2. Grinding and pelleting significantly reduced rumen digestion of organic matter and structural carbohydrate (P less than 0.05) and the synthesis of rumen VFA (P less than 0.01), whilst significantly more digestion occurred in the hind gut, although this was not sufficient to prevent a decline in over-all digestibility on the pelleted diets (P less than 0.05). The magnitude of all responses was much larger on the Italian ryegrass diet. 3. Net microbial protein synthesis was 15% less on the pelleted diets but efficiency of microbial protein synthesis was unaffected (mean 188 g/kg rumen digested organic matter). Pelleting reduced the degradation of dietary protein from 69% to 47%, and dietary protein represented significantly more of the total protein flowing to the duodenum on the pelleted diets (chopped 28%, pelleted 41%). 4. Over-all, grinding and pelleting reduced total absorbed energy supply by 10% but increased absorbed protein supply by 15% which may contribute to some of the improvements seen in the net energy value of pelleted diets.

The digestion by cattle of grass silage made with formic acid and formic acid-formaldehyde.

1. A primary growth crop of perennial ryegrass (Lolium perenne L., cv. S23) was partially wilted and ensiled after the application of either formic acid-water (1:3, w/v; 7.11 l/t fresh herbage; control diet C) or formic acid-formaldehyde (1:1, 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). The two silages were fed separately and a third diet comprising formaldehyde-treated silage, supplemented with urea (20 g/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 cannulas in two 3 x 3 Latin Square experiments, and measurements were made of the digestion of energy, carbohydrate 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 less than 0.05, P less than 0.05 and P less than 0.01 respectively) by treatment with formaldehyde, but cellulose and neutral-detergent fibre digestibility were unaffected. 4. Within the rumen the digestion of organic matter, cellulose and neutral-detergent fibre were unaffected by formaldehyde treatment or supplementation 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 less than 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 less than 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 less than 0.001) in cattle fed formic acid-formaldehyde silage diets compared with the control silage due to the reduction in degradation of protein at ensiling and in the rumen.

The utilization of chopped and pelleted lucerne (Medicago sativa) by growing lambs.

1. The efficiency of utilization of the dietary energy and nitrogen contained in a dried lucerne (Medicago sativa cv. Chartainvilliers) given either chopped (CL) or ground (1.96 mm screen) and pelleted (PL), was measured in a comparative slaughter experiment. Growing lambs were given equal amounts of digestible energy in the chopped or pelleted form at each of the three planes of nutrition for a period of 100 d. 2. The initial energy, fat and protein content of both the carcass and the total body of the test lambs was estimated from regression equations between fasted (18 h) live weight and these components, derived from a group of twenty-three comparable lambs. The final energy, fat and protein content of the test lambs was determined directly by chemical analyses. 3. The metabolizable energy (ME) content of the diets was derived at each plane of nutrition from measured faecal and urinary losses and estimated methane losses. The depression in ME content with grinding and pelleting the dried lucerne was small (CL 8.69 MJ/kg dry matter (DM), PL 8.42 MG/kg DM). 4. The efficiency of utilization of the ME of the dried lucerne for growth and fattening was higher (P less than 0.01) when given in the ground pelleted form (0.533), than in the chopped form (0.284). The net energy value of the PL (3.5 MJ/kg DM) was higher than that of CL (2.2 MJ/kg DM). 5. Thus lambs fed on PL grew faster and had a higher caracass weight gain, carcass protein and fat retention than lambs fed on CL. The composition of the carcass was not altered by the physical processing treatment. 6. Digestion studies with these same CL and PL diets had shown that grinding and pelleting depressed digestion in the forestomachs and increased digestion in the small intestine compared with the chopped form. The increased efficiency of utilization of the gross energy and ME and the higher net energy value of PL was attributed primarily to a change in the site of digestion within the alimentary tract. Associated with this change was a higher value for absorbed amino acids : absorbed energy and an increased apparent absorption of methionine for lambs fed on PL. The difference in the energy costs of eating and ruminating the CL and PL was small.

Estimates of maintenance requirement of growing lambs.

1. Total body energy retention (ER) and metabolizable energy intake (MEI) values from experiments with 231 lambs (Suffolk male x (Border Leicester male x Cheviot female) female) housed indoors and given thirteen forage diets were used to estimate the metabolizable energy (ME) required for maintenance. 2. ER was measured using the comparative slaughter technique, and the lambs were fed at several planes of nutrition above maintenance between 2 and 5 months of age. 3. The daily ER and MEI results were scaled to live weight (kg0.75) and linear regression lines fitted to the values for individual diets. Extrapolation of the fitted lines to zero ER gave estimates of maintenance requirement ranging from 141 to 466 kJ ME/kg0.75 per d and values for the efficiency of utilization of ME for growth and fattening (Kf) of 0.25-0.53 (mean 0.39). 4. An alternative analysis constrained the estimated maintenance requirement to be the same for all diets. An iterative search procedure indicated minimal residual variation at 339 kJ/kg0.75 per d. This common value of ME for maintenance gave Kf values ranging from 0.30 to 0.54 (mean 0.39). 5. The implications of the technique were considered together with some discussion of the variability of the estimate. Allowing the minimum RSD to vary by 10% gave a maintenance requirement of between 231 and 408 kJ/kg0.75 per d.